LIGHT AND ITS SOURCES—WHAT IS LIGHT?—VELOCITY OF LIGHT—REFLECTION AND REFRACTION—RELATIVE VALUE OF LIGHTS.

The subject of Light and the science of Optics are so interesting to all of us that some short history of light is necessary before we can enter upon the scientific portion of the subject. The nature of the agent (as we may term light) upon which our sight depends has employed man’s mind from a very early period. The ancients were of opinion that the light proceeded from the eye to the object looked at. But they discovered some of the properties of light. Ptolemy of Alexandria, who was born A.D. 70, made some attempts to discover the law of Refraction; and we are informed that Archimedes set the Roman fleet on fire with burning-glasses at Syracuse. The Arabian treatise of Alhagen, in 1100 A.D., contains a description of the eye and its several parts; and the writer notices refraction and the effects of magnifying glasses (or spectacles). Galen, the physician, practically discovered the principle of the stereoscope, for he laid down the law that our view of a solid body is made up of two pictures seen by each eye separately.

Still the science of optics made little progress till the law determining the path of a ray of light was made known, and the laws of refraction discovered. Refraction means that a ray is deflected from its straight course by its passage from one transparent medium to another of different density. The old philosophers found out the theory of sound, and they applied themselves to light. Newton said light consisted of minute particles emanating from luminous bodies. Huyghens and Euler opposed Newton’s theory of the emission of light; and it was not till the celebrated Thomas Young, Professor at the Royal Institution, grappled with the question that the undulating or wave theory of light was found out. He based his investigations upon the theory of sound waves; and we know that heat, light, and sound are most wonderfully allied in their manner of motion by vibration. But he was ridiculed, and his work temporarily suppressed by Mr. Brougham.

Light, then, is a vibratory motion (like sound and heat), a motion of the atoms of our ether. But how is the motion transmitted? Sound has its medium, air; and in a vacuum sounds will be very indistinctly heard, if heard at all. But what is the medium of communication of light? It is decided that light is transmitted through a medium called ether, a very elastic substance surrounding us. The vibrations, Professor Tyndall and other philosophers tell us, of the luminous atoms are communicated to this ether, or propagated through it in waves; these waves enter the pupil of the eye, and strike upon the retina. The motion is thus communicated by the optic nerve to the brain, and then arises the great primary faculty, Consciousness. We see light, the waves of which, or ether vibrations, are transversal; air waves or sound vibrations are longitudinal.

We have spoken of radiant heat. Light acts in the same way through the ether; and when we consider Sound we shall learn that a certain number of vibrations of a string give a certain sound, and the quicker the vibration the shriller the tone. So in light. The more quickly the waves of luminosity travel to our eye, and the faster they strike it, the greater the difference in the colour, or what we call colour. Light as we see it is composed of different colours, as visible in the rainbow. There are seven primary colours in the sunlight, which is white. These can be divided or “dispersed,” and the shortest rays of the spectrum are found to be violet, the longest red. It has been calculated that 39,000 red waves make an inch in length. Light travels at a rate of nearly 190,000 miles a second, so if we multiply the number of inches in that distance by the number of red waves, we shall have millions of millions of waves entering the eye in a single second of time. The other waves enter more rapidly still, and “the number of shocks corresponding to the impression of violet is seven hundred and eighty-nine millions of millions” per second! Or taking the velocity of light at 186,000 miles in a second, it would be six hundred and seventy-eight millions of millions (Tyndall). There may be other colours which we cannot see because the impressions come too rapidly upon the retina; but the violet impression has been thus accurately determined. See page 168.

We have seen that heat is a kind of motion of particles in a body—a vibratory motion which, instead of being apparent to the ear, is apparent to the eye in rays of light. Thus heat, sound, and light are all intimately connected in this way. We have also learnt that rays of light radiate and travel with tremendous speed to our eyes, but without any shock. There is no feeling connected with the entrance of light to the eye any more than there is any sensation of sound when entering the ear, except when the light is vividly and very suddenly revealed, or when a very piercing sound is heard. Then the nerves are excited, and a painful sensation is the result; but under ordinary circumstances we are not physically conscious of the entrance of light or sound.

Heat and light are considered to be one and the same thing in different degrees of intensity. The sources of light are various. The sun and fixed stars, heat, electricity, many animals, and some plants, as well as decaying animal matter, give out light. There are luminous and non-luminous bodies. The moon is non-luminous, as she derives her light from the sun, as does the earth, etc.

Light is distributed in rays. These rays are straight in all directions. The velocity of light is almost inconceivable. It travels at a rate of 186,500 miles a second. The latest computation with electric light has given a rate of 187,200 miles a second; but the blue rays in the light experimented on probably account for the difference, for blue rays travel quicker by one per cent. than red rays. RÖmer first found out the velocity of light, which comes to us from the sun—ninety millions of miles—in eight minutes. Fizeau calculated the velocity by means of a wheel, which was set moving with tremendous speed by making the light pass between the teeth of the wheel and back again.

When rays of light meet substances they are deflected, and the phenomena under these circumstances are somewhat similar to the phenomena of heat and sound. There are three particular conditions of rays of light: (1) they are absorbed; (2) they are reflected; (3) they are refracted.

Firstly. Let us see what we mean by light being absorbed; and this is not difficult to understand, for any “black” substance shows us at once that all the sunlight is taken in by the black object, and does not come out again. It does not take in the light and radiate it, as it might heat. The rose is red, because the rays of light pass through it, and certain of them are reflected from within. So colour may be stated to be the rays thrown out by the objects themselves—those they reject or reflect being the “colour” of the object.

Secondly. Bodies which reflect light very perfectly are known as mirrors, and they are termed plane, concave, or convex mirrors, according to form. A plane mirror reflects so that the reflected ray d i forms the same angle with the perpendicular as the incident ray r i; in other words, the angle of incidence is always equal to the angle of reflection, and these rays are perpendicular to the plane from which they are reflected. The rays diverge, so that they appear to come from a point as far behind the mirror as the luminous point is in front, and the images reflected have the same appearance, but reversed. There is another law, which is that “the angular velocity of a beam reflected from a mirror is twice that of the mirror.” The Kaleidoscope, with which we are all familiar, is based upon the fact of the multiplication of images by two mirrors inclining towards each other.

A concave mirror is seen in the accompanying diagram, and may be called the segment of a hollow sphere—V W. The point C is the geometrical centre, and O C the radius; F is the focus; the line passing through it is the optical axis; O being the optical centre. All perpendicular rays pass through C. All rays falling in a direction parallel with the optical axis are reflected and collected at F. Magnified images will be produced, and if the object be placed between the mirror and the focus, the image will appear at the back; while if the object be placed between the geometrical centre and the focus, the image will appear to be in front of the mirror.

We can understand these phenomena by the accompanying diagrams. Suppose a ray A n passes from one object, A B, at right angles, it will be reflected as n A C, the ray A C being reflected to F. These cannot meet in front of the mirror, but they will if produced meet at a, and the point A will be reflected there; similarly B will be reflected at b, and thus a magnified image will appear behind or at the back of the mirror’s surface. In the next diagram the second supposed case will produce the image in the air at a b, and if a sheet of paper be held so that the rays are intercepted, the image will be visible on the sheet. In this case the perpendicular ray, A n, is reflected in the same direction, and the ray, a c, parallel with the axis is reflected to the focus. These rays meet at a and corresponding rays at b, when the image will be reproduced; viz., in front of the mirror.

The concave mirror is used in the manufacture of telescopes, which, with other optical instruments, will be described in their proper places. We will now look at the Refraction of light.

Bodies which permit rays of light to pass through them are termed transparent. Some possess this property more than others, and so long as the light passes through the same medium the direction will remain the same. But if a ray fall upon a body of a different degree of density it cannot proceed in the same direction, and it will be broken or refracted, the angle it makes being termed the angle of refraction.

For instance, a straight stick when plunged into water appears to be broken at the point of immersion. This appearance is caused by the rays of light taking a different direction to our eyes. If in the diagram (fig. 89) our eye were at o, and the vessel were empty, we should not see m; but when water is poured into the vessel the object will appear higher up at n, and all objects under water appear higher than they really are.

One may also place a piece of money at the bottom of a basin, and then stoop down gradually, until, the edge of the basin intervening, the coin is lost to view. If an operator then fills the basin with water, the piece of money appears as though the bottom had been raised. The glass lenses used by professors may be very well replaced by a round water-bottle full of water. A candle is lighted in the darkness, and on holding the bottle between the light and a wall which acts as a screen, we see the reflected light turned upside down by means of the convergent lens we have improvised (fig. 90). A balloon of glass constitutes an excellent microscope. It must be filled with perfectly clear, limpid water, and closed by means of a cork. A piece of wire is then rolled round its neck, and one end is raised, and turned up towards the focus; viz., to support the object we wish to examine, which is magnified several diameters. If a fly, for instance, is at the end of the wire, we find it is highly magnified when seen through the glass balloon (fig. 91). By examining the insect through the water in the balloon, we can distinguish every feature of its organism, thanks to this improvised magnifier. This little apparatus may also serve to increase the intensity of a luminous focus of feeble power, such as a lighted candle. It is often employed in this manner by watchmakers. If a bottle full of water is placed on a table, and exposed to the rays of the sun, the head of a lucifer match being placed in the brightest centre of light caused by the refracted rays, the match will not fail to ignite. I have succeeded in this experiment even under an October sun, and still more readily in warm weather.

In the Conservatoire des Arts in Paris a visitor will always notice a number of people looking at the mirrors in the “optical” cabinets. These mirrors deform and distort objects in a very curious manner, and people find much amusement in gazing into them till they are “moved on” by the attendants. Such experiments create great interest, and a very excellent substitute for these may be found in a coffee-pot or even in a large spoon, and all the grotesque appearance will be seen in the polished surface. The least costly apparatus will sometimes produce the most marvellous effects. Look at a soap-bubble blown from the end of a straw. When the sphere has a very small diameter the pellicule is colourless and transparent; but as the air enters by degrees, pressing upon all parts of the concave surface equally, the bubble gets bigger as the thickness decreases, and then the colours appear,—feeble at first, but stronger and stronger as the thickness diminishes. The study of soap-bubbles and of the effects of the light is very interesting. Newton made the soap-bubble the object of his studies and meditations, and it will ever hold its place amongst the curious phenomena of the Science of Optics. But before going into all the phases of Lights and Optics we will proceed to explain the structure of the eye, as it is through that organ that we are enabled to appreciate light and its marvellous effects.

It is often considered an embarrassing matter to fix precisely the value of two lights. Nothing, however, can be easier in reality, as we will show. In comparing different lights, it is necessary to bear in mind the amount of waste, the colour of the light, the luminous value of the source, and the steadiness of the flame. The luminous value of a lamp-burner is generally equalled by that of a wax candle, and we will take as an example one of those at six to the pound. Very precise appliances are used for this experiment when great exactness is required; but it is easy to calculate in a simple manner the differences in ordinary lights. Supposing we desire to test the value of light given by a lamp and a wax candle, they must both be placed on the table at an equal height, B and A, (fig. 93), in front of some opaque body, A, and then a large sheet of paper must be fixed as vertically as possible to form a screen. When B and A are lighted, two shadows, E and F, are produced, to which it is easy to give exactly the same intensity, by advancing or withdrawing one of the two sources of light. The intensities of the two lights will then be inversely proportional to the squares of the measured distances, AB and AC. By a similar careful calculation it has been possible to draw up a table of the relative values of various ordinary lights. We have not included here the electric light, which has recently attracted so much attention, because this system of lighting can hardly be said to have yet penetrated the domain of domestic life; but when we consider electricity, as we hope to do in a future part, we intend to study this question fully, for there is no doubt that electricity is becoming more and more adapted to our daily life.

The measurement of intensity of light is called Photometry, and the instruments used are Photometers. Bunsen’s instrument consists of a screen of writing-paper, saturated in places with spermaceti to make it transparent. A sperm candle is placed on one side, and the light to be compared on the other. The lights are provided with graduated bars, and these lights are then removed farther and farther from the screen till the spots of grease are invisible. The relative intensities are as the squares of the distance from the screen.

We append a table showing the comparative cost of light given by Dr. Frankland at the Royal Institution some few years ago. The standard of comparison was 20 sperm candles burning for 10 hours at the rate of 120 grains an hour:—

There are many other interesting experiments connected with Light,—Spectrum Analysis, etc., etc.,—all of which we will defer for a time until we have examined the Eye and some effects produced upon it by Light, illustrated by numerous diagrams in the pages next following.