Sir Norman Lockyer

CHAPTER XXV.
THE GENERAL FIELD OF PHYSICAL INQUIRY.

We have now gone down the stream of time, from Hipparchus to our own days. We find now enormous telescopes which enable us to see and examine celestial bodies lying at distances so great that the mention of them conveys little to the mind. We find also perfect systems of determining their places. The following chapters will show, however, that modern astronomy has not been contented with annexing those two branches of physics which have enabled us to make the object-glass and the clock, and another still which enables us to make that clock record its own time with accuracy.

These applications of Science have been effected for the purpose either of determining with accuracy the motion and positions of the heavenly bodies or of enabling us to investigate their appearances under the best possible conditions. The other class of observations to which we have now to refer, have to do with the quantity and the quality of the vibrations which these bodies impart to the ether, by virtue of which vibrations they are visible to us.

We began by measurement of angles, we end with a wide range of instruments illustrating the application of almost every branch of physical as well as of mathematical science. In modern observatories applications of the laws of Optics, Heat, Chemistry and Electricity, are met with at every turn.

Each introduction of a new instrument, or of a new method of attack, has by no means abolished the preexisting one; accretion rather than substitution has been the rule. On the one hand, measurement of angles goes on now more diligently than it did in the days of Hipparchus, but the angles are better measured, because the telescope has been added to the divided arc. Time is as necessary now as it was in the days of the clepsydra, but now we make a pendulum divide its flow into equal intervals and electricity record it. On the other hand, the colours of the stars are noted as carefully now as they were before the spectroscope was applied to the telescope, but now we study the spectrum and inquire into the cause of the colour. The growth of the power of the telescope as an instrument for eye observations has gone on, although now almost all phenomena can be photographically recorded.

The uses to which all astronomical instruments may be put may be roughly separated into two large groups:—

I. They may be used to study the positions, motions, and sizes of the various masses of matter in the universe. Here we are studying celestial mechanics or mechanical astronomy, and with these we have already dealt.

II. They maybe used to study the motions of the molecules of which these various masses are built up, to learn their quality, arrangement, and motions. Here we are studying celestial physics, or physical astronomy.

It is with this latter branch that we now have to do.

First we have to deal with the quantity and intensity of the ethereal vibrations set up by the constituent molecules of these distant bodies. We wish to compare the quantity of light given out by one star with that given out by another. We wish, say, to compare the light of Mars with the light of Saturn; we are landed in the science of photometry, which for terrestrial light-sources has been so admirably investigated by Rumford, Bouguer, and others.

Here we deal with that radiation from each body which affects the eye—but by no means the total radiation. This is a point of very considerable importance.

Modern science recognises that in the radiation from all bodies which give us white light there is so great a difference of length of wave in the vibrations that different effects are produced on different bodies. Thus white light is a compound thing containing long waves with which heat phenomena are associated, waves of medium length to which alone the eye is tuned, and short waves which have a decided action on some metallic salts which are unaffected by the others.

To thus examine the constituents of a beam of light a lantern, with a lime-light or electric light, may be used for throwing a constant beam; we may then produce an image of the cylinders of lime or the carbon points in the lantern on a piece of paper or a screen, and our eyes will tell us that this is an instance of how the radiations from any incandescent substance are competent to give us light. We receive all the rays to which our eyes are tuned and we see a white image on the screen. We shall see also that the light is more intense than that of a candle, in other words that the radiation from the light-sources we have named is very great.

Fig. 164.—Thermopile and Galvanometer.

Now let us insert in front of the lantern a piece of deep red glass, that is, glass which allows only the red constituents of the white light to pass. Now if a thermo-electric pile, Fig. 164, be introduced into the beam we shall see that the needle of the galvanometer will alter its position. Now, why does the needle turn? This is not the place for giving all the details of this instrument, but it is sufficient to say (1) that the needle moves whenever a current of electricity flows through the coil of wire surrounding the needles, and (2) that the pile consists of a number of bars of antimony and bismuth joined at the alternate ends, and whenever one end of the pile is heated more than the other, a current of electricity is caused to flow. Such is the delicacy of the instrument, that the heat radiated from the hand, held some yards away from it, is sufficient to set the needle swinging violently; this then acts as a most delicate thermometer. In this case it shows that heat effects are produced by the red constituents of the light from the lamp.

Now replace the thermopile by a glass plate coated with a salt of silver in the ordinary way adopted by photographers. No effect will be produced.

Replace the red glass by a blue one. If the light is now allowed to fall on the photographic plate, its effect is to decompose, or alter the arrangement of, the atoms of silver, so that on applying the developing solution, the silver compound is reduced to its metallic state on the places where the light has acted; and thus, if the image of the light-source has been focussed on the plate, a photograph of it is the result. If the thermopile is brought into the beam it will be now as insensitive to the blue light as the photographic plate was to the red light in the former case. We have therefore three kinds of effects produced, viz., light, heat, and chemical or actinic action, and when light is passed through a prism, these three different radiations, or energies, are most developed in three different portions of the spectrum.

If indeed a small spectrum be thrown on the screen and the different colours are examined with the thermopile, it will be found that as long as we allow it to remain at the blue end of the spectrum, there will be no effect on the galvanometer, but if instead of holding it at the blue end we bring it towards the red, the galvanometer needle is deflected from its normal position, to that it had when the red rays fell on it, showing that it is beyond all doubt the red rays and not the blue to which it is sensitive. Where then in the spectrum are the rays which affect the photographic plate? We can at once settle this point. If one be placed in the spectrum for a short time, and then developed, it will be found to be affected only in the part on which the blue rays have fallen. Indeed to demonstrate this no lamp is necessary.

If for half-an-hour or so two pieces of sensitive paper are placed in the daylight, one covered with red glass, and the other with violet, so that the sunlight is made to travel, in the one case, through red glass, and in the other through violet, it will be found that the violet light will act, and produce a darkening of the paper, while the red glass will preserve the paper below it from all action. This is a proof that the blue end of the spectrum has another kind of energy, a chemical energy, by means of which certain chemicals are decomposed, this is the basis of photography.

These different qualities of light have been utilized by the astronomer. He attaches a thermopile to his telescope and establishes a celestial thermometry. The radiations repay a still more minute examination, and aided by the spectroscope, he is able to study with the utmost certitude the chemical condition of the heavenly host, while the polariscope enables him to acquire information in still another direction. Nor does he end here. He replaces his eye by a sensitive plate, which not only enables him to inquire into the richness of the various bodies in these short waves, but actually to obtain images of them of most marvellous beauty and exactness.

These various lines of work we have to consider in the remaining chapters.

CHAPTER XXVI.
DETERMINATION OF THE LIGHT AND HEAT OF THE STARS.

One branch of observatory work is that of determining the relative magnitude of stars, the word magnitude being of course used in a conventional sense for brightness. There are, moreover, stars which vary in brightness or magnitude from time to time; these are called variable stars, and the investigation of the amount and period of variation opens up another use for the equatorial, and an instrument is required for finding the value of the amount of light given by a star at any instant; in fact, a photometer is necessary. The methods of determining the brilliancy of stars are so similar in principle to those employed for ordinary light-sources that the ordinary methods of photometry may be referred to in the first instance. We may determine the relative brilliancy of two or more lights, or we may employ a standard light and refer all other lights to that.

Rumford’s photometer, Fig. 165, is based upon the fact that if the intensity of the shadows of an opaque body be equal, the lights throwing the shadows are equal. Hence the lights are moved towards or from a screen until the shadows are equal; then if the distances from the screen are unequal the lights are unequal, and the intensities vary in the inverse ratio of the squares of the distances.

This method is practically carried out in the telescope by reducing the aperture till the stars become invisible, and noting the apertures at which each vanishes in turn.

The most simple method of doing this is that used by Dawes, which is simply an adjustable diaphragm limiting the available area of the object-glass; we can thus view a star, and gradually reduce the aperture until the star is just visible, or until it just disappears, the latter limit being perhaps the most accurate and most usually used; the aperture is read off on the scale attached.

Fig. 165.—Rumford’s Photometer.

The photometer of Mr. Knobel is, however, a very handy one; it consists of a plate of metal having a large V-shaped piece with an angle of 60° cut out of it; another plate slides over the first in such a manner that its edge forms a base for the V-shaped opening, thus forming an equilateral triangular hole, which is adjustable at pleasure by moving the second plate. The edge of the moveable plate is divided so that the size of the base of opening is known at once, and its area easily calculated.

The annexed woodcut will give an idea of the second method which is possible.

Fig. 166.—Bouguer’s Photometer.

Let the gas flame be supposed to represent a constant light at constant distance; then the intensity of the light to be experimented upon (represented by the candle) is determined by moving it towards or from the mirror till the illumination of both the halves of the porcelain screen is equal. The instrument by which this kind of investigation is carried out by astronomers has been introduced by ZÖllner, and is called the Astrophotometer.

In this the star is compared with a small image of a portion of the flame of a lamp attached to the telescope. It being found that, though the total light emitted by the flame varies with its size, the intensity of the brightest part does not, appreciably. Two artificial stars are formed by means of a pin-hole, a double concave lens, and a double convex lens. These appear in the field by reflexion from the front and back faces of a plate of glass alongside the image of the real star, the light of which passes through the plate. The intensity of the artificial star is varied, first by changing the pin-hole, and finally by two Nicol’s prisms, the colour being first matched with that of the star by means of a third Nicol, with a quartz plate between it and the first of the other two Nicols. The instrument is provided with object-glasses of various sizes (and diaphragms) up to 2¾ inches, and, if fainter stars are to be examined, it can be screwed on to the eyepiece of an equatorial instrument. A second arrangement, like the first, but without the quartz plate arrangement, forms an artificial star from moonlight, for comparison of the light of that body with the artificial star.

So far there is no difficulty, but this measure must be interpreted into magnitude, and we must know what magnitude a star is which just disappears with a given aperture of, say, one inch, and secondly, the ratio of light between the magnitudes, or how much less light is received from a star of the next magnitude in proportion to the given one. If now we were able to start a new scale of magnitude, it would be easy to say that a star just visible with an inch aperture on a fine night shall be called a ninth magnitude star, and fix a certain number of ninth magnitude stars for reference, so that the errors induced by hazy nights and variable eyes might be eliminated. An observer on a bad night could limit his aperture on a known star, when he might find that double the area given by an aperture of one inch was required as a limit for one of the stars of reference, and in that case he would know that half the usual amount of light from every star was stopped by atmospheric causes, and he would make the requisite corrections throughout his observations. We might also say that a star of a whole magnitude, greater or less than another, shall give us half or double the amount of light—in fact, that this shall be the ratio between magnitudes. We are not, however, able to make these rules, for an arbitrary scale has been adopted for years, and we can only reduce this scale to a law, in such a manner as not to interfere greatly with the generally received magnitudes.

Amongst the brighter stars there is a close agreement in the estimate of magnitude by different observers, but amongst the higher magnitudes a difference appears. Sir J. Herschel and Admiral Smyth, for instance, go into much higher numbers of magnitudes than Struve; the limit of Admiral Smyth’s vision with his 6-inch telescope was a 16th magnitude, while the limit of Struve’s vision with a 9½-inch telescope he calls a 12th magnitude; the estimates of the latter observer are, however, gaining greater adoption. In order to reduce the relative magnitude to a law, Mr. Pogson^[21] took stars differing largely in magnitude, and compared the amount of light from each, and so reduced the ratio between the magnitudes given by Knott and all the best observers.

From this he found that a mean of 2·4 represented the ratio, and for reasons given he adopted the quantity 2·512 as a convenient ratio; as he states, “the reciprocal of ½ log. R (in his paper R = the ratio 2·512), a constant continually occurring in photometric formulÆ, is in this case exactly 5.”

So far the ratio is established. The next thing is the basis from which to commence reckoning; this Mr. Pogson fixed by reference to Argelander’s catalogued stars, estimated by him at about the 9th magnitude, and with these, comparison is made with the star whose light is measured, and the above constant of ratio applied, which at once gives the magnitude of the measured star. To do this, in Mr. Pogson’s words: “If then any observer will determine for himself the smallest of Argelander’s magnitudes, just visible by fits, on a fine moonless night, with an aperture of one inch, and call this quantity L, or the limit of vision for one inch, the limit l, for any other aperture, will be given by the simple formula, l = L + 5 × log. aperture.” The value of L founded by Mr. Pogson is 9·2; that is, a star of 9·2 magnitude, according to Argelander, is limited by 1-inch aperture, with Mr. Pogson’s eye. On different nights and with different eyes, this number, or the magnitude limited, must vary, and it varies from exactly the same causes that produce variation in the light of the stars to be measured, so that we are independent of transparency of the air, at least within considerable limits. Having found the value of L for any night, we turn the telescope on a star to be measured, then alter the aperture if we employ the first method, until the limit is found, and insert the value in the equation, the value of l, or the star’s magnitude, then at once appears. By this means a number of well-known stars of all magnitudes may be settled for future reference and comparison with variable stars.

The comparison stars then being fixed upon, and their magnitude accurately known, there is not much difficulty in comparing any variable star with one or more of those of approximately the same magnitude. By this means a number of independent estimates of the magnitude of the variable is obtained free from errors from the disturbing effects of mist or moonlight, which affects both the stars of comparison and variable alike. If we call the stars of comparison A B C D, we enter the comparisons somewhat as follows; (variable) 2 &rt A, 4 &lt B, 1 &lt C, 7 &rt D, the number showing how many tenths of a magnitude the variable is more or less bright than each comparison star, and the magnitude of the latter being known, we get several values of the magnitude of the variable, a mean of which is taken for the night. In order to show clearly to the eye the variations of a star, and to compute the periods of maximum and minimum, a graphical method is adopted: a sheet of cross-ruled paper is prepared, on which the dates of observation are represented by the abscissÆ, and the corresponding observed magnitudes by the ordinates. Dots are then made representing the several observations, and a free-hand curve drawn amongst the dots, which at once gives the probable magnitude at any epoch in the period of observation, the change of the curve from a bend upwards to downwards, or vice versÂ, indicating a maximum or minimum of magnitude.

So much then for the method of determining the intensity of the visible radiation. The next point to consider is the intensity of the thermal radiations—we pass from photometry to thermometry. The thermopile will in the future be an astronomical instrument of great importance. We need not go into its uses in other branches of physics, we shall here limit ourselves to the astronomical results which have been already obtained. Lord Rosse used a pile of this kind, made of alternate bars of bismuth and antimony. He attacked the moon, and by observing it from new to full, and from full to new, he got a distinct variation of the amount of heat, according as the moon was nearest to the epoch of full moon, or further from that epoch. As the moon was getting full, he found the needle moved, showing heat, and, after the full, it went down again and found its zero again at new. By differential observations Lord Rosse showed that this little instrument, at the focus of his tremendous reflector, was able to give some estimate of the heat of the moon, which may be 500 degrees Fahr. at the surface.

It may be said that the moon is very near us, and we ought to get a considerable amount of heat from it; but the amount is scarcely perceptible without delicate instruments. Still the instrument is so delicate, that the heat of the stars has been estimated. A pile of very similar construction to the one just mentioned has been attached by Mr. Stone to the large equatorial at Greenwich. The instrument consists of two small piles about one-tenth of an inch across the face; the wires from each are wound in contrary directions round a galvanometer, so that when equal currents of electricity are passing they counteract each other, and the needle remains stationary. It only moves when the two currents are unequal; we have then a differential galvanometer, showing the difference of temperature of the faces of the two piles; the image of a star is allowed to fall half-way between the two piles—then on one pile and then on another; then matters are reversed, and a mean of the galvanometer readings taken, beginning with zero when the image of a star was exactly between the two piles. The result was this, that the heat received from Arcturus, when at an altitude of 25°, was found to be just equal to that received from a cube of boiling water, three inches across each side, at the distance of 400 yards.

Arcturus is not the only star which has been observed in this way; in another star, Vega, which is brighter than Arcturus, it has been demonstrated that the amount of heat which it gives out, when at an altitude of 60°, is equal to that from the same cube at 600 yards, so that Mr. Stone shows beyond all question, that Arcturus gives us more heat than Vega.

This opens a new field, for if we get heat effects different from the effects on the eye, the stars ought to be catalogued with reference to their thermal relations as well as their visual brightness. Another valuable application of this method is due to Professor Henry, of Washington. Professor Henry imagined that, by means of a thermo-electric pile placed at the eyepiece of the telescope, so that a sun-spot, or a part of the ordinary surface, could be brought on the face of the pile, he could tell whether there was a greater, or less radiation of heat from a spot, than from any other part; and he was able with the thermopile to show that there was a smaller radiation of heat from the spots than from the other parts of the sun’s surface.

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21.Monthly Notices, R.A.S., vol. xvii., p. 17.

CHAPTER XXVII.
THE CHEMISTRY OF THE STARS: CONSTRUCTION OF THE SPECTROSCOPE.

In the addition of chemical ideas to astronomical inquiries, we have one of the most fruitful and interesting among the many advances of modern science, and one also which has made the connection between physics and astronomy one of the closest.

To deal properly with this part of our book, as the constitution of one of the heavenly bodies can be studied in the laboratory as well as in the observatory, we have to describe physical instruments and methods, as well as the more purely astronomical ones.

In a now rare book published in London in the year 1653, that is to say, some years before Sir Isaac Newton made his important observations on the action of a prism on the rays of light—observations which have been so very rich in results—is given Kepler’s treatise on Dioptrics. From this one finds that the great Kepler had done all he could to try to investigate the action of a three-cornered piece of glass.

It has been considered, that, because Newton was the first to teach us much of its use, he was the first to investigate the properties of the prism. This is not so. Fig. 167 is an illustration taken from this book, by which Kepler shows that if we have a prism and pass light through it, we get three distinct results when a ray (F) falls on the prism. He shows that the first surface reflects a certain amount of light, (D I), and that this is uncoloured, because it does not pass through the glass, and that the remainder is refracted by the glass and part emerges at E, coloured like the rainbow. Then he goes on to show that the second surface of the prism also reflects some light internally, and that there is a certain amount of light leaving the prism at M, and going to K.

Fig. 167.—Kepler’s Diagram.

By means of a very few experiments Newton was able to show how much knowledge could be got by examination of the prism. The first proposition in Newton’s Optics is an attempt to prove that light, which differs in colour, differs also in degree of refrangibility. We shall recollect from the fifth chapter what this term means, for it was there shown that whenever a ray of light enters obliquely a medium denser than that in which it had been travelling, it is bent towards the perpendicular to the surface, in fact it is refracted, and those rays which are most refracted by the same substance with the same angle are said to be more refrangible than others. Newton’s experiment was very simple. He took a piece of paper, one half of which was coloured red and the other half blue; and this was placed on a stand horizontally, in the light from a window, with a prism between it and the eye.

Fig. 168.—Newton’s Experiment showing the different Refrangibilities of Colours.

He went on to show, that when he allowed the beam of sunlight to fall upon the paper, strongly illuminating the red and blue portions, making at the same time all the rest of the room as dark as possible (so that the operation was not impeded by extraneous light), when he held a prism in a particular way, he found that the red and the blue occupied different positions when looked at through the prism. When the prism is held as shown, the red is seen below and the blue above. If the prism be turned with the refracting edge downwards, the red is seen above and the blue below. When the refracting edge is upwards, it is very clear that if the violet is seen uppermost it must be because the violet ray is more refracted, and when the red ray is uppermost, with the refracting edge of the prism downwards, it is because the red ray is the least refracted.

There are other experiments to which he alludes, and by which Sir Isaac Newton considered he had proved that lights which differ in colour differ also in degrees of refrangibility.

Newton at one step went to the sun, and his second theorem is “The light of the sun consists of rays of different refrangibility,” and then he enters into the proof by experiment. The light from the sun passes through a hole in the window-shutter and through the prism which throws a spectrum on a screen. We now see the full meaning of the different degrees of refrangibility. There he had a long band of light of all colours, the red at one end and the blue at the other, showing that the different colours are unequally refracted, or turned from their course. In this way Sir Isaac Newton determined whether the law, that light which differed in colour differed also in refrangibility, held true with regard to the sun; and he clearly showed that in this case also the light differs in refrangibility, in exactly the same way as the red light and the blue light had done in his experiment with the pieces of paper. He was soon able to prove to himself that the circular aperture was not the best thing he could use, because in the spectrum he had a circle of colour representing every ray into which the light could be broken up. If we put a bit of red glass in the path of the rays we get an image of the hole in red; if we use other coloured glasses, we have a circle for each particular colour; all these images overlap, and the sum total gives us an extremely mixed spectrum, something quite different from what is seen when we introduce a slight alteration, which curiously enough was delayed for a great many years.

Sir Isaac Newton recognised the difficulties there were in getting a pure spectrum by means of a circular aperture, but although he used afterwards an oblong opening instead of a circular aperture, in which we had something more or less like what we now use, namely, a “slit”—a narrow line of light; he does not seem to have grasped the point of the thing, because in one of his theorems he says he also tried triangular openings. We shall show how important it is that we should not only have an oblong opening as proposed by Newton, but that that oblong opening should be of small breadth.

The moment we exchange the circular aperture for the oblong opening of Newton, we get a spectrum of greater purity, and, as in the case of the circular opening the purity depended on the size of the circle, so also in the case of the oblong opening the purity of the spectrum depends very much on the breadth of the oblong opening.

We thus sort out the red, orange, yellow, green, blue, and violet; they are no longer mixed as they are when we employ a circular opening. If we attempt the same experiment with red glass interposed we get something more decided than before; we have no longer a circular patch of light, but an oblong one in the red; in fact, the exact form of the aperture, or slit, through which we have allowed the light to pass through the prism and lens to form an image.

Fig. 169.—Wollaston’s first Observation of the Lines in the Solar Spectrum.

Now although Newton made these important observations on sunlight, he missed one of the things, in fact we may say the thing, which has made sunlight and starlight of so much importance to Astronomy. The oblong opening which Newton used varied from one-tenth to one-twentieth of an inch in width; but Dr. Wollaston in 1812—we had to wait from 1672 till 1812 to get this apparently ridiculously small extension—used such a narrow slit as we have mentioned, and he found that when he examined the light of the sun with a prism before the eye, he got results of which Newton had never dreamt.

Dr. Wollaston not only found the light of the sun differing in refrangibility; but in the different colours of the solar light he found a number of dark lines, which are represented by the black lines across the spectrum in Fig. 169.

Fig. 170.—Copy of Fraunhofer’s first Map of the Lines in the Solar Spectrum.

Fig. 171.—Student’s Spectroscope.

In the year 1814 Fraunhofer examined the spectrum by means of the telescope of a theodolite, directing it towards a distant slit, with a prism interposed. In this manner he observed and mapped 576 lines, the appearance of the spectrum to him being represented in Fig. 170. From this time they were called the “Fraunhofer lines.” It need scarcely be said that from the time of Wollaston until a few years ago these strange mysterious lines were a source of wonder to all observers who attempted to attack the problem. The difference between the simple prism and slit which Newton, Wollaston, and Fraunhofer used to map these lines, and the modern spectroscope, as used with or without the telescope, is due to a suggestion of Mr. Simms in 1830.

Let us refer to a modern spectroscope. Fig. 171 represents a form usually used for chemical analysis. The only difference between the spectroscope and the simple prism in Newton’s experiment is this, that in the one case the light falls directly from the slit through the prism on a screen and is viewed there; and in the other the eye is placed where the screen is, and looks through the prism and certain lenses at the slit.

The great improvement which Mr. Simms suggested was this simple one. He said, “It would surely be better that the light which passes through the prism or prisms independently of the number I use, should, if possible, pass through them as a parallel beam of light; and therefore, instead of putting the slit merely on one side of a prism and the eye on the other, I will, between the slit and the prism, insert an object-glass,” as shown in Fig. 172; so that the slit of the spectroscope is the representative of the hole in the shutter.

Fig. 172.—Section of a Spectroscope, showing the Path of the Ray from the Slit.

The slit is exactly in the focus of the little object-glass, C, or collimating lens, as it is called; so that naturally the light is grasped by this lens, and comes out in a parallel beam, and travels among the prism or prisms, quite irrespective of course of their number. This parallel beam, in order to be utilized by the eye after it has passed through the system of prisms, is again taken up by another object-glass and reduced from its parallel state into a state of convergence, and brought to a focus which can be examined by means of an eyepiece.

The red rays from the slit come to a focus at R, and the blue at B, forming there their respective images of the slit, and between B and R are a number of other images of the slit, painted in every colour that is illuminating it, thus forming a spectrum which is viewed by the eyepiece. In fact, the object-glass and eyepiece constitute a telescope, through which the slit is viewed, and the collimating lens makes the light parallel, just as if it had come from a distant object, and fit to be utilized in the telescope. This is the principle to be observed in the construction of every spectroscope.

We have now given an idea of the general nature of the instrument depending on this important addition made by Mr. Simms, which is the basis of the modern spectroscope, and it is obvious that if we want considerable dispersion, we can either increase the number of prisms, or increase their dispersive power.

We have already shown in a previous chapter that the dispersion depends on the angle of the prisms, and that the calculations necessary for making the object-glass of a telescope were based upon an observation made by passing light through a prism of a particular angle made of the same glass as that of which the proposed object-glass was to be constructed. Then, again, we took the opportunity of showing that with very dense substances greater dispersion could be obtained. We showed how the prism of dense flint glass overpowered the dispersion of the prism of the crown glass, and how the combination gave us refraction without dispersion.

Fig. 173.—Spectroscope with Four Prisms.

Fig. 173 is a drawing of a spectroscope containing four prisms. It is a representation of that used by Bunsen and Kirchhoff when they made their maps of the solar spectrum: it is so arranged that the light after passing through the slit goes through the collimating lens, and then through the prisms; it is afterwards caught by the telescope lens and brought to a focus in front of the eyepiece. It is very important, when we have many prisms, to be able to arrange them so that whether we use one part of the spectrum or the other, each prism shall be in the best condition for allowing the light to traverse it; that is to say, that it shall be in the position of minimum deviation, when the angles of incidence and emergence are equal, and each surface refracts the ray equally. They can be arranged so, that as the telescope is moved to observe a new part of the spectrum, every prism will be automatically adjusted.

To insure this the prisms are united to form a chain so that they all move together, and each has a radial bar to a central pin which keeps them at the proper angle.

Fig. 174.—Automatic Spectroscope (Grubb’s form).

There is another arrangement which is very simple, in which we get the condition of minimum deviation by merely mounting the prisms on a spring, and then moving the spring with the telescope, in the same way as the telescope moves the other automatic arrangement.

Fig. 175.—Automatic Spectroscope (Browning’s form).

For some observations, especially solar observations, in which the light is very intense, it is extremely important, in fact essential, to reduce the brilliancy of the spectrum; and of course this enables us, in the case of the sun especially, to increase the dispersion almost without limit, by having a great number of prisms, or even using the same twice over, in the following manner:

On the spectroscope there is a number of prisms so arranged that the light comes from the slit, and travels through the lower portion of the prisms; it then strikes against the internal reflecting surface of a right-angled prism at the back of the last prism, Fig. 176, and is sent, up to another reflecting surface, and then comes back again through the same prisms along an upper storey, and then is caught by means of a telescope above the collimator, on the slit of which the sun’s image is allowed to fall.

Fig. 176.—Last Prism of Train for returning the Rays.

This contrivance, suggested by the author and Prof. Young independently, is now largely used. Fig. 177 shows an ordinary spectroscope so armed. The light from the slit traverses the upper portions of the prisms; it is then thrown down by the reflecting prism seen behind the collimator, then, returning along the lower part, it is received by a right-angled prism in front of the object-glass of the observing telescope.

Instead of the rays of light being reflected back through the upper storey of the prisms, another method has been adopted; the last prism is in this case a half prism, and the last surface on which the rays of light fall is silvered; the rays then are returned on themselves, and, when the instrument is adjusted, come to a focus on the inside of the slit plate, forming there a spectrum, any part of which can, by moving the prisms, be made to fall on a small diagonal reflecting prism on one side of the slit, by which it is reflected to the eyepiece. In this arrangement the collimating lens becomes its own telescope lens on the return of the ray.

Fig. 177.—Spectroscope with returning Beam.

There is another form of spectroscope, called the direct vision, which is largely used for pocket instruments. The principle of it is that the light passing through it is dispersed but not turned from its course, just the reverse of the achromatic combination of the object-glass; a crown-glass prism is cemented on a flint one of sufficient angle that their deviative powers reverse each other but leave a certain portion of the flint-glass dispersion uncorrected; since, however, the dispersive power of the flint-glass is to a great extent neutralized, therefore, in order to make the instrument as powerful as one of the ordinary construction, a number of flint-glass prisms are combined with crown-glass ones, as shown in Fig. 178.

Fig. 178.—Direct Vision Prism.

There is another form of direct-vision prism, called the Herschel-Browning, in which the ray is caused to take its original course on emerging by means of two internal reflections.

CHAPTER XXVIII.
THE CHEMISTRY OF THE STARS (CONTINUED): PRINCIPLES OF SPECTRUM ANALYSIS.

We have next to say something about the principles on which the use of the spectroscope depends; if we look through one we can readily observe how each particular ray of light paints an image of the slit. Thus, if we are dealing with a red ray of light, that ray, after passing through the prisms, will paint a red image of the slit; if the light be violet, the ray will paint a violet image of the slit, and these images will be separated, because one colour is refracted more than the other. Now it follows from this that when the slit is illuminated by white light, white light being white because it contains all colours, we get an infinite number of images of slits touching or overlapping each other, and forming what is called a continuous spectrum.

Hence it is that if we examine the light of a match or candle, or even the electric light, we get such a continuous spectrum, because these light sources emit rays of every refrangibility. Modern science teaches us that they do so because the molecules—the vibrations of which produce, through the intermediary of the ether, the sensation of light on our optic nerve—are of a certain complexity.

In the preceding list of light sources the sun was not mentioned, because its light when examined by Wollaston and Fraunhofer, was found to be discontinuous. Now it is clear that if in a beam of light there be no light of certain particular colours, of course we shall not find the image of the slit painted at all in the corresponding regions of the spectrum. This is the whole story of the black lines in the spectrum of the sun and in the spectra of the stars.

Here and there in the spectrum of these there are colours, or refrangibilities, of light which are not represented in light which comes from those bodies, and therefore there is nothing to paint the image of the slit in that particular part of the spectrum; we get what we call a dark line, which is the absence of the power of painting an image.

But then it may be asked, How comes it that the prism and the spectroscope are so useful to astronomers? In answer we may say, that if we knew no more about the black lines in the spectra of the sun and stars than we knew forty years ago, the spectroscope ought still to be an astronomical instrument, because it is our duty to observe every fact in nature, even if we cannot explain it. But these dark lines have been explained, and it is the very explanation of them, and the flood of knowledge which has been acquired in the search after the explanation, which makes the spectroscope one of the most valuable of astronomical instruments.

Many of us are aware of the magnificent generalizations by which our countrymen, Professors Stokes and Balfour Stewart, and ÅngstrÖm, Kirchhoff and Bunsen, were enabled to explain those wonderful lines in the solar spectrum.

These lines in the solar spectrum are there because something is at work cutting out those rays of light which are wanting, and they explained this want by showing to us that around the sun and all the stars there are absorbing atmospheres containing the vapours of certain substances cooler than the interior of the sun or of the stars.

These philosophers also showed us, that we can divide radiation and absorption into four classes, and that we can have general radiation and selective radiation, and general absorption and selective absorption, so that the phenomena that we see in our chemical and physical laboratories and our observatories may all be classed as general and selective radiation, or general and selective absorption.

Let us explain these terms more fully. Kirchhoff showed us that from incandescent solid and liquid bodies we get a continuous spectrum; thus from the carbon poles of an electric lamp we get a complete spectrum. That is called a continuous spectrum, and it is an instance of continuous radiation, which we get from the molecular complexity of solids or liquids, and likewise, from dense gases or vapours. When we examine vapours or gases which are not very dense we get an indication of selective radiation—that is to say, the light one gets from these substances, instead of being spread broadcast from the red to the violet, will simply fall here and there on the spectrum; in the case of one vapour we may get a yellow line—a yellow image of the slit—and in the case of another vapour, we may get a green one; the light selects its point of appearance, and does not appear all along the spectrum.

Fig. 179.—Electric Lamp. y, z, wires connecting battery of 50 Grove or Bunsen elements; G, H, carbon holders; K, rod, which stops a clockwork movement, which when going makes the poles approach until the current passes; A, armature of a magnet which by means of K frees the clockwork when not in contact; E, electro-magnet round which the current passes when the poles are at the proper distance apart, causing it to attract the armature A.

This selective radiation is due to a simplification of the molecular structure of the vapours, the simpler states are less rich in vibrations, and therefore instead of getting rays of all refrangibilities we only get rays of some.

Fig. 180.—Electric Lamp arranged for throwing a spectrum on a screen. D, lens; E E´, bisulphide of carbon prisms.

Very striking experiments showing the spectra of bodies may be made with an electric lamp armed with a condenser and a narrow slit; by means of a lens this slit is magnified on a screen. Then one or two prisms of glass containing bisulphide of carbon are placed in the beam after it has traversed the lens, which draw out the image of the slit into a spectrum. We can then place a piece of sodium on the lower carbon pole, and when the poles are brought together it will be volatilized, and its vapour rendered luminous. Its spectrum on the screen will be seen to consist of four lines only, the yellow line being for more brilliant than the rest. Sodium was selected on account of the simplicity of its spectrum.

Fig. 181.—Comparison of the line spectra of Iron, Calcium, and Aluminium, with Common Impurities. Copy of a photograph, in which by dividing the slit of the spectroscope into sections, and admitting light from the various light sources through them in succession, spectra of different elements are recorded on the same photographic plate.

If we put another metal, say calcium, in the place of the sodium, there will appear on the screen the characteristic lines of that metal. A number of distinct images of the slit in different colours is seen; if we are well acquainted with the spectrum of any metal, and see it with the spectroscope, it is easy to at once recognise it. Fig. 181 shows at one glance the spectra (1) of iron, (2) of calcium, and (3) of aluminium; and will clearly indicate the great difference there is between the radiation spectra of the rare vapours of each of the metallic elements.

Fig. 182.—Coloured Flame of Salts in the flame of a Bunsen’s Burner.

The electric light is only required where great brilliancy is essential, as for showing spectra on a screen. A Bunsen’s burner is the best instrument for studying the spectra of metallic salts. By its means the nature of a salt can be easily studied with a hand spectroscope, and in this way an almost infinitesimal quantity can be detected.

These are instances of selective radiation. We will now turn to absorption. If we first get a continuous spectrum from our lantern and then interpose substances in the path of the beam, we can examine their effects on the light. If we first use a piece of neutral-tinted glass, which is a representative of a great many substances which do, for stopping light, what solids and liquids do for giving light—namely, it cuts off a portion of every colour; the spectrum on the screen will be dimmed; here we have a case of general absorption. If, instead of this, we hold in the beam a vessel containing magenta, a dark band in the spectrum is seen, and if we put a test-tube in its place containing iodine vapour, a number of well-defined lines pervading the spectrum is observed. In these cases clearly, the magenta in one case, and the iodine vapour in the other, have cut off certain colours, and so the slit is not painted in these colours, and dark lines or bands appear. These are instances of selective absorption, certain rays are selected and absorbed, while others pass on unheeded. The easiest method of performing these absorption experiments in the case of liquids is to place the substance in a test-tube in front of the slit of the spectroscope, as shown in Fig. 183, and point the collimator to a strong light.

Besides the absorption by liquids, the vapours of the metals also absorb selectively, and if a tube containing a piece of sodium and filled with hydrogen (so that the metal will not get oxidized) is placed in the path of the rays, and the sodium heated, the spectrum is at first unaffected, but as the sodium gets hot and its vapour comes off, we can mark its effect on the spectrum. We see a dark line beginning to appear in the yellow, finally the whole light of that particular colour is absorbed, and we have a dark line in its place. To sum up then:—

We get from solids, when heated, general radiation, and when they act as absorbers, we get general absorption; from gases and vapours we get selective radiation and selective absorption.

Fig. 183.—Spectroscope arranged for showing Absorption.

Now it at once strikes any one performing these experiments that the dark line of yellow sodium appears in the same place in the spectrum as the bright one, and this is so. When the absorption by sodium vapour is examined by the spectroscope, it is then seen to consist of two well-defined lines close together, and when the radiation is examined, it is found to consist of two bright ones, and the absorption and radiation lines, the dark and bright ones, are found to exactly agree in position in the spectrum, showing that the substance that emits a certain light is able to absorb that same light, so that it matters not whether a body is acting as an absorber or radiator, for still we recognize its characteristic lines. In 1814 Fraunhofer strongly suspected the coincidence of the two bright sodium lines with the dark lines in the sun; afterwards Brewster, Foucault, and Miller showed clearly the absolute coincidence; and Professor Stokes in 1852 came to the conclusion that the double line D, whether bright or dark, belonged to the metal sodium, and that it absorbed from light passing through it the very same rays which it is able, when incandescent, to emit. The phenomena rendered visible to us by the spectroscope have their origin, as we have said, in molecular vibration, and the reason of the identical position of the light and dark lines, and indeed the whole theory of spectrum analysis, may be shortly stated as follows:—

The spectroscope tells us that when we break a mass of matter down to its finest particles, or, as some people prefer to call them, ultimate molecules, the vibrations of these ultimate parts of each different kind of matter are absolutely distinct; so that if we get the ultimate particle, say of calcium, and observe its vibrations we find that the kind of vibration or unrest of one substance—of the calcium, for instance—is different from the kind of unrest or mode of vibration—which is the same thing—of another substance, let us say sodium. Mark well the expression, ultimate molecule, because the vibrations of the larger molecular aggregations are absolutely powerless to tell us anything about their chemical nature. When we bring down a substance to its finest state, and observe, by means of the prism, the vibrations it communicates to the ether, we find that, using a slit in the spectroscope and making these vibrations paint different images of the slit, we get at once just as distinct a series of images of the slit for each substance as we should get a distinct sequence of notes if we were playing different tunes on a piano.

Next, this important consideration comes into play—whenever any element finds itself in this state of fineness, and therefore competent to give rise to these phenomena, it will give rise to them in different degrees according to certain conditions. The intensest form is observed when we employ electricity. In a great many cases the vibrations may be rendered very intense by heat. The heat of a furnace or of gas will, for instance, in a great many cases, suffice to give us these phenomena; but to see them in all their magnificence—their most extreme cases—we want the highest possible temperatures, or better still, the most extreme electric energy. What we get is the vibration of these particles rendered visible to our eye by the bright images of the slit or by their bright “lines.”

But that is not the only means we have of studying these states of unrest. We can study them almost equally well if, instead of dealing with the radiation of light from the particles themselves, we interpose them between us and a light source of more complicated molecular structure, and hotter or more violently excited than the particles themselves. From such a source the light would come to us absolutely complete; that is to say, a perfectly complete gamut of waves of light, from extreme red to extreme violet. When we deal with these particles between us and a light-source competent to give us a continuous spectrum, then we find that the functions of these molecules are still the same, but that their effect upon our retinas is different. They are not vibrating strongly enough to give us effectively light of their own, but they are eager to vibrate, and, being so, they are employed, so to speak, in absorbing the light which otherwise would come to our eyes. So that whether we observe the bright spectrum of calcium or any other metal, or the absorption spectrum under the conditions above stated, we get lines exactly in the same part of the chromatic gamut, with the difference that when we are dealing with radiation we get bright lines, and when dealing with absorption we get dark ones.

It was such considerations as these by which the presence of sodium was determined in the sun. Soon followed the discovery of coincidence of other dark lines with the bright lines of numbers of our elements, and we had maps made by Kirchhoff, and Bunsen, and ÅngstrÖm, in which almost every dark line is mapped with the greatest accuracy.

The dark lines in the spectra of the stars, and the light ones in nebulÆ, comets, and meteorites have also yielded to us a knowledge more or less accurate of the elements of which these celestial bodies are built up.

These radiations and absorptions are the A B C of spectrum analysis, and they have their application in every part of the heavens which the astronomer studies with the spectroscope. But although it is the A B C it is not quite the whole alphabet. After Kirchhoff and Bunsen had made their experiments showing that we might differentiate between solids, liquids, gases, and vapours, by means of their spectra, and say, here we have such a substance, and there another, either by its spectrum when it is incandescent or from the absorption lines produced by it on a continuous spectrum when it is absorbing, PlÜcker and Hittorf showed that not only were the spectra very different among themselves, but there were certain conditions under which the spectrum of the same substance was not always the same; and although they did not make out clearly what it was, they showed that it depended either on the pressure of the gas or vapour, or the density, or the temperature. And other observations since then indicate that we get changes in spectra which are due to pressure, and not to temperature per se; so that we have another line of research opened to us by the fact, that not only are the spectra of different substances different, but that the spectra of the same substances are different under different conditions.

Fig. 184.—Geissler’s Tube.

Fig. 184 represents a hydrogen tube, called a Geissler’s tube—a glass tube with hydrogen in it and two platinum wires, one passing into each bulb, by which a current of electricity can be passed through the gas. In this case we use hydrogen gas in a state of extreme tenuity. If now one of these tubes be connected with a Sprengel pump, we can alter the condition of tenuity at pleasure, either reducing the contents of the tube or increasing them by admitting hydrogen from a receiver, by a tap connected to the tubing of the air-pump; we can thus considerably increase the amount of gas in the tube and bring it to something like atmospheric pressure. We shall find the colour of the gas through which the spark passes varies considerably as we increase the pressure of the hydrogen in the tube. The hydrogen at starting is nearly as rare as it can be, and if more hydrogen be let in we shall see a change of colour from greenish white to red; the hydrogen admitted has increased the pressure and the colour of the spark is entirely changed. It is a very brilliant red colour, the colour of the prominences round the sun.

It may be asked, probably, whether there are any applications of this experiment to astronomical observation. It is of importance to the astronomer to get the differences of the spectra of the same substance under different conditions, and it is found as important to get these differences between the spectra of the same substance, as those between the spectra of different substances.

There is another experiment which will show another outcome of this kind of research. Change of colour in the spark is accompanied by a considerable difference in the spectrum—that is to say, it is clear, to refer back to the colour of the hydrogen when the light was green, that we should get some green in the spectrum, and when the light became red, there would be some change or increase of light towards the red end of the spectrum. We see that that is perfectly true; but there is not only a change produced by the different pressures, as shown by the different colours; but if we carry the analysis still further—if, instead of dealing with the whole of the spectrum, we examine particular lines, we find in some cases that there are very great changes in them. If, for instance, we examine the bluish-green line given by hydrogen, we shall find it increase in width as the pressure increases. This kind of effect can be shown on the screen by means of the electric lamp. We place some sodium on the carbon poles in the lamp, and have an arrangement by which we can use either twenty or fifty cells at pleasure. The action of a number of cells upon the vapour of sodium in the lamp is this: the more cells we work with, the greater is the quantity of the sodium vapour thrown out, and associated with the greater quantity of vapour is a distinct variation of the light—in fact, an increase in the width and brightness of the yellow lines on the screen.

Fig. 185.—Spectrum of Sun-Spot.

Now just to give an illustration of the profitable application of this: we know, for instance, from other sources, strengthened by this, that in certain regions of the sun, called sun-spots, there are greater quantities of sodium vapour present than in others, or it exists there at greater pressure. If that be so, we ought to get the same sort of result from the sun as we get on the screen by varying the density of the sodium vapour. That is so. We do get changes exactly similar to the changes on the screen, only of course it is the dark lines we see, and not the bright ones: the dark lines of sodium are widened out over a sun-spot, Fig. 185, showing its presence in greater quantity, or at greater pressure.

Fig. 186.—Diagram explaining Long and Short Lines.

Besides the widening of the lines due to pressure, there is something else which must be mentioned. While experimenting with the spark taken between two magnesium wires focussed on the slit of the spectroscope by a lens, the lines due to the metal were found to be of unequal lengths. Now, as the lines are simply images of the slit, the lengths of the lines depend on the length of the slit illuminated, so that in this case it appeared that the slit was not illuminated to an equal extent by all the colours given out by magnesium vapour, but that the vapour existed in layers round the wires, the lower ones giving more colours, and so also more lines, than the upper ones further from the wire, as is represented in Fig. 186; this is only meant to give an idea of the thing, and is not, of course, exactly what is seen. S is the slit of the spectroscope, P the image of one of the magnesium poles; the other, being at some little distance away, does not throw its image on the slit, and therefore does not interfere. The circles shown are intended to represent the layers of vapour giving out the spectrum; on the right the lower layers give A, B, and C, the next A and B, and the upper ones only B. Now we may reason from this that the layers next the poles are denser than those further off, and give a more complicated spectrum than the others; and also, if the quantity of vapour of any metal is small, we may only get just these longest lines.

Of late, experiments have been made in England on other metals—for instance, aluminium and zinc, and their compounds; and it is found that, when the vapour is diluted, as it were, one gets only the longest line or lines; and in the compounds, where the bands due to the compound compose the chief part of the spectrum, the longest line or lines of the metal only appear. Now what is the application of this? In the sun are found some of the dark lines of certain metals, but not all; for instance, there are two lines in the solar spectrum corresponding to zinc, but there are twenty-seven bright lines from the metal when volatilized by the electric spark. Why should not these also have their corresponding dark lines in the sun? The answer is, that the non-corresponding lines of the metal are the short ones, and only exist close to the metal where the vapour is dense; and in the sun the density is not sufficient to give these lines. Here, then, we have at once a means of measuring the quantity of vapour of certain metals composing the sun. It was thought that aluminium was not in the sun, as only two lines of the metal out of fourteen corresponded to black lines in the solar spectrum. It is now known that these two are the longest lines, and that aluminium probably exists in the sun, and zinc, strontium, and barium must also be added. These probably exist in small quantities, insufficiently dense to give all the lines seen from a spark in the air.

Fig. 187.—Comparison of the Absorption Spectrum of the Sun with the Radiation Spectra of Iron and Calcium, with Common Impurities.

There is also another quite distinct line of inquiry in which the spectroscope helps us.

Imagine yourself in a ship at anchor, and the waves passing you, you can count the number per minute; now let the vessel move in the direction whence the waves come, you would then meet more waves per minute than before; and if the vessel goes the other way, less will pass you, and by counting the increase or decrease in the number passing, you might estimate the rates at which you were moving. Again, suppose some moving object causes ripples on some smooth water, and you count the number per minute reaching you, then if that object approach you, still moving, and so producing waves at the same rate, the number of ripples a minute will increase, and they will be of course closer together; for as the object is approaching you, every subsequent ripple is started, not from the same place as the preceding one, but a little nearer to you, and also nearer to the one preceding, on whose heels it will follow closer. By the increase in the number of ripples, and also the decrease in the distance between them, one can estimate the rate of motion of the object producing them, for the decrease in distance between the ripples is just the distance the object travels in the time occupied between the production of two waves, which was ascertained when the object was stationary.

Now let us apply this reasoning to light. Light, we now hold, is due to a state of vibration of the particles of an invisible ether, or extremely rare fluid, pervading all space; and the waves of light, although infinitesimally small, move among these particles.

Now we know that it is the length of the waves of light which determines their refrangibility or colour, and therefore anything that increases or diminishes their length alters their place in the spectrum; and as waves of water are altered by the body producing them moving to or from the observer, so the waves of light are changed by the motion of the luminous body; and this change of refrangibility is detected with the spectroscope. By measuring the wave-length of let us say the F line, and the new wave-length as shown by the changed position, we can estimate the velocity at which the light source is approaching or receding from us.

This application, as we shall see in the next chapter, enables us to determine the rate at which movements take place in the solar atmosphere. It also gives us the power of measuring the third co-ordinate of the motion of stars. We can, by the examination of their positions, measure the motion at right angles to our line of sight, and so determine their motion with reference to the two co-ordinates, R.A. and Dec., or Lat. and Long., and just in the same way as we can measure the velocity of the solar gases to or from us, so we can measure the motion of the stars to or from us, thereby giving us the third co-ordinate of motion.

It need scarcely be said that by the introduction of the spectroscope a new method of observation, and a new power of gaining facts, has dawned, and the sooner it is used all over the world with the enormous instruments which are required, the better it will be for science.

---

These then are some of the chief points of spectroscopic theory which makes the spectroscope one of the most powerful instruments of research in the hands of the modern astronomer.

CHAPTER XXIX.
THE CHEMISTRY OF THE STARS (CONTINUED): THE TELESPECTROSCOPE.

We have now to speak of the methods of using these spectroscopes for the purpose of astronomical observations. For a certain class of observations of the sun no telescope is necessary, but some special arrangements have to be made.

Thus while Dr. Wollaston and Fraunhofer were contented with simple prisms, when Kirchhoff observed the solar spectrum, and made his careful maps of the lines, he used an instrument like Fig. 173, and for the purpose of comparing the spectrum of the sun with that of each of the chemical elements in turn, he used a small reflecting prism, covering one-half of the slit, Fig. 188, so that any light thrown sideways on to the slit would be caught by this prism, and reflected on to the slit as if it came from an object near the source of light at which the spectroscope is pointing, so that one-half of the slit can be illuminated by the sun, while the other is illuminated by another light; and on looking through the eyepiece one sees the two spectra, one above the other; so that we are able to compare the lines in the two spectra.

The sunlight, whether coming from the sun itself or a bright cloud, is reflected, into the comparison prism, Fig. 189, of the spectroscope. An instrument called a heliostat can be used for this, reflecting the light either directly into the prism or through the medium of other reflectors.

Fig. 188.—Comparison Prism, showing the path of the Ray.

The heliostat is a mirror, mounted on an axis, which moves at the same rate as the sun appears to travel, so that wherever the sun is, the reflector, once adjusted, automatically throws the beam into the instrument, so that the light of the moving sun can be observed without moving the spectroscope.

Fig. 189.—Comparison Prism fixed in the Slit.

An average solar spectrum is thus obtained, and, by means of a prism over one-half of the slit, it was quite possible for Kirchhoff and Bunsen to throw in a spectrum from any other source for comparison, and so they compared the spectra of the metals and other elements with the solar spectrum, and tested every line they could find in the spectra. They first found that the two lines of sodium corresponded with the two lines called D in the spectrum, then that the 460 lines of iron corresponded in the main with dark lines in the solar spectrum; and so they went on.

Fig. 190.—Foucault’s Heliostat.

There is, however, a method of varying the attack on this body altogether, by means of the spectroscope and telescope. We saw that Kirchhoff and Bunsen contented themselves with an average spectrum of the sun—that is to say, they dealt with the general spectrum which they got from the general surface of the sun, or reflected from a cloud or any other portion of the sky to which they might direct the reflector; but by means of some such an arrangement as is shown in Fig. 192, we can arrange our spectroscope so that we shall be able to form an image of the sun by the object-glass of a telescope, on the slit, and allow it to be immersed in any portion of the sun’s image we may choose. We then have a delicate means of testing what are the spectroscopic conditions of the spots and of those brighter portions of the sun which are called faculÆ, and the like. And it is known that, by an arrangement of this kind, it is even possible to pick up, without an eclipse, those strange things which are called the red prominences, or the red flames, which have been seen from time to time during eclipses.

If we wish to observe any of the other celestial bodies, we must employ a telescope and form an image on the slit, or else use the heavenly body itself as a slit. In the former case spectroscopes must be attached to telescopes, and hence again they must be light and small, unless a siderostat is employed.

In the latter case the prism is placed outside the object-glass, and the true telescope becomes the observing telescope.

Fraunhofer, at the beginning of the present century, was the first to observe the spectra of the stars by placing a large prism outside the object-glass, three or four inches in diameter, of his telescope, and so virtually making the star itself the slit of the spectroscope; and in fact he almost anticipated the arrangement of Mr. Simms, and satisfied the conditions of the problem. The parallel light from the star passed through the prism, and by means of the object-glass was brought to a focus in front of the eyepiece, so that the spectrum of the star was seen in the place of the star itself.

This system has recently been re-invented, and the accompanying woodcut, Fig. 191, shows a prism arranged to be placed in front of an object-glass of four inches aperture. It is seen that the angle of the prism is very small. The objection to this method of procedure is that the telescope has to be pointed away from the object at an angle depending upon the angle of the prism.

Fig. 191.—Object-glass Prism.

In the other arrangement we have the thing managed in a different way: we have the object-glass collecting the light from the star and bringing it to a focus on the slit, and it then passes on to the prisms, through which the light has to pass before it comes to the eye. In this combination of telescope and spectroscope we have what has been called the telespectroscope; one method of combination is seen in the accompanying drawing of the spectroscope attached to Mr. Newall’s great refractor; but any method will do which unites rigidity with lightness and allows the whole instrument to be rotated with smoothness.

Fig. 192.—The Eyepiece End of the Newall Refractor (of 25 inches aperture), with Spectroscope attached.

For solar observation, as there is light enough to admit of great dispersion, many prisms are employed, as shown in Fig. 192; or the prisms may be made so tall that the light may be sent backwards and forwards many times by means of return prisms, to which reference has been already made.

For the observation of those bodies which give a small amount of light, fewer prisms must be used, and arrangements are made for the employment of reference spectra, i.e., to throw the light coming from different chemical elements into the spectroscope, in order that we may test the lines; whether any line of Sirius, for instance, is due to the vapour of magnesium, as Kirchhoff tested whether any line in the sunlight was referable to iron or the other vapours which he subsequently studied.

Fig. 193.—Solar Spectroscope (Browning’s form).

Fig. 194.—Solar Spectroscope (Grubb’s form).

Fig. 195.—Side view of Spectroscope, showing the arrangement by which the light from a spark is thrown into the instrument by means of the reflecting prism, e, by a mirror F. (Huggins.)

Fig. 196.—Plan of Spectroscope. T, eyepiece end of telescope, B interior tube, carrying A, cylindrical lens; D, slit of spectroscope; G, collimating lens; h h, prisms; Q, micrometer. (Huggins.)

Fig. 197.—Cambridge Star Spectroscope Elevation.

Fig. 198.—Cambridge Spectroscope Plan.

These are shown in Fig. 195. e is a reflecting prism, and F is another movable reflector to reflect the light from a spark passed between two wires of the metal to be compared, and to throw it on the prism, which reflects the light through the slit of the spectroscope to the prisms and eye; if the instrument were in perfect adjustment and turned on a star, and a person were to place his eye to the spectroscope, he would see in one-half of the field of view the spectrum of the star with dark lines, and in the other half the spectrum of the vapour with its bright lines; and if he found the bright lines of the vapour to correspond with any particular dark line of the spectrum of the star, he would know whether the metal exists at that star or not; so this little mechanical arrangement at once tells him what there is at the star, whether it be iron or anything else.

In Figs. 197 and 198 is shown another form of stellar spectroscope, that of the Cambridge (U.S.) observatory; it is the same in principle as that just described.

A direct vision star spectroscope is shown in Fig. 199.

Fig. 199.—Direct-vision Star Spectroscope. (Secchi.)

A new optical contrivance altogether has to be used when star spectra are observed.

The image of a star is a point, and if focussed on the slit will of course give only an extremely narrow spectrum; to obviate this a cylindrical lens is employed, which may be placed either before the slit or between the eyepiece and the eye. If placed before the slit, it draws out the image of the star to a fine line which just fits the slit, so that a sufficient portion of the slit is illuminated to give a spectrum wide enough to show the lines, or the slit may be dispensed with altogether.

In stellar observations, when the cylindrical lens is used in front of the slit, special precautions should be taken so as to secure that the position of the cylindrical lens and slit in which the spectrum appears brightest should be used. In any but the largest telescopes the spectra of the stars are so dim that unless great care is used the finer lines will be missed. A slit is not at all necessary for merely seeing the spectra; indeed they are best seen without one. If a slit be used, it should lie in a parallel and not in a meridian; under these circumstances slight variations in the rate of the clock are of no moment.

In this and in other observational matters it is good to know what to look for, and there are great generic differences between the spectra of the various stars. In Fig. 200 are represented spectra from the observations of Father Secchi. In the spectrum of Sirius, a representative of Type I., very few lines are represented, but the lines are very thick; and stars of this class are the easiest to observe.

Next we have the solar spectrum, which is a representative of Type II., one in which more lines are represented. In Type III. fluted spaces begin to appear; and in Type IV., which is that of the red stars, nothing but fluted spaces is visible, and this spectrum shows that there is something different at work in the atmosphere of those red stars to what there is in the simpler atmosphere of the first—of Type I. These observations were first attempted, and carried on with some success, by Fraunhofer, and we know with what skill and perseverance Mr. Huggins has continued the work in later years, even employing reference spectra and determining their chemical constitution as well as their class.

Fig. 200.—Types of Stellar Spectra (Secchi).

We need scarcely say that the same arrangement, minus the cylindrical lens, is good for observing the nebulÆ and such other celestial objects as comets and planets.

For all spectrum work, it has to be borne in mind that the best definition is to be had when the actual colour under examination is focussed on the slit. With reflectors, of course, there is no difference of focus for the different colours. As the best object-glasses are over-corrected for chromatic aberration, the red focus is generally inside and the blue one outside the visual one. It is not necessary to move the whole spectroscope to secure this; all collimators should be provided with a rack and pinion giving them a bodily movement backwards and forwards.

This precaution is of especial importance in the case of solar observations, to which we have next to refer.

If in any portion of the sun’s image on the plate carrying the slit we see a spot, all we have to do is to move the telescope, and with it of course the sun’s image, so that the slit is immersed in the image of the spot; if, however, we wish to observe a bright portion of the sun, we can immerse this slit in the bright portion. Again, if we wish to examine the chromosphere of the sun, we simply have to cover half the slit with the sun, and allow the other part of the slit to be covered by any surroundings of the sun, and, so to speak, to fish round the edge; the lower half of the slit, say, is covered by the sun itself, and therefore we shall get from that half the ordinary solar spectrum; the upper half is, however, immersed in the light reflected from our atmosphere, giving a weak solar spectrum, so that we get a bright and feeble spectrum side by side. But besides the atmospheric light falling on the upper part of the slit, the image of anything surrounding the sun falls there also, and its spectrum is seen with the faint solar spectrum, and we find there a spectrum of several bright lines. Now, as an increase of dispersive power will spread out a continuous spectrum and weaken it, we may almost indefinitely weaken the atmospheric spectrum, and so practically get rid of it, still leaving the bright-line spectrum with the lines still further separated; so that if it were not for our atmosphere, we should get only the spectrum of the sun and that of its surroundings; one a continuous spectrum with black lines, and the other consisting of bright lines only.

Now if we suppose these observations made—if the precaution to which we have alluded be not taken, the spectrum of the sun-spot will differ but little from that of the general surface, and the chromospheric lines will scarcely be visible.

If the precaution be taken, in the case of the spot it will be found that every one of the surrounding pores is also a spot; and if the air be pure the spectrum will be full of hard lines running along the spectrum, just like dust lines, but emphatically not dust lines, because they change with every movement of the sun. The figure of the spot spectrum on p. 415 will show what is meant. Fig. 201 will show the appearance of the chromospheric line when the blue-green light is exactly focussed; the boundary of the spectrum of the photosphere approaches in hardness that at the end of the slit.

By measuring the lengths of the lines we can estimate the height of the vapours producing them; we find from this that magnesium is usually present to a height of a few hundred miles, and that hydrogen extends to between 3,000 and 4,000 miles; in some positions of the slit the hydrogen lines are seen to start up to great heights, showing the presence of flames or prominences extending in height to sometimes 100,000 miles.

Fig. 201.—Part of Solar Spectrum near F.

If, without changing the focus, we open the slit wider, and throw the sun’s image just off the slit, so that the very bright continuous spectrum no longer dazzles the eye, we shall be able to see these flames whenever they cross the opening, for the image of the slit is focussed on the eye, and the sun and its flames are focussed on the slit, so if we virtually remove the slit by opening it wide, we see the flames; still the limit of opening is soon approached, and the flood of atmospheric light soon masks them. The red hydrogen line of the spectrum is the best for viewing them, although the yellow or blue will answer. We may also place the sun’s image so that the slit is tangential to it, in which case a greater length of the hydrogen layer, or chromosphere, as it is called, is visible, although its height is limited by the opening of the slit.

By these means we are able to view a small part of the chromosphere at a time, and to go all round the sun in order to obtain a daily record of what is going on. If, however, we throw the image of the sun on a disc of metal of exactly the same size, we eclipse the sun, but allow the light of the chromosphere to pass the edge of the disc; this of course is masked by the atmospheric light, but if the annulus, or ring of chromosphere, be reduced sufficiently small, it can be viewed with a spectroscope in the place of a slit, in fact it is virtually a circular slit on which the chromosphere rests. By this means nearly the whole of the chromosphere can be seen at once. This is accomplished as follows:—

The image of the sun is brought to focus on a diaphragm having a circular disk of brass in the centre, of the same size as the sun’s image, so that the sun’s light is obstructed and the chromospheric light is allowed to pass. The chromosphere is afterwards brought to a focus again at the position usually occupied by the slit of the spectroscope; and in the eyepiece is seen the chromosphere in circles corresponding to the “C” or other lines.

A lens is used to reduce the size of the sun’s image, and keep it of the same size as the diaphragm at different times of the year; and other lenses are used in order to reduce the size of the annulus of light to about ? inch, so that the pencils of light from either side of it may not be too divergent to pass through the prisms at the same time, in order that the image of the whole annulus may be seen at once. There are mechanical difficulties in producing a perfect annulus of the required size, so one ½ inch in diameter is used, and can be reduced virtually to any size at pleasure.

From what has been said it is easy to see that we really now get a new language of light altogether, and a language which requires a good deal of interpretation.

Fig. 202.—Distortions of F line on Sun.

We have still, indeed, to consider some curious observations which are now capable of being made every day when anything like a sun-storm is going on, by means of the arrangement in which the spectroscope simply deals with the light that comes from a small portion of the sun instead of from all the sun. If we make the slit travel over different portions of the sun on which any up-rushes of heated material, or down-rushes of cold material, or other changes, are going on from change of surface temperature, the Fraunhofer lines, which we have before shown to be straight, instead of being so, appear contorted and twisted in all directions. On the other hand, if we examine the chromosphere under the same conditions, we find the bright lines contorted in the same manner. The usually dark lines, moreover, sometimes appear bright, even on the sun itself; sometimes they are much changed in their relative positions with reference to the solar spectrum. The meaning of these contortions has already been hinted at (p. 420).

It was there shown that every colour, or light of every refrangibility, is placed by the prisms in its own particular position, so if a ray of light alters its position in the spectrum it must change its colour or refrangibility, so the light producing the F line in the one case, and the absent light producing the dark line in the other, differ slightly in colour, or are rather more or less refrangible than the normal light from hydrogen. In the case when the F line is wafted towards the blue end of the spectrum, the light falling on the slit is rather more refrangible than usual; and in the middle drawing, Fig. 203, where the F line bifurcates, the slit is supplied with two kinds of light differing slightly in refrangibility. Not only does the light radiated by a substance change in this way, but the light absorbed by that substance also changes, hence the contortions of the black lines are due to a similar cause.

Here, therefore, we have evidence of a change of refrangibility, or colour, of the light coming from the hydrogen surrounding the sun. This change of refrangibility is due to the motion of the solar gases, as explained in the last chapter.

So we find that the hydrogen producing the light giving us one of the forms of the F line, shown in Fig. 203, is moving towards us at the rate of 120 miles a second, while that giving the other form is moving away from us.

Let us see how these immense velocities are estimated. By means of careful measurements, ÅngstrÖm has shown on his map of the solar spectrum the absolute length of the waves of light corresponding to the lines; thus the length of the wave of light of hydrogen giving the F line is 4860
10000000 of a millimeter. In Fig. 203 the dots on either side of the F line show the positions, where light would fall, if it differed from the F light by 1, 2, 3, or 4 ten-millionths of a millimeter, so that in the figure the light of that part of the line wafted over the fourth dot is of a wave-length of 4 ten-millionths of a millimeter less than that of the normal F light, which has a wave-length 4860
10000000 of a millimeter. The F light therefore has had its wave-length reduced by 4
4860 = 1
1215 part; and in order that each wave may be decreased by this amount, the source of the light must move towards us with a velocity of 1
1215 of the velocity of light, which is 186,000 miles per second, and 1
1215 of 186,000 is about 150; this then is the velocity, in miles per second, at which the hydrogen gas must have been moving towards us in order to displace the light to the fourth dot, as shown in the figure.

In previous chapters we have considered the lessons that we can learn from light—from the vibrations of the so-called ether—when we put questions to it through various instruments as interpreters. There is still another method of putting questions to these same vibrations, and the instrument we have now to consider is the Polariscope.

The spectroscope helped us to inquire into the lengths of the luminiferous waves; from the polariscope we learn whether there is any special plane in which these waves have their motion.

The polariscope is an instrument which of late years has become a useful adjunct to the telescope in examining the light from a body in order to decide whether it is reflected or not, and to ascertain indirectly the plane in which the rays reflected to the eye lie. The action of the instrument depends upon the fact that light which consists solely of vibrations perpendicular to a given plane is said to be completely polarized in that plane. Light that contains an excess of vibrations perpendicular to a given plane is said to be partially polarized in that plane.

It was Huyghens that discovered the action of Iceland spar in doubly refracting light; and the light which passed the crystal was called polarized light at the suggestion of Newton, who, it must be remembered, looked upon light as something actually emitted from luminous bodies; these projected particles were supposed, after passage through Iceland spar, to be furnished with poles analogous to the poles of a magnet, and to be unable to pass through certain bodies when the poles were not pointing in a certain direction. It was not until the year 1808 that Malus discovered the phenomenon of polarization by reflection. He was looking through a double-refracting prism at the windows of the Luxembourg Palace, on which were falling the rays of the setting sun. On turning the prism he noticed the ordinary and extraordinary images alternately become bright and dark. This phenomenon he at once saw was in close analogy to that which is observed when light is passed through Iceland spar. At first he thought it was the air that polarized the light, but subsequent experiments showed him that it was due to reflection from the glass.

Let us examine some of the phenomena before we proceed to show the use astronomers make of them.

It is the property of some crystals, such as tourmaline, when cut parallel to a given direction, called the optic axis of the crystal, to absorb all vibrations or resolved parts of vibrations perpendicular to this line, transmitting only vibrations parallel to it.

A similar absorption of vibrations perpendicular to a given direction may be effected by various other combinations, of which one, Nicol’s prism, is in most common use. Any of these arrangements may be used as an analyzer with the telescope, for determining whether the light is completely or partially polarized, and in either of these cases which is the plane of polarization. The plane containing the direction of the rays and the line in the analyzer to which the transmitted vibrations are parallel, is called the plane of analyzation: all the light which reaches the eye consists of vibrations in the plane of analyzation. As we rotate the analyzer, we rotate equally the plane of analyzation. If we find a position of the plane of analyzation for which the light received by the eye is a maximum, we know that the light from the object is partially or completely polarized in a plane perpendicular to the plane of analyzation when in this position. To determine whether the polarization is partial or complete, we must turn the analyzer through an angle of 90° from this position: if we now obtain complete darkness, we know that there are no vibrations having a resolved part parallel to the plane of analyzation in this position, or that the light is completely polarized in this plane: if there be still some light visible, the polarization is only partial.

To explain this a little more fully, we may compare the vibrations or waves of light to waves of more material things: we may have the vibrating particles of the ether moving up and down as the particles do in the case of a wave of water, or the particles may move horizontally as a snake does in moving along the ground. We may consider that ordinary light consists of vibrations taking place in all planes, but if it passes through or is reflected by certain substances at certain angles, the vibrations in certain planes are, as it were, filtered out, leaving only vibrations in a certain plane. This light is then said to be polarized, and its plane of polarization is found by its power of passing through polarizing bodies only when they are in certain positions.

If, for instance, a ray of ordinary light is passed through a crystal of tourmaline, the vibrations of the filtered ray will only lie in one plane; if then a second crystal of tourmaline be held in a similar position to the first, the ray will pass through it unaffected; but if it be turned through a quarter of a circle about the ray as an axis, the ray will no longer be able to pass, for being in a position at right angles to the first, it will filter out just the rays that the first allows to pass. For illustration, take a gridiron: if we attempt to pass a number of sheets of paper held in all positions through it, only those in a certain plane, viz., that of the rods forming the gridiron, could be passed through, and those that would go through would also go through any number of gridirons held in a similar position. But if another gridiron be placed so that its bars cross those of the first, the sheets of paper could no longer pass, and it is evident that if we could not see or feel the paper, we could tell in what plane it was by the position in which the gridiron must be held to let it pass, and having found the paper to be, say horizontal, we know that the bars of the first gridiron are also horizontal. So with light, we can analyze a ray of polarized light and say in what plane it is polarized.

The example of the gridiron, however, does not quite represent the action of the second crystal; for if the bars of the second gridiron are turned a very small distance out of coincidence with those of the first, the sheets of paper would be stopped; but with light, the intensity of the ray is only gradually diminished, until it is finally quenched when the axes of the crystals are at right angles to each other.

Light is polarized by transmission and by reflection. We have already, when we were discussing the principle involved in the double-image micrometer, seen how a crystal of Iceland spar divides a ray into two parts at the point of incidence. Now these two rays are oppositely polarized, that is to say, the vibrations take place in planes perpendicular to each other; the vibrations of the incident light in one plane are refracted more than the vibrations in the opposite plane, and we have therefore two rays, one called the ordinary ray, and the other the extraordinary ray. Fig. 204 shows a ray of light, S I, incident on the first crystal at I; it is then divided up into the ordinary ray I R and the extraordinary one I R´; a screen is then interposed, stopping the extraordinary ray and allowing the ordinary one to fall on the second crystal at I. If then this crystal be in a similar position to the first, this ray, vibrating only in one plane, will pass onwards as an ordinary ray, I R; there being no vibrations in the perpendicular plane to form an extraordinary ray, there will be only one circle of light thrown on the screen at O by the lens. But, if the second crystal be turned round the line S S as an axis, the plane of vibration of the ray falling on its surface will no longer coincide with the plane in which an ordinary ray vibrates in the crystal, and it therefore becomes split up into two, one vibrating in the plane as an ordinary ray, and the other in that of an extraordinary ray; we have therefore the ray I R´ in addition to the first, and consequently a second circle on the screen at E´. As the crystal rotates, the plane of extraordinary refraction becomes more and more coincident with the plane of vibration of the incident ray, until, when it has revolved through 90°, it coincides with it exactly; it then passes through totally as an extraordinary ray, and as the refractive power of the crystal is greater for vibrations in this plane, we get all the light traversing the direction I R and falling on the screen at E´, and there being then no light ordinarily refracted, the circle O disappears. Fig. 205 shows the relative brightness of the circles E and O as they revolve round the centre S of the screen, the images produced by the ordinary and the extraordinary ray becoming alternately bright and dark as the crystal is rotated. Fig. 206 shows the images on the screen when the ordinary ray is stopped by the first screen instead of the extraordinary one.

A crystal of tourmaline acts in a like manner to Iceland spar, but the ordinary ray is rapidly absorbed by the crystal, so that the extraordinary ray only passes. There is an objection to the use of it, as it is not very transparent, and a Nicol’s prism is now generally used for polarizing light. It is constructed out of a rhombo-hedron of Iceland spar cut into two parts in a plane passing through the obtuse angles, and the two halves are then joined by Canada balsam. The principle of construction is this: the power of refracting light possessed by Canada balsam is less than that possessed by Iceland spar for the ordinary ray, and greater in the case of the extraordinary ray; in consequence, the ordinary ray is reflected at the surface of junction, while the extraordinary ray passes onwards through the crystal.

It is manifest then that if two Nicols are used instead of two simple crystals, represented in Fig. 204, there will be only one spot of light on the screen, which is due to the extraordinary ray, and as in certain positions this no longer passes (for the ordinary ray, which appears in the place of the extraordinary when the crystal is used, cannot pass through the Nicol), no light at all passes in such positions, so that we can use the second Nicol as an analyzer to ascertain in what plane the light is polarized.

Light is also polarized by reflection from the surface of a transparent medium. When a ray of ordinary light falls on a plate of glass at an angle of 54° 55´ with the normal, the reflected ray is perfectly polarized, and at other inclinations the polarization is incomplete. Here then is polarization by reflection. Fig. 207 shows an apparatus for producing this phenomenon. The light foiling on the first mirror from E is reflected through the tube as a polarized beam, and this is analyzed by the other mirror (I), whose plane can be rotated round the axis of the tube. The angle of polarization differs with different substances according to their refractive power, for polarization of the reflected ray is perfect only when the angle of incidence is such that the reflected ray is at right angles to the refracted one.

As a result of what we have said, the light of the sun reflected from the surface of water or from the glass of a window is polarized, and although it may be dazzling to the eye, it is reduced, or even entirely cut off, when falling at the polarizing angle, by looking through the transparent Nicol’s prism or plate of glass held in certain positions and acting as an analyzer. On rotating the analyzer there is an alternation of intensity, and by looking at the window through a crystal of Iceland spar as an analyzer, two images would be seen which would alternate in brightness as the crystal is rotated. So also there is a difference in the intensity of the light from the sky when the analyzer is rotated, showing that the light reflected from the watery and dust particles in the air is polarized, and by the position of the analyzer we find that it is polarized in the plane we should expect if it be, as it is, reflected from the sun.

It will be asked, however, what is the astronomical use of determining whether light has an excess of vibrations in any given direction?

To this we may reply that light that is reflected from any body is generally partially polarized in the plane of reflection, and that if we find that the light received from any body is partially polarized in a given plane, we may conclude that it has very likely been reflected in that plane.

Hence then in the case of any celestial body the origin of the light of which is doubtful, the polariscope tells us whether the light is intrinsic or reflected.

It tells us more than this, it tells us the plane in which the reflection has taken place. As the polarization takes place, when it does take place, at the celestial body, all we have to do is to attach an analyzer to the telescope.

A careful application of the above principles has shown that the light from the sun’s corona is partially polarized, and in the same plane as it would be if reflected from small particles in the neighbourhood of the sun: so also a portion of the light of Coggia’s Comet was found to be polarized, and therefore we say that it reflected sunlight in addition to its own proper light.

In what has been hitherto said we have only considered the use of a Nicol, or glass plates, or crystal of Iceland spar as an analyzer, and by the variation of brightness the presence and plane of polarization have been determined; but unless the polarization is somewhat decided, it could not be detected by this method. Advantage is therefore taken of the fact that a plate of quartz rotates the plane of polarization of a ray passing through it, and it rotates the more refrangible colours more than the others, and some crystals rotate the plane one way, and others in the opposite direction: the crystals are therefore called respectively right- and left-handed quartz; the thicker the quartz the greater the angle through which the plane of polarization is twisted.

This supplies us with a most delicate apparatus, which we next describe. A crystal of right- and a crystal of left-handed quartz are taken and cut to such thickness that a ray of any colour, say green, has its plane turned through 90° on passing through each of them. They are then cut into the form of a semicircle and placed side by side. Any change of the angle of polarization will now affect each plate differently. In one plate the colours will change from red to violet, in the other from violet to red.

If now a ray of polarized light, say vibrating in a vertical plane, falls on them, the green rays will have their plane of vibration turned through 90° by each crystal, and the vibration of the green from both crystals will then be in the horizontal plane. Nicol’s prism interposed between the quartz plates and the eye, so as to allow horizontal vibrations to pass, will show the green from both crystals of equal intensity; the rays of other colours, being turned through a greater or less angle than 90°, will not be vibrating horizontally, and will therefore only partially pass through, so green will be the prevailing colour. If now the plane of vibration of the original ray be turned a little out of the vertical, the ray, on the red side of the green, will appear in one half, and that on the violet side of the green in the other: so that immediately the plane of polarization changes, the plates transmit a different colour, and the apparatus must be twisted round through just the same angle as the polarized ray in order to get the crystals of the same colour. It is therefore obvious that the angle made by a polarized ray with a fixed plane is easily ascertained in this manner.

There is also another instrument for detecting polarization which is perhaps more commonly used than the biquartz: it is generally called Savart’s analyser, and is extremely sensitive in its action. On looking through it at any object emitting ordinary light, the white circle of light limited by the aperture of the instrument only is seen; but if any polarized light should happen to be present, a number of parallel bands, each shaded from red to violet, make their appearance; on rotating the instrument a point is found when a very slight motion causes the bands to vanish and others to appear in the intermediate spaces, and knowing the position required for the change of bands with light polarized in a known plane, say the vertical plane, it is easy to find how far the plane of polarization of any ray is from the vertical, by the number of degrees through which the instrument must be turned to change the bands. The construction of the instrument, and especially its action, is not easy to understand without a considerable knowledge of optics, but it may be stated that a plate of quartz is cut, in a direction inclined at 45° to its axis, into two parts of the same thickness; one part is then turned through a right angle and placed with the same surfaces in contact as before; these are fixed in the instrument so that the light shall traverse them perpendicularly to the plane of section; the light then passes through a Nicol’s prism as an analyser to the eye. The lines observed, “black centred” in one position, and “white centred” in the position at right angles to this, are always in the direction before referred to. The delicacy of the test supplied by this arrangement increases as this direction is more nearly parallel or perpendicular to the plane of polarization of the ray under examination.

We come now last of all to that branch of the work of the physical astronomer which bids fair in the future to replace all existing methods of observation.

In the introductory chapter we referred to the introduction of photographic records of astronomical phenomena as marking an epoch in the development of the science. In the last ones we have to dwell briefly on the modus operandi of the various methods by which the eye is thus being gradually replaced.

The point of celestial photography is that it not only enables us to determine form and place, absolutely irrespective of personal equation so far as the eye is concerned, but that, properly done, it gives us a faithful and lasting record of the operation, so that it is not forgotten; Mr. De La Rue has called the photographic plate the retina which does not forget, and an excellent name it is.

We may pass over altogether the ordinary photographic processes, which have been carried on with a degree of skill and patience which is beyond all praise, and confine our attention exclusively to the instrumental processes. Be it remembered, we have no longer to consider the visual rays, but the so-called chemical rays, which lie at the violet end of the spectrum.

We must also recollect that, in a former chapter, we have seen that the optician’s business was to throw aside the violet rays altogether—to discard them, caring nothing for them, because, so far as the visible form of the objects is concerned, they help very little. But we shall see in a moment that, if we wish to use refractors for photographing, we must abolish this idea, and undo everything we did to get a perfect telescope to see the body, because in the case of the photographic processes employed at present, the visible rays have as little to do with building up the image on the photographic plate as the blue rays have to do with building up the image on the retina of the eye. We shall see presently how admirably this has been done by Mr. Rutherfurd. If, however, we use reflectors instead of refractors, we are able to utilize all the rays by means of the same mirror without alteration, as the focus is the same for all rays, so that a reflector is equally good for all classes of observation.

Let us first consider the cases in which the plate is made to replace the retina with the ordinary telescope. We shall see in the sequel that whether the spectroscope, polariscope, or other physical instrument be added to the telescope—when we pass, that is to say, from mechanical to physical astronomy—the plate can still replace the eye with advantage.

The body of the telescope, with the object-glass or mirror at one end and the plate at its focus in place of the eyepiece, forms the camera, corresponding to those we find in photographic studies. The plate-holder shown in section in the accompanying figure is therefore the only addition required to make a telescope into a camera for ordinary work. Fig. 208.

A is a screw of such a size that it can be inserted into the eyepiece end of the telescope; the sensitive plate is held between a lid at the back, which opens for the plate to be inserted, and a slide in front, which is drawn out so as to expose the face of the plate to the object. A piece of ground glass of extreme fineness is inserted in the slide, on which the object is focussed before the sensitive plate is put in. It is easy then by the eyepiece focussing-screw to put this nearer or further away from the object-glass, so that the image is thrown sharply on the ground glass. When that is done the ground glass is taken away, and the sensitive plate put there in its place, and then exposed as required, so that the methods are similar to the ordinary photographic process.

We have here an arrangement that enables us to photograph the moon, stars, and planets. M. Faye has proposed that for the transit circle also the photographic method should be applied, the chronograph registering the time of the instantaneous opening of the slide, instead of the time the star is seen to transit, so that the position of the star with respect to the wires is registered at a certain known time; therefore, not only for physical astronomy have we the means of making observations without an observer at all, but also for position observations.

Every one knows sufficient of photography to be aware that, if we wish to secure the image of a faint object, such as a faint star or a faint part of the moon, we must expose the plate for some little time, as we have to do in ordinary photography if the day is dull, and therefore the larger the aperture of the telescope the more light passes; and the shorter the focus is, and the more rapid the process, the shorter will be the exposure; if the focus is short, the image will be small; but as we can magnify the image afterwards, rapidity becomes of greater moment, as the shorter the time of exposure is the less atmospheric and other disturbances and errors in driving the telescope come into play. Still, if we photograph the moon or other object, we do not wish to limit ourselves to the size of the original negative obtained at the focus. If the negative is well defined—that is, if it possesses the quality of enlargeableness—there is no difficulty in getting enlarged prints.

The method of enlarging photographs is very simple; all that is required is a large camera, the negative to be copied being placed nearer the lens than the prepared paper, so that the image is larger than the original. Fig. 209 shows an enlarging camera: the body, A, can be made of wood, or better still, of a soft material, bellows-fashion, so that the length can be altered at pleasure. In the end, at B, is fixed a lens—an ordinary portrait lens will do, but a proper copying lens is preferable; and E is a piece of wood with a hole in its centre, over which the negative is placed, the distance of E to B being also adjustible; then, by altering the lengths of B E and B C, the image of the negative can be made to appear of suitable size. At the end, C, a piece of sensitive paper is placed, and the light of the sun being allowed to fall through the negative and lens, the paper soon becomes printed, and can be toned and fixed as an ordinary paper positive. The camera may be carried on a rough equatorial mounting, consisting of an axis pointing to the pole, and pulled round with the sun by attaching a string to an equatorial telescope, moved by clockwork; or a heliostat can be used with more advantage, thereby allowing the camera to be stationary; a good enlarging lens is a very desirable thing, for most lenses seem to distort the image considerably.

If we wish to obtain a large direct image of the moon, we must, as said before, employ a telescope of as long a focal length as possible; for reasons just mentioned, this is not always desirable. If, however, large images can be obtained as good as small ones, they can of course be enlarged to a much greater size. The primary image of the moon taken by Mr. De La Rue’s exquisite reflector is not quite an inch in diameter. In one of Mr. Rutherfurd’s telescopes of fifteen feet focus, the image of the moon is somewhat larger—about one and a half inch in diameter. In Mr. Newall’s magnificent refractor, the focal length of which is thirty feet, the diameter is over three inches. In the Melbourne reflector the image obtained is larger still.

In celestial photography we have not only to deal with faint objects. With the sun the difficulty is of no ordinary character in the opposite direction, because the light is so powerful that we have to get rid of it. Now there are two methods of doing this, and as in a faint object we get more light by increasing the aperture, so with a bright light like that of the sun we can get rid of a large amount of it by reducing the aperture of our telescope; but it is found better to reduce infinitesimally the time of exposure, and methods have been adopted by which that has been brought down to the one-hundredth part of a second.

Let us show the simple way in which this can be done by the means of an addition to an ordinary plate-holder.

Fig. 208 shows the ordinary plate-holder, like those used generally for photography. What is termed the instantaneous slide, B, Fig. 210, consists of a plate with an adjustible slit in it inserted between the object itself and the focus. This can be drawn rapidly across the path of the rays by means of a spring, D; we can bring it to one side, and fix it by a piece of cotton, E, and then we can release it by burning the cotton, when the spring draws it rapidly across. The velocity of the rush of the aperture across the plate, and the time of exposure, can be determined by the strength of the spring and the aperture of the slit. If the velocity is too great, we can alter the size of the slit, C. If we absorb some of the superabundant light by means of yellow glass, or some similar material, we can keep the opening wide enough to prevent any bad effects of diffraction coming into play.

The light of the sun is so intense that another method may be employed. Instead of having the plate at the focus of the object-glass we may introduce a secondary magnifier in the telescope itself, and thus obtain an enlarged image, the time necessary for its production being still so short (1
50th of a second) that nothing is lost from the disturbances of the air.

A telescope with this addition is called a photoheliograph. The first instrument of this kind was devised by Mr. De La Rue, and for many years was regularly employed in taking photographs of the sun at Kew.

Some astronomers object to this secondary magnifier, and to obtain large images use very long focal lengths, and of course a siderostat is employed. In this way Professor Winlock obtained photographs of the sun which have surpassed the limits of Mr. Newall’s refractor; the negatives have a good definition, and show a considerable amount of detail about the spots; they were taken by a lens, inserted at the end of a gas-pipe forty feet long. The pipe was fixed in a horizontal position, facing the north, and at the extreme north part of it was the lens, a single one of crown glass, with no attempt to correct it. In front of it was a siderostat, moved by a clock, reflecting the light down the tube, so that the image of the sun could be focussed on the ground glass at the opposite end.

One will see the importance of shortening the time for even the brightest object. Those who are favoured with many opportunities of looking through large telescopes know that the great difficulty we have to deal with is the atmosphere; because we have to wait for definition, and the sum total of the photograph of any one particular thing depends upon these atmospheric fits. If we require to photograph an object, it will be obvious that the more fits we have, the worse it will be, because we get a number of images partially superposed which would otherwise give as good an effect as we could get by an ordinary eye observation. It is therefore most important to reduce the interval as much as possible.

The process used should therefore be the most rapid attainable; any work on photography will give a number of processes of different degrees of rapidity, but a process that suits one person’s manipulation may prove a failure in another’s, and the general principles are the only rules suitable for all. First, the glass plate should be carefully cleaned, the collodion lightly coloured, the bath strong and neutral, certainly not acid, and the developer fairly strong. Pyrogallic acid and silver should not be used for intensifying; a good intensifier is made by adding to a solution of iodide of potassium, strength one grain to the ounce of water, a saturated solution of bichloride of mercury, drop by drop, until the precipitate at first formed ceases to be re-dissolved; use this after fixing.

Now let us inquire what has been done by this important adjunct to ordinary means of observing. We may say that celestial photography was founded in the year 1850 by Professor Bond, who obtained a daguerreotype of the moon about that date. An immense advance has been made, but not so great as there might have been if the true importance of the method had been recognized as it ought to have been; and if we study the history of the subject we find that till within the last few years we have to limit ourselves to the works of two men who, after Bond, set the work rolling. Several observers took it up for a time; but the work requires much both of time and money, and different men dropped off from time to time. There remained always steadfast one Englishman and one American—Mr. De La Rue and Mr. Rutherfurd. The magnificent work Mr. De La Rue has done was begun in 1852. He was so anxious to see whether England could not do something similar to what had been done in America, that, without waiting for a driving clock, he thought he would see whether photographs of the moon could be taken by moving the telescope by hand. He soon found that he was working against nature—that nature refused to be wooed in this way; the moon in quite a decided manner declined to be photographed, and we waited five years till Mr. De La Rue was armed with a perfect driving clock. Mr. Rutherfurd was waiting for the same thing in America.

At last, in 1857, Mr. De La Rue got a driving clock to his reflector of thirteen inches aperture, and began those admirable photographs of the moon which are now so well known. Since the above date the moon has been photographed times without number, and Mr. De La Rue has made a series which shows the moon in all her different phases. They are remarkable for the beautiful way in which the details come out in all parts of the surface. We must recollect that these pictures of which we have spoken, some of them a yard in diameter, were first taken on glass about three inches across, the image covering the central inch. At the same time the British Association granted funds for the photographic registration of sun-spots at the Kew Observatory, where the sun was photographed every day for many years.

Encouraged by success, Mr. De La Rue, in 1858, attacked the planets Jupiter and Saturn, and some of the stars. He discovered that photographs of the moon can be combined in the stereoscope so that the moon shows itself perfectly globular.

To accomplish this result it was necessary to photograph her at different epochs, so that the libration, which gives it the appearance of being turned round slightly and looking as it would do to a person several thousand miles to the right or left of the telescope, should be utilized. These two views when combined give the appearance of solidity just as the image of a near object combined by the two eyes gives that appearance. The reason of this appearance of solidity is easily seen by looking at an orange or ball first with one eye and then with the other, when it is noticed that each eye sees a little more of one side than the other; and it is the combination of these slightly dissimilar images that gives the solid appearance.

If we examine two of these photographs combined for the stereoscope, we see that they have the appearance of being taken from two stations a long distance apart. One shows a little more of the surface on one side than the other. They are obtained in different lunations, when the moon, in the same phase, has turned herself slightly round, showing more of one side. In this way we have a distinct effect due to libration. In the year 1859 Mr. De La Rue found that sun-pictures could be combined stereoscopically in the same manner.

When we turn to the labours of Mr. Rutherfurd, we find him in 1857 armed with a refractor of 11¼ inches aperture; the actinic focus, or rather the nearest approach to a focus, was 7
10ths of an inch from the visual focus. With this telescope, without any correction whatever, he, in 1857 and 1858, obtained photographs of the moon which, when enlarged to five inches in diameter, were well defined. He also obtained impressions of stars down to as far as the fifth magnitude, and also of double stars some 3? apart—for instance, ? Virginis was photographed double. The ring of Saturn and belts of Jupiter were also plainly visible, but ill-defined. The satellites of Jupiter failed to give an image with any exposure, while their primary did so in five or ten seconds. The actinic rays, instead of coming to a point and producing an image of a satellite, were spread over a certain area and thereby rendered too weak to impress the plate.

In the summer of 1858 Mr. Rutherfurd combined his first stereograph of the moon independently of Mr. De La Rue’s success in England.

Mr. Rutherfurd then commenced an inquiry of the greatest importance, which will in time bring about a revolution in the processes employed.

In 1859 he attempted, by placing lenses of different curvatures between the object-glass and the focus, to bring the chemical rays together, leaving the visual rays out of the question; this had the effect of shortening the focus considerably and improving the photographs; but he found that, except for the middle of the field, this method would not answer. He therefore in 1860 attempted another arrangement, and one which he found answered extremely well for short telescopes.

Between the lenses of the object-glass of a 4½-inch refractor he put a ring which separated the lenses by three-quarters of an inch, and reduced the power of the flint-glass lens, which corrects the crown-glass for colour, so that the combination became achromatic for the violet rays instead of for the yellow. With this lens he was successful to a certain extent: he obtained even better results than with the 11¼ inch; but eventually he rejected this method, which we may add has recently been tested by M. Cornu, who thinks very highly of it.

He next attempted a silver-on-glass mirror in 1861; in the atmosphere of New York it only lasted ten days; he gave it up; and he then very bravely, in 1864, attacked the project de novo, and began an object-glass of a telescope which should be constructed so as to give best definition with the actinic rays, just as ordinary object-glasses are made to act best with the visual rays.

He found that in order to bring the actinic portion of the rays to a perfect focus, it was necessary that a given crown-glass lens should be combined with a flint, which will produce a combined focal length of about ? shorter than would be required to satisfy the conditions of achromatism for the eye. This combination was of course absolutely worthless for ordinary visual observation; his new lens when finished was 11¼ inches aperture and a little less than 14 feet focal length. With this he obtained impressions of ninth magnitude stars, and within the area of a square degree in the Proesepe in Cancer twenty-three stars were photographed in three minutes’ exposure. Castor gave a strong impression in one second, and stars of 2? distance showed as double. But even with this method Mr. Rutherfurd was not satisfied. Coming back to the 11¼-inch object-glass which he had used at first, he determined to see whether or not the addition of a meniscus lens outside the front lens would not give him the requisite shortness of the focus and bring the actinic rays absolutely together. By this arrangement he got a telescope which can be used for all purposes of astronomical research, and he has also eclipsed all his former photographic efforts.

Having in the previous chapter dealt with some of the pioneer work, we come finally to consider some of the applications which in the last years have occupied most attention.

With regard to the sun, we need scarcely say that Messrs. De La Rue and Stewart have been enabled, by the photographic method, to give us data of a most remarkable character, showing the periodicity of the changes on the sun’s surface, and so establishing their correlation with magnetic and other physical phenomena.

These photographic researches, following upon the eye observations of Schwabe, SpÖrer, Carrington and others, have opened up to us a new field of inquiry in connection with the meteorology of the globe; and it is satisfactory to learn that photoheliographs are now daily at work at Greenwich, Paris, Potsdam, and the Mauritius, and that shortly India will be included in the list.

Quite recently, the importance of these permanent records of the solar surface has been demonstrated by Dr. Janssen, the distinguished director of the Physical Observatory at Meudon, in a very remarkable manner.

It seems a paradox that discoveries can be made depending on the appearance of the sun’s surface by observations in which the eye applied to the telescope is powerless; but this is the statement made by Dr. Janssen himself, and there is little doubt that he has proved his point.

Before we come to the discovery itself let us say a little concerning Dr. Janssen’s recent endeavours. Among the six large telescopes which now form a part of the equipment of the new Physical Observatory recently established by the French government at Meudon, in the grounds of the princely Chateau there, is one to which Dr. Janssen has recently almost exclusively confined his attention. It is a photoheliograph giving images of the sun on an enormous scale—compared with which the pictures obtained by the Kew photoheliograph are, so to speak, pigmies, while the perfection of the image and the photographic processes employed are so exquisite, that the finest mottling on the sun’s surface cannot be overlooked by those even who are profoundly ignorant of the interest which attaches to it.

This perfection of size and image have been obtained by Dr. Janssen by combining all that is best in the principles utilised in one direction by Mr. De La Rue, and in the other by Mr. Rutherfurd, to which we have before referred. In the Kew photoheliograph, which has done such noble work in its day that it will be regarded with the utmost veneration in the future, we have first a small object-glass corrected after the manner of photographic lenses, so as to make the so-called actinic and the visual rays coincide, and then the image formed by this lens is enlarged by a secondary magnifier constructed, though perhaps not too accurately, so as to make the actinic and visual rays unite in a second image on a prepared plate. Mr. Rutherfurd’s beautiful photographs of the sun were obtained in a somewhat different manner. In his object-glass, as we have seen, he discarded the visual rays altogether and brought only the blue rays to a focus, but when enlargements were made, an ordinary photographic lens—that is, one in which the blue and yellow rays are made to coincide—was used.

Dr. Janssen uses a secondary magnifier, but with the assistance of M. Pragmowski he has taken care that both it and the object-glass are effective only for those rays which are most strongly photographic. Nor is this all; he has not feared largely to increase the aperture and focal length, so that the total length of the Kew instrument is less than one-third of that in operation in Paris.

The largely-increased aperture which Dr. Janssen has given to his instrument is a point of great importance. In the early days of solar photography the aperture used was small, in order to prevent over-exposure. It was soon found that this small aperture, as was to be expected, produced poor images in consequence of the diffraction effects brought about by it. It then became a question of increasing the aperture while the exposure was reduced, and many forms of instantaneous shutters have been suggested with this end in view. With these, if a spring be used, the narrow slit which flashes across the beam to pay the light out into the plate changes its velocity during its passage as the tension of the spring changes. Of this again Dr. Janssen has not been unmindful, and he has invented a contrivance in which the velocity is constant during the whole length of run of the shutter.

By these various arrangements the plates have now been produced at Meudon of fifteen inches diameter, showing details on the sun’s surface subtending an angle of less than one second of arc.

So much for the modus operandi. Now for the branch of solar work which has been advanced.

It is more than fifteen years ago since the question of the minute structure of the solar photosphere was one of the questions of the day. The so-called “mottling” had long been observed. The keen-eyed Dawes had pointed out the thatch-like formation of the penumbra of spots, when one day Mr. Nasmyth announced the discovery that the whole sun was covered with objects resembling willow-leaves, most strangely and effectively interlaced. We may sum up the work of many careful observers since that time by stating that the mottling on the sun’s surface is due to dome-like masses, and that the “thatch” of the penumbra is due to these dome-like masses being drawn, either directly or in the manner of a cyclone, towards the centre of the spot. In fact the “pores” in the interval between the domes are so many small spots, while the faculÆ are the higher levels of the cloudy surface. The fact that faculÆ are so much better seen near the limb proves that the absorption of the solar atmosphere rapidly changes between the levels reached by the upper faculÆ and the pores.

Thus much premised, we now come to Dr. Janssen’s discovery.

An attentive examination of his photographs shows that the surface of the photosphere has not a constitution uniform in all its parts, but that it is divided into a series of figures more or less distant from each other, and presenting a peculiar constitution. These figures have contours more or less rounded, often very rectilinear, and generally resembling polygons. The dimensions of these figures are very variable; they attain sometimes a minute and more in diameter.

While in the interior of the figures of which we speak the grains are clear, distinctly terminated, although of very variable size, in the boundary the grains are as if half effaced, stretched, stained; for the most part, indeed, they have disappeared to make way for trains of matter which have replaced the granulation. Everything indicates that in these spaces, as in the penumbrÆ of spots, the photospheric matter is submitted to violent movements which have confused the granular elements.

We have already referred to the paradox that the sun’s appearance can now be best studied without the eye applied to the telescope. This is what Dr. Janssen says on that point.

“The photospheric network cannot be discovered by optical methods applied directly to the sun. In fact, to ascertain it from the plate, it is necessary to employ glasses which enabled us to embrace a certain extent of the photographic image. Then if the magnifying power is quite suitable, if the proof is quite pure, and especially if it has received rigorously the proper exposure, it will be seen that the granulation has not everywhere the same distinctness; that the parts consisting of well-formed grains appear as currents which circulate so as to circumscribe spaces where the phenomena present the aspect we have described. But to establish this fact, it is necessary to embrace a considerable portion of the solar disc, and it is this which it is impossible to realise when we look at the sun in a very powerful instrument, the field of which is, by the very fact of its power, very small. In these conditions we may very easily conclude that there exist portions where the granulation ceases to be distinct or even visible; but it is impossible to suppose that this fact is connected with a general system.”

But it is not alone with the uneclipsed sun that the new method enables us to make discoveries. The extreme importance of photography in reference to eclipse observations cannot be over estimated. Most of our best observations of eclipses have been wrought by means of photography. The time of an eclipse is an exciting time to astronomers; and it is important that we should have some mechanical operation which should not fail to record it.

The first eclipse photograph was taken in 1851. In 1860, chiefly owing to the labours of Mr. De La Rue, our knowledge was enormously increased. The Kew photoheliograph was the instrument used, and the series of pictures obtained showed conclusively that the prominences belonged to the sun. In 1868 the prominences were again photographed. In 1869 the Americans attacked the corona, and their suggestion that the base of it was truly solar has been confirmed by other photographs taken in 1870, 1871, and 1875. Although to the eye the phenomena changed from place to place, to the camera it was everywhere the same with the same duration of exposure.

It is not to be wondered at, then, that on the occasion of the last transit of Venus, which may be regarded as a partial eclipse of the sun, photography was suggested as a means of recording the phenomena.

Science is largely indebted to Dr. Janssen, Mr. De La Rue, and others for bringing celestial photography to aid us in this branch of work also. While on the one hand astronomers have to deal with precious moments, to do very much in very little time, in circumstances of great excitement; the photographer on the other goes on quietly preparing and exposing his plates, and noting the time of the exposure, and thus can make the whole time taken by the planet in its transit over the sun’s disc one enormous base line. His micrometrical measures of the position of the planet on the sun’s disc can be made after all is over. It was suggested by Dr. Janssen that a circular plate of sufficient size to contain sixty photographs of the limb of the sun, at the points at which Venus entered and left it could be moved on step by step round its centre, and so expose a fresh surface to the sun’s image focussed on it, say every second. In this way the phenomena of the transit were actually recorded at several stations.

With reference to the moon, we have said enough to show that if we wish to map her correctly, it is now no longer necessary to depend on ordinary eye observations alone; it is perfectly clear that by means of an image of the moon, taken by photography, we are able to fix many points on the lunar surface. Still, although we can thus fix these and use them as so many points of the first order, as one might say, in a triangulation, there is much that photography cannot do; the work of the eye observer would be essential in filling in the details and giving the contour lines required to make a map of the moon.

The accompanying drawings on the same scale show that up to the present, for minute work, the eye beats the camera.

The light of the moon is so feeble in blue rays that a long exposure is necessary for a large image, and during the exposure all the errors in the rate of the clock are magnified.

We need not enlarge on the extreme importance of what Mr. Rutherfurd has been doing in photographing star clusters and star groups. It is doubly important to astronomy, and starts a new mode of using the equatorial and the clock; in fact, it gives us a method by which observations may be photographically made of the proper motion of stars, and even the parallax of stars may be thus determined independently of any errors of observers. Mr. Rutherfurd shows that the places of stars can be measured by a micrometer on a plate in the same way as by ordinary observation; hence photography can be made use of in the measurement of position and distance of double stars.

As an instance of the extreme beauty of the photographs of stars produced by a proper instrument, it may be stated that with the full aperture of the 11¼-inch object-glass corrected only for the ordinary rays, Mr. Rutherfurd found that he required an exposure of more than ten seconds to get an image of the bright star Castor; but now, instead of requiring ten seconds, he can get a better image in one. The reason of this is, that, with the object-glass corrected only for the visual rays, the chemical ones are spread over a certain small area instead of coming to a point, and so, of course, the intensity is reduced; but when the chemical rays all come to one point the intensity is greater, since the image of the star is smaller and the action more intense.

Let us follow Mr. Rutherfurd a little in his actual work. First, a wet plate is exposed for four minutes. This gives stars down to the tenth magnitude. But there may be points on the plate which are not stars, hence a second impression is taken on the same plate after it has been slightly moved. All points now doubled are true stars. Now for measures of arc. Another photograph is taken, and the driving clock is stopped; the now moving stars down to the fourth magnitude are bright enough to leave a continuous line, the length of this in a very accurately known interval, say two minutes, enables the arc to be calculated.

Next comes the mapping. The negative is fixed on a horizontal divided circle on glass illuminated from below. Above it is a system of two rails, along which travels a carrier with two microscopes, magnifying fifty diameters. By the one in the centre, with two cross wires in the field of view, the photograph is observed; by the other, armed with a wire micrometer, a divided scale on glass which is fixed alongside the rail is read. Suppose we wish to measure the distance between two stars on the plate. The plate is rotated, so that the line which joins them coincides with that which is described by the optical axis of the central microscope marked by the cross wires when the carrier runs along the rails. This microscope is then brought successively over the two stars, and the other microscope over the scale reads the nearest division, while the fractions are measured by the micrometer. Hence, then, the fixed scale, and not a micrometer screw, is depended upon for the complete distance. In this way the distance between the stars on the plate can be measured to the 1
500 part of a millimetre.

So far then we have shown how photography has been called in to the aid of the astronomer, and how, by means of photography, pictures of the different celestial bodies have been obtained of surpassing excellence. Now, photography is also the handmaiden to the spectroscope in the same way as it is the handmaiden to the telescope. Not only are we able to determine and register the appearance of the moon and planets, but, day by day, or hour by hour, we can photograph a large portion of the solar spectrum; and not only so, but the spectrum of different portions of the sun: nay, even the prominences have been photographed in the same manner; while more recently still, Drs. Huggins and Draper have succeeded in photographing the spectrum of some of the stars. We owe the first spectrum of the sun, showing the various lines, to Becquerel and Draper; the finest hitherto published we owe to Mr. Rutherfurd.

This magnificent spectrum extends from the green part of the spectrum right into that part of the spectrum called the ultra-violet. Of course it had to be put together from different pictures, because there is a different length of exposure required for the different parts; the exposure of any particular part of the spectrum must be varied according to the amount of chemical intensity in that part. If the line G was exposed, say for fifteen seconds, the spectrum near the line F would require to be exposed for eight minutes, and at the line H, which is further away from the luminous part of the spectrum than G, there the exposure requisite would be two or three minutes.

In order to obtain a photograph of the average solar spectrum, the camera replaces the observing telescope, and a heliostat is used, as in the ordinary way. The beam, however, should be sent through an opera-glass in order to condense it, and thereby to render the exposure as short as possible.

Further, if an electric lamp be mounted as shown in Fig. 216, observations, similar to those originally made by Kirchhoff, of the coincidence on the various metallic lines with the Fraunhofer ones, can be permanently recorded on the photographic plate. The lens between the lamp and the heliostat is for the purpose of throwing an image of the sun between the carbon poles. The lens between the lamp and spectroscope then focuses both the poles and the image of the sun on to the slit. The spectrum of the sun is first obtained by uncovering a small part of the slit and allowing the image of the sun to fall on this uncovered portion, the lamp not being in action. When this has been done the light of the sun is shut off. The metal to be studied is placed in the lower pole; the adjacent portion of the slit is uncovered, that at first used being closed in the process. The current is then passed to render the metal incandescent. After the proper exposure the plate is developed and the spectra are seen side by side. Fig. 187 is a woodcut of a plate so obtained.

If the spectrum of any special part of the sun, or the prominences, has to be photographed, then either a siderostat must be employed, or a camera is adjusted to the telespectroscope, as shown in Fig. 217.

For the stars, of course, much smaller dispersion must be used, but the method is the same; and what has already been said by way of precaution about the observation of stellar spectra applies equally to the attempt to obtain spectrum photographs of these distant suns.