John Henry Pepper

This branch of the phenomena of light includes some of the most remarkable and gorgeous chromatic effects; at the same time, regarded philosophically, it is certainly a most difficult subject to place in a purely elementary manner before the youthful minds of juvenile philosophers, and unless the previous chapter on the diffraction of light is carefully examined, the rationale of the illustrations of polarized light will hardly be appreciated. We have first to ask, "What is polarized light?" The answer requires us again to carry our thoughts back to the consideration of the undulatory theory of light, already illustrated and partly explained at page 262, and page 330.

After perusing this portion of the subject, it might be considered that waves of light were constituted of one motion only, and that an undulation might be either perpendicular or horizontal, according to circumstances. (Fig. 320.)

Fig. 320. Fig. 320.

No. 1. A wire bent to represent a perpendicular vibration, which if kept in the latter position, will only pass through a perpendicular aperture.—No. 2. A wire bent to represent a horizontal wave which will only pass through a horizontal aperture.

This simple condition of the waves of light could not, however, be reconciled theoretically with the actual facts, and it is necessary in regarding a ray of light, to consider it as a combination of two vibrating motions, one of which, for the sake of simplicity, may be considered as perpendicular, and the other horizontal; and this idea of the nature of an undulation of light originated with the late Dr. Young, who while considering the results of Sir D. Brewster's researches on the laws of double refraction, first proposed the theory of transversal (cross-wise) vibration. Dr. Young illustrated his theory with a stretched cord, which if agitated or violently shaken perpendicularly, produces a wave that runs along the cord to the other end, and may be often seen illustrated on the banks of a river overhung with high bushes; the bargemen who drive the horses pulling the vessel by a rope, would be continually stopped by the stunted thick bushes, but directly they approach them, they give the horse a lash, and then violently agitate the rope vertically, which is thrown into waves that pass along the rope, and clear the bushes in the most perfect manner. (Fig. 321.)

Fig. 321. Fig. 321.

Bargeman throwing his tow-rope into waves to get it over the thick bushes.

Now if a similar movement is made with the stretched rope from right to left, another wave will be produced, which will run along the cord in an horizontal position, and if the latter is compared with the perpendicular undulation, it will be evident that each set of waves will be in planes at right angles to and independent of each other. This is supposed to be the mechanism of a wave of common light, so that if a section is taken of such an undulation, it will be represented by a circle a b c d (Fig. 322), with two diameters a b, and c d; or a better mechanical notion of a wave of com mon light is acquired from the inspection of another of Mr. Woodward's cardboard models. (Fig. 323.)

Fig. 322. Fig. 322.

A section of a wave of common light made up of the transversal vibration, a b and c d.

Fig. 323. Fig. 323.

Model of a wave of common light.

The existence of an alternating motion of some kind at minute intervals along a ray is, says Professor Baden Powell, "as real as the motion of translation by which light is propagated through space. Both must essentially be combined in any correct conception we form of light. That this alternating motion must have reference to certain directions transverse to that of the ray is equally established as a consequence of the phenomena; and these two principles must form the basis of any explanation which can be attempted." A beam of common light is therefore to be regarded as a rapid succession of systems of waves in which the vibrations take place in different planes.

If the two systems of waves are separated the one from the other, viz., the horizontal from the perpendicular, they each form separately a ray of polarized light, and as Fresnel has remarked, common light is merely polarized light, having two planes of polarization at right angles to each other. To follow up the mechanical notion of the nature of polarized light, it is necessary to refer again to Woodward's card wave model (Fig. 323), and by separating the two cards one from the other it may be demonstrated how a wave of common light reduced to its skeleton or primary form is reducible into two waves of polarized light, or how the two cards placed together again in a transversal position form a ray of common light. (Fig. 324.)

Fig. 324. Fig. 324.

No. 1. Common light, made up of the two waves of polarized light, Nos. 2 and 3.

The query with respect to the nature of polarized light being answered, it is necessary, in the next place, to consider how the separation of these transversal vibrations may be effected, and in fact to ask what optical arrangements are necessary to procure a beam of polarized light? Light may be polarized in four different ways—viz., by reflection, single refraction, double refraction, and by the tourmaline—viz., by absorption.

Polarization by Reflection, and by Single Refraction.

Fig. 325. Fig. 325.

No. 1. a is the lime light. b. The condenser lenses. c. The beam of common light. Here the glass plates are removed.—No. 2. a. Lime light. b. The condenser lenses. c c. The bundle of plates of glass at an angle of 56° 45´. d is the ray of light polarized by reflection from the glass plates, c c, and e is the beam of polarized light by single refraction, having passed through the bundle of plates of glass, c c.

In the year 1810, the celebrated French philosopher, Mons. Malus, while looking through a prism of Iceland spar, at the light of the setting sun, reflected from the windows of the Luxemburg palace in Paris, discovered that a beam of light reflected from a plate of glass at an angle of 56 degrees, presented precisely the same properties as one of the rays formed by a rhomb of Iceland spar, and that it was in fact polarized. One of the transversal waves of polarized light of the common light, being reflected or thrown off from the surface of the glass, whilst the other and second transversal vibration passed through the plate of glass, and was likewise polarized in another plane, but by single refraction, so that the experiment illustrates two of the modes of polarizing light-—viz., by reflection, and by single refraction. This important elementary truth is beautifully illustrated by Mr. J. T. Goddard's new form of the oxy-hydrogen polariscope, by which a beam of common light traverses a long square tin box without change; but directly a bundle of plates of glass composed of ten plates of thin flattened crown glass, or sixteen plates of thin parallel glass plates used for microscopes, are slid into the box at an angle of 56° 45´, then the beam of common light is split into two beams of polarized light, which pursue their respective paths, one passing by single refraction through the glass, and the other being reflected, and rendered apparent by opening an aperture over the glass plates, and then again by using a little smoke from brown paper, the course of the rays becomes more apparent.

The same truth is well illustrated by the cardboard model wave and a wooden plane with horizontal and perpendicular slits, placed at an angle of 56° 45´, as at Fig. 326.

Fig. 326. Fig. 326.

a a. Model in wood of a bundle of plates of glass at an angle of 56° 45´. b. Beam of common light, with transversal vibration. c. Light polarized by reflection. d. Light polarized by refraction.

POLARIZATION BY DOUBLE REFRACTION.

The name of Double-refracting or Iceland Spar is given to a very clear, limpid, and perfectly transparent mineral, composed of carbonate of lime, and found on the eastern coast of Iceland. Its crystallographic features are well described by the Rev. Walter Mitchell in his learned work on mineralogy and crystallography, and it is sufficient for the object of this article to state that it crystallizes in rhombs, and modifications of the rhomboidal system. It must not be confounded with rock or mountain crystal, which, under the name of quartz, crystallizes in six-sided prisms with six-sided pyramidal tops; quartz being composed of silica, or silicic acid and calcareous spar of carbonate of lime. Very large specimens of the latter mineral are rare and valuable, and the lion of specimens of calcareous, or double-refracting spar, is now in the possession of Professor Tennant, the eminent mineralogist of the Strand. It is nine inches high, seven and three-quarters inches broad, and five and a half inches thick; its estimated value being 100l. This beautiful specimen has been photographed, and its stereograph illustrates in a very striking manner the double refracting properties of the spar.

If a printed slip of paper is placed behind a rhomb of Iceland spar, two images of the former are apparent, and the stereograph already alluded to shows this fact very perfectly, at the same time illustrates the value of the stereoscope. Out of the stereoscope the words "Stereoscopic Magazine" appear doubled, but seem to lie in the same plane; but directly the picture is placed in the instrument, then it is clearly seen that one image is evidently in a very different plane from the other. The double-refracting power of this mineral is illustrated by holding a small rhomb of Iceland spar, placed in a proper brass tube before the orifice as at Fig. 327, from which the rays of common light are passing; if an opaque screen of brass perforated with a small hole is introduced behind the rhomb, then, instead of one circle of light being apparent on the screen, two are produced, and both the rays issuing in this manner are polarized, one being termed the ordinary and the other the extraordinary ray. (Fig. 327.)

Fig. 327. Fig. 327.

a. The condensers. b. The hole in the brass screen or stop. c. The rhomb of Iceland spar. o. The ordinary, and e the extraordinary, ray, both of which are polarized light.

The polarizing property of the rhomb is perhaps better shown by the next diagram, where a b represents the obtuse angles of the Iceland spar, and a line drawn from a to b, would be the axis of the crystal. The incidental ray of common light is shown at c, and the oppositely polarized transmitted rays called the ordinary ray o, and extraordinary ray e, emerge from the opposite face of the rhomboid. If a black line is ruled on a sheet of paper as at k k, and examined by the eye at c, it appears double as at k k and j j. (Fig. 328.)

Fig. 328. Fig. 328.

Rhomb of Iceland spar.

The cardboard model is again useful in demonstrating the polarization of light by double refraction, and if a model of a rhomb of Iceland spar is made of glass plates, one face of which has an aperture like a cross, and the other a horizontal and perpendicular slit, as at Nos. 1 and 2 (Fig. 329), the production of the ordinary and extraordinary rays is demonstrated in a familiar manner, and is easily comprehended.

Fig. 329. Fig. 329.

No. 1. One face of the model rhomb to admit the transversal vibration, represented by the cardboard model.—No. 2. The opposite face of the rhomb, from which issue the polarized, ordinary, and extraordinary rays.—No. 3. Side view of the model.

In Newton's "Optics" we find the following description of Iceland spar:—"This crystal is a pellucid fissile stone, clear as water or crystal of the rock (quartz), and without colour.... Being rubbed on cloth it attracts pieces of straw and other light things like amber or glass, and with aquafortis it makes an ebullition.... If a piece of this crystalline stone be laid upon a book, every letter of the book seen through it will appear double by means of a double refraction."

POLARIZATION BY THE TOURMALINE.

This mineral was first discovered during the sixteenth century, in the island of Ceylon, afterwards in Brazil, and since that period at various localities in the four quarters of the globe. In the Grevillian collection purchased many years ago by government for the British Museum, there is a fine specimen of red tourmaline valued at 500l. The green tourmaline is named Brazilian emerald, and the Berlin blue tourmaline is called Brazilian sapphire; the mineral chiefly consists of sand (silica) and alumina, with a small quantity of lime, or potash, or soda, boracic acid, and sometimes oxide of iron or manganese. When light is passed through a slice of this mineral it is immediately polarized, one of the transversal vibrations being absorbed, stopped, or otherwise disposed of, the other only emerging from the tourmaline, consequently it is one of the most convenient polarizers, although the polarized light partakes of the accidental colour of the mineral. Green, blue, and yellow tourmalines are bad polarizers, but the brown and pink varieties are very good, and it is a most curious fact that white tourmaline does not polarize. (Fig. 330.)

The mineral crystallizes in long prisms, whose primitive form is the obtuse rhomboid, having the axis parallel to the axis of the prism. The term axis with reference to the earth, as shown at page 16, is an imaginary single line around which the mass rotates, but in a crystal it means a single direction, because a crystal is made up of a number of similar crystals, each of which must have its axis, thus the whitest Carrara marble reduced to fine powder, moistened with water and placed under a microscope, is found to consist chiefly of minute rhomboids, similar to calcareous spar. The smallest crystal of this mineral is divisible again and without limit into other rhombs, each of which possesses an axis. (Fig. 331.)

If a plate of tourmaline is held before the eye whilst looking at the sun (like the gay youth in Hogarth's picture who is being arrested whilst absorbed with the wonders of a tourmaline, which was, in the great painter's time, a popular curiosity,) it may be turned round in all directions without the slightest difference in the appearance of the light, which will be coloured by the accidental tint of the crystal, but if a second slice of tourmaline is placed behind the other, there will be found certain directions in which the light passes through both the slices, whilst in other positions the light is completely cut off.

When the axes of both plates coincide, the light polarized by one tourmaline will pass through the other, but if the axes do not coincide, and are at right angles to each other, then the polarized light is entirely stopped, and the rationale of this will be appreciated at once if a tourmaline is regarded (mechanically) as if it were like a grating with perpendicular bars through which the polarized light will pass. Any number of such gratings with the bars parallel would not stop the polarized light, but if the second grating is turned round ninety degrees, the bars will be at right angles to those of the first grating, and the perpendicular wave of polarized light cannot pass. (Fig. 332.)

Splendid Chromatic effects produced by Polarized Light.

Having discussed the various modes of obtaining polarized light, the next step is to arrange an apparatus by which certain double refracting crystals, and other bodies, shall divide a ray of polarized light, and then by subsequent treatment with another polarizing surface, the divided rays are caused to interfere with each other, and afford the phenomena of colour. Bodies that refract light singly, such as gases, vapours or liquids, annealed glass, jelly, gums, resins, crystallized bodies of the tessular system, such as the cube and octohedron, do not afford any of the results which will be explained presently, except by the influence of pressure, as in unannealed glass, or a bent cold glass bar. By compression or dilatation, they are changed to double refractors of light. The bodies that possess the property of double refraction (though not to the visible extent of Iceland spar), are all other bodies such as crystallized chemicals, salts, crystallized minerals, animal and vegetable substances possessing a uniform structure, such as horn and quill; all these substances divide the ray of polarized light into two parts, and by placing a thin film of a crystal of selenite (which is one of the best minerals that can be used for the purpose) in the path of the beam of polarized light, coming either from the glass plates, as in No. 2, (Fig. 325), page 338, or from a slice of tourmaline, and then receiving it through the ordinary focusing lenses or object-glasses of the oxy-hydrogen microscope, no colour is yet apparent in the image of the selenite on the screen, until another tourmaline, or a bundle of glass plates, is placed at an angle of 56° 45´, and at right angles to the plane of reflection of the first set of plates; then the most gorgeous colours suddenly appear over all parts of the film of selenite as depicted on the screen, like other objects shown by the oxy-hydrogen microscope. (Fig. 333.)

Goddard's oxy-hydrogen polariscope is one of the most convenient, because either the reflected or refracted polarized rays can be rendered available; it consists of the apparatus shown at Fig. 325, page 338, and to this is added a low microscope power, and stage to hold the selenite or other objects, with another bundle of sixteen plates of the thin microscopic glass or mica, called the analyser. A slice of tourmaline, or a Nicol's prism may be employed, instead of the second bundle of reflecting plates. When the ray of polarized light reflected from the first set of glass plates enters the doubly refracting film of selenite, which is about the fortieth or fiftieth part of an inch in thickness, it is split into the ordinary and extraordinary rays, and is said to be dipolarized, and forms two planes of polarized light, vibrating at right angles to each other. When the latter are received on another bundle of plates of glass called the analyser, at an angle of 56° 45´, but at right angles to the first set of glass plates, they interfere, because in the passage of the two rays from the selenite they have traversed it in different directions, with different velocities; one of these sets of waves will therefore, on emerging from the opposite face of the selenite be retarded, and lie behind the other; but being polarized in different planes, they cannot interfere until their planes of polarization are made to coincide, which is effected by means of the second bundle of glass plates called the analyser; and when this is brought into a position at right angles to the first set of reflecting glass plates, half the ordinary wave interferes with half the extraordinary wave; and being transmitted through the analyser, produces, say red and orange, whilst the remaining halves also interfere, and being reflected, afford the complementary colours green and blue. (Fig. 334.) The term complementary is intended to define any two colours containing red, yellow, and blue, because the three combined together produce white light; for example, the complementary colour to red would be green, because the latter contains yellow and blue; the complementary colour to orange would be blue, because the former contains red and yellow. Any two colours, therefore, which together contain red, yellow, and blue are said to be complementary; and if this principle was better understood, ladies would never commit such egregious blunders as they occasionally do in the choice of colours for bonnets and dresses, and select a blue bonnet to be worn with a green dress, or vice versÂ. By rotating the analyser, the reflected and refracted rays change colours, and if the former is red and the latter green, by moving the analyser round 90°, the reflected rays change to green and the refracted to red; at 180° the colours again change places; at 270° the reflected ray will be again green, and the refracted red; to be once more brought back at 360° to the original position, viz., reflected rays red, refracted green. The thickness of the films of selenite determines the particular colour produced.

If the selenite is of a uniform thickness, one colour only is obtained, and by ingeniously connecting pieces of various thicknesses (in the same forms as stained glass for cathedral windows), the most beautiful designs were made by the late Mr. J. T. Cooper, jun., which have since been manufactured in great quantity and variety by Mr. Darker, of Paradise-street, Lambeth. The colours of these selenite objects are seen by placing them in front of a piece of black glass, fixed at the polarizing angle, and then examining the design with a slice of tourmaline, or still better with a single-image Nicol prism, when the most brilliant colours are obtained, and varied at every change of the angle of the analyser.

Selenite, or sparry-gypsum, is the native crystallized sulphate of lime, which contains water of crystallization (CaO, SO₃, 2H₂O). It frequently occurs imbedded in London clay, and is called quarry glass by the labourers who find it at Shotover Hill, near Oxford, and also in the Isle of Sheppey.

At a very early period, before the discovery of glass, selenite was used for windows; and we are told that in the time of Seneca, it was imported into Rome from Spain, Cyprus, Cappadocia, and even from Africa. It continued to be used for this purpose until the middle ages, for Albinus informs us, that in his time, the windows of the dome of Merseburg were of this mineral. The first greenhouses, those invented by Tiberius, were covered with selenite. According to Pliny, beehives were encased in selenite, in order that the bees might be seen at work.

The late Dr. Pereira has placed the phenomena already described in the form of a most instructive diagram, which we borrow from his elaborate work on "Polarized Light." (Fig. 336.)

The chromatic effects described are not confined to selenite objects only, but are obtained from glass, provided the particles are in a state of unequal tension, as in masses of unannealed glass of various forms. (Fig. 337.) Consequently, polarized light becomes a most valuable means for ascertaining the condition of particles otherwise invisible and inappreciable. One of the most beautiful experiments can be made with a bar of plate-glass, which refracts light singly until pressure is applied to the centre, in order to bend it into an arch or curve, when the appearance presented in Fig. 338 is apparent.

A quill placed in the polarizing apparatus is also discovered to be in a state of unequal tension by the appearance of coloured fringes within it, which change colour at every movement of the analyser.

Another series of beautiful appearances present themselves when a ray of white polarized light is made to pass perpendicularly through a slice of any crystallized substance with a single axis; if the analyser consist of a slice of tourmaline, a number of concentric coloured rings are rendered visible with a black cross in the centre, which is replaced with a white one on moving the tourmaline through each quadrant of the circle.

Crystals of Iceland spar present this phenomenon in great beauty; and if the crystal (such as nitre) has two axes of double-refraction, a double-system of coloured rings is apparent, with the most curious changes and combinations of the black and white crosses with them. (Fig. 339.)

Crystal of nitre with two axes, as seen in polarized light.

Mr. Goddard has recommended the optical arrangement (Fig. 340) for showing the rings with great perfection, as also the number of rings that increase in some crystals (the topaz, for example), with the divergence of the rays of polarized light passing through them.

Mr. Woodward's table and oxy-hydrogen polariscope and microscope, made by Smith and Beck, of Coleman-street, is well adapted, from its simplicity and perfection, to exhibit all the varied and beautiful effects of polarized light; and we only regret that want of space prevents us describing it in detail, although the reader may see the body of the apparatus at page 123, where the modifications of the oxy-hydrogen light are described and figured; and the polarizing apparatus would be placed, of course, in front of the light issuing from the lantern.

Finally, the question of utility (the cui bono) may be considered in answer to the query, What is the use of polarized light?

The value to scientific men of a knowledge of the nature of this modification of common light cannot be overrated. It has given the philosopher a new kind of test, by which he discovers the structure of things that would otherwise be perfectly unknown; it has given the astronomer increased data for the exercise of his reasoning powers; whilst to the microscopist the beauty of objects displayed by polarized light has long been a theme of admiration and delight, and has served as a guide for the identification of certain varieties of any given substance, such as starch.

A tube provided with a polarizer of tourmaline, or a single-image Nicol prism, is invaluable to the look-out at the mast-head in cases where vessels are navigating either inland or sea water, where the presence of hidden rocks is suspected, because the polarizer rejects all the glare of light arising from unequal reflection at the surface of water, and enables the observer to gaze into the depths of the sea and to examine the rocks, which can only be perfectly visible by the refracted light coming from their surfaces through the water.

Professor Wheatstone has invented an ingenious polarizing clock for showing the hour of the day by the polarizing power of the atmosphere. Birt, Powell, and Leeson have each invented instruments for examining the circular polarization of fluids, by which a more intimate knowledge of the relative values of saccharine solutions may be obtained, besides unfolding other truths important to investigators in this branch of science.

And last, but not least, it was with the assistance of polarized light that Dr. Faraday established the relation that exists between light and magnetism, and through the latter, with the force of electricity; and the next figure indicates the necessary apparatus required to repeat this highly important physical truth—viz., the deviation of the plane of polarization of light by the influence of the magnetic force from a powerful electro-magnet. (Fig. 341.)

By another and equally beautiful experiment at the London Institution, Professor Grove demonstrated the production of all the other kinds of force from light, using the following arrangement for the purpose:

A prepared daguerrÉotype plate is enclosed in a box full of water having a glass front with a shutter over it; between this glass and the plate is a gridiron of silver wire; the plate is connected with one extremity of a galvanometer coil, and the gridiron of wire with one extremity of a Breguet's helix; the other extremities of the galvanometer and helix are connected by a wire, and the needles brought to zero. As soon as a beam of either daylight or the oxy-hydrogen light is, by raising the shutter, permitted to impinge upon the plate, the needles are deflected. Thus, light being the initiatory force, we get

Chemical action on the plate,
Electricity circulating through the wires,
Magnetism in the coil,
Heat in the helix,
Motion in the needle.

Such, then, are some of the glorious phenomena that we have endeavoured to explain in this and the preceding chapters on light. Here we have noticed specially how completely we owe their appreciation to the sense of sight operating through the eye, the organ of vision. Well may those who have lost this divine gift speak of their darkness as of a lost world of beauty to be irradiated only by better and more enduring light; and most feelingly does Sir J. Coleridge speak on this point when he says:—

"Conceive to yourselves, for a moment, what is the ordinary entertainment and conversation that passes around any one of your family tables; how many things we talk of as matters of course, as to the understanding and as to the bare conception of which sight is absolutely necessary. Consider, again, what an affliction the loss of sight must be, and that when we talk of the golden sun, the bright stars, the beautiful flowers, the blush of spring, the glow of summer, and the ripening fruit of autumn, we are talking of things of which we do not convey to the minds of these poor creatures who are born blind, anything like an adequate conception. There was once a great man, as we all know, in this country, a poet—and nearly the greatest poet that England has ever had to boast of—who was blind; and there is a passage in his works which is so true and touching that it exactly describes that which I have endeavoured, in feeble language, to paint. Milton says:—

The great poet, when intent upon his work, sought for celestial light to accomplish it. And this brings me to that part of the labours of our Blind Institutions upon which I dwell the most and which, after all, is the greatest compensation we can afford to the inmates for the affliction they suffer; and that is, the means we provide for them to read the blessed Word of God, which they can read by day as well as by night, for light in their case is not an essential."