CHAPTER XXIV.

Fig. 307.Fig. 307.

A compound achromatic lens, composed ofc c, the double-convex lens of crown-glass, andf f, the plano-concave lens of flint-glass.

This instrument has now attained a popularity quite equal to, if it does not surpass, that formerly enjoyed by the kaleidoscope, and without entering upon the much-vexed question of priority of discovery, it is sufficient again to mention with the highest respect the names of Sir David Brewster and Professor Wheatstone as identified with the discovery and use of this most pleasing optical instrument.

The principle of the stereoscope (meaning,solid I see) is copied from nature:i.e., when both eyes are employed in the examination of an object, two separate pictures, embracing dissimilar forms, are impressed upon the retinæ, and produce the effect of solidity; if the pictures formed at the back of the eyes could be examined by another person with a stereoscope, they would come together, and also produce the effect of solidity.

Stereoscopic pictures are obtained by exposing sensitized paper in the camera to the picture of an object taken in two positions, or two cameras are employed to obtain the same result. If the latter mode is adopted, the stereoscopic pictures must not be taken from positions too widely separated from each other; or else, when the two pictures are placed in the stereoscope, they will stand out with a relief that is quite unnatural, and the object will appear like a very reduced solid model, instead of having the natural appearance presented by pictures which have been taken at positions too distant from each other.

Sir David Brewster says, "In order to obtain photographic pictures mathematically exact, we must construct a binocular camera which willtake the pictures simultaneously, and of the same size; that is, by a camera with two lenses of the same aperture and focal length, placed at the same distance as the two eyes. As it is impossible to grind and polish two lenses, whether single or achromatic, of exactly the same focal lengths, even if we had the very same glass for each, I propose to bisect the lenses, and construct the instrument with semi-lenses, which will give us pictures of precisely the same size and definition. These lenses should be placed with their diameters of bisection parallel to one another, and at a distance of 2½ inches,which is the average distance of the eyes in man; and when fixed in a box of sufficient size, will form a binocular camera, which will give us at the same instant, with the same lights and shadows, and of the same size, such dissimilar pictures of statues, buildings, landscapes, and living objects, as will reproduce them in relief in the stereoscope." Thus with a single camera provided with semi-lenses, or two lenses of the same focal length, stereoscopic pictures can be obtained.

To bring the images of the two pictures together, and produce the effect of solidity; either of two instruments may be employed. The reflecting stereoscope is the invention of Professor Wheatstone. The refracting or lenticular stereoscope that of Sir David Brewster.

The former is constructed by placing two upright boards on a wooden stand at a moderate distance from each other; the stereoscopic pictures are attached to these boards, which may be made to move up or down, and if the pictures are held in grooves, they may be pulled right or left at pleasure, and thus four movements are secured—viz., upward, downward, right, or left. Between the two stereoscopic pictures are placed two looking-glasses, so adjusted that their backs form an angle of ninety degrees with each other. (Fig. 308.)

Fig. 308.Fig. 308.

Wheatstone's reflecting stereoscope.

The pictures are illuminated at night by a lamp or gas flame placed at the back of the mirrors, which, when fixed together, have the same shape as a prism; indeed, Professor Wheatstone substituted a prism for the mirrors, and thus paved the way for the invention of the lenticular stereoscope.

The stereoscopic effect is obtained by bringing the eyes close to the inclined mirrors, so that the two reflected images coincide at the intersection of the optic axis; the coincidence of the images is further secured by moving either picture a little to the right or left, and if the upright boards move bodily in grooves to or from the centre mirror, the greatest nicety of adjustment is procured.

During the last three years of the author's directorship of the Polytechnic—viz., in 1856, 1857, 1858—nearly the whole of the pictures shown by the dissolving-view apparatus were coloured photographs from Mr. Hine's original pictures, painted two feet square in blue and white, and reduced on the glass to about six inches square. The collodion film being frequently thick and difficult to penetrate with light, was etched and scratched away where required, and filled in with colour, and when these pictures were looked at withoneeye only, they appeared to be almost solid or stereoscopic on the disc.

The lenticular stereoscope consists of a box of a pyramidal shape, open at the base, and provided with grooves in which are placed the stereoscopic pictures; if the latter are taken on glass the base of the box is held directly against the light, but if they are daguerreotypes or paper pictures, then a side light is reflected upon them by means of a lid covered in the inside with tinfoil, which is raised or lowered at pleasure from the top part of the box. Two semi-lenses are now fitted into the narrow part of the box, and are placed at such a distance from each other that the centres of the semi-lenses correspond with the pupil of the eyes, and this distance has already been stated to amount to 2½ inches. (Fig. 309.)

Fig. 309.Fig. 309.

Brewster's lenticular stereoscope.

The principle of the lenticular stereoscope is perhaps better seen by reference to the next diagram, in which the centres of the semi-lenses (i.e., a lens cut in half) are placed at 2½ inches apart, with theirthinedges towards each other, and marked,a b, Fig. 310. The centres of the two stereoscopic picturesc dcorrespond with the centres of the lenses, and the rays of lightdivergingfromc dfall upon the semi-lenses, and being refracted nearlyparallelare, by the prismatic form of the semi-lenses, deflected from their course, and leave the surfaces of the lenses in the same direction as if they actually emanated frome; and as all images of bodies appear to come in a straight line from the point whence they are seen, the two pictures are superimposed on each other, and together produce the appearance of solidity, so that a stereoscopic result is obtained when thespectral imagesof the two stereoscopic pictures are made to overlap each other. By taking one of the semi-lenses in each hand, and looking at the two pictures, the over-lappingof thespectral imagesbecomes very apparent, so that the combinedspectral images, and not thepicturesthemselves, are seen when we look into a stereoscope. (Fig. 310.)

Fig. 310.Fig. 310.

Sir David Brewster says, "In order that the two images may coalesce without any effort or strain on the part of the eye, it is necessary that the distance of the similar parts of the two drawings be equal to twice the separation produced by the prism. For this purpose measure the distance at which the semi-lenses give the most distinct view of the stereoscopic pictures, and having ascertained by using one eye the amount of the refraction produced at that distance, or the quantity by which the image of one of the pictures is displaced, place the stereoscopic pictures at a distance equal to twice that quantity—that is, place the pictures so that the average distance of similar parts in each is equal to twice that quantity. If this is not correctly done, the eye of the observer will correct the error by making the images coalesce, without being sensible that it is making any such effort. When the dissimilar stereoscopic pictures are thus united, the solid will appear standing as it were in relief between the two plane representations."

M. Claudet, whose name has long been celebrated in connexion with the art of photography, has described an instrument by which a single picture is made to simulate the appearance of solidity, and he states that by means of this arrangement a number of persons may observe the effect at the same time. The apparatus required is very simple, consisting of a large double convex lens, and a screen of ground glass. Theobjecta, Fig. 311, is highly illuminated, and placed in the focus of a double convex lensb, when an image of the object is projected, and will be found suspended in the air in the conjugate focus of the lens atc, and from this point the rays of light will diverge as from a real object, which will be seen by separate spectators atd dande e; and if the screen of ground glass is placed atg g, the image will appear with all the effect of length, breadth, and depth, which belong to solid bodies. (Fig. 311.)

Fig. 311.Fig. 311.

The stereomonoscope.

An image formed on ground glass in this manner can be seen only in the direction of the incident rays, and the stereoscopic effect is not apparent when the image is received on a calico or transparent screen, on account of the rays being scattered in all directions.

This arrangement is an important modification of the other, and consists of a screen of ground glass (a b, Fig. 312), and two convexlenses (c d, ande f) arranged in such a manner that they will project images of the stereoscopic pictures,g h, at the same point on the screen,a b.

It might be thought that a confusion of images would result from projecting two pictures on one point,p—viz., the focus of the two lenses; but as each photograph can be seen only in the direction of its own rays, it follows that if the eyes are so placed that each receives the impression of one stereoscopic picture, the two images must coalesce, and a stereoscopic effect will be the result, as is apparent atk kandl l; so that several persons may look at the stereoscope at one time. (Fig. 312.)

Fig. 312.Fig. 312.

The stereomoscope.

This curious optical instrument, as its name implies, produces a false image by the refracting power of prisms, and is the invention of Professor Wheatstone. When used with both eyes, the same as the stereoscope, it inverts the relief of a solid body, and makes it appear exactly as if it were an intaglio, or sunk beneath the line surrounding it. For instance, a terrestrial globe when looked at through the pseudoscope appears to be concave, like Wyld's Globe in Leicester-square, instead of convex. A vase with raised ornaments upon it looks as if it had been turned (to reverse the usual expression) outside in, andthe whole of its convexity is turned to concavity; and of course a face seen under these circumstances looks very curious. (Fig. 313.) The cause is perhaps somewhat difficult to understand; but by taking other and more simple examples of the same effect, the principle may be gradually comprehended.

Fig. 313.Fig. 313.

Horizontal section of the pseudoscope, showing ata btwo prisms placed against a block of wood about two inches long and one inch and a half wide, and cut out in the centre to admit the nose atd. The eyes are supposed to be looking at the globe,c, in the direction of the arrows.e e.Brass plates blackened, which shut out the side light, and assist in keeping the prisms in position.

Sir David Brewster, in his "Letters on Natural Magic," remarks that "one of the most curious phenomena is thatfalseperception in vision by which we conceive depressions to be elevations, and elevations depressions—or by which intaglios are converted into cameos, and cameos into intaglios. This curious fact seems to have been observed at one of the early meetings of the Royal Society of London, when one of the members, in looking at a guinea through a compound microscope of new construction, was surprised to see the head upon the coin depressed, while other members could only see it embossed, as it really was.... The best method of observing this deception is to view the engraved seal of a watch with the eye-piece of an achromatic telescope, or with a compound microscope, or any combination of lenses which inverts the objects that are viewed through it; a single convex lens will answer the purpose, provided we hold the eye six or eight inches behind the image of the seal formed in its conjugate focus."

After bringing forward various interesting experiments in further explanation of the cause, Sir D. Brewster states it to be his belief that the illusion is the result of an operation of our own minds, whereby we judge of the forms of bodies by the knowledge we have acquired of light and shadow. Hence, the illusion depends on the accuracy and extent of our knowledge on this subject; and while some persons are under its influence, others are entirely insensible to it. This statement is borne out by experience, as the author, whilst Resident Director of the Polytechnic, had four of Wheatstone's pseudoscopes placed in the gallery, with proper objects behind them; and he frequently noticed that some visitors would look through the instrument and see no alteration of the convex objects, whilst others would shout with delight, and call their friends to witness the strange metamorphosis, who in their turn might disappoint the caller by being perfectly insensible to its strange effects.

The pseudo-effects of vision are not confined to the results already explained, but are to be observed especially whilst travelling in a coach, when the eyes may be so fixed as to give the impression of movement to the trees and houses, whilst the coach appears to stand still. In railway carriages, after riding for some time and then coming to a stand still, if another train is set slowly in motion by the one at rest, it frequently happens that the latter appears to be moving instead of the former.

The analysis of light has been explained in a previous chapter, and it has been shown how the spectrum is produced. Colour, however, may be obtained by other means, and the property enjoyed by certain bodies, of absorbing certain coloured rays in preference to others, offers another mode of decomposing light.

The property of absorption is shown to us in every kind of degree by innumerable natural and artificial substances; and by examining the spectrum through a wedge of blue glass, Sir David Brewster was enabled to separate the seven colours of the spectrum into the three primary colours, red, yellow, and blue, which he proved existed at every point of the spectrum, and by over-lapping each other in various proportions, produce the compound colours of orange, green, indigo, and violet.

Connected with this property is the remarkable effect produced by coloured light on ordinary colours, and the sickly hue cast upon the ghost in a melodrama, or the fiery complexion imparted to the hair of Der Freischutz, or the jaundiced appearance presented by every member of a juvenile assembly when illuminated with a yellow light from the salt and burning spirit of "snapdragon," are too well known to require a lengthened description here.

If a number of colours are painted on cardboard, or groups of plants, flowers, flags, and shawls, are illuminated by a mono-chromatic light, and especially the light procured from a largetowtorch well supplied with salt and spirit, the effect is certainly very remarkable; at the same time it shows how completely substances owe their colour to the light by which they are illuminated, and it also indicates why ladies cannot choose colours by candle light, unless of course they propose to wear the dress only at night, when it is quite prudent to see the colours in a room lit with gas; and this fact is so well known that with the chief drapers, such as at Messrs. Halling, Pearce, and Stone's, Waterloo House, a darkened room lit with gas is provided during the daytime to enable purchasers of coloured dresses to judge of the effect of artificial light upon them. Whilst the flowers, &c., are lighted up with the yellow light, a magical change is brought about by throwing on suddenly the rays from the oxy-hydrogen light, when the colours are again restored; or if the latter apparatus is not ready, the combustion of phosphorus in a jar of oxygen will answer the same purpose. The light obtained from the combustion of gas affords an excess of the yellow or red rays of light, which causes the difference between candlelight and daylight colours already alluded to.

In this part of the subject it is absolutely necessary to return to the theory of undulations with which the present subject was commenced. The inflection of light offers a third method by which rays of light may be decomposed and colour produced. The phenomena are extremely beautiful, although the explanation of them is almost too intricate for a popular work of this kind.

The cases where colour is produced by inflection are more numerous than might at first be supposed; thus, if we look at a gaslight or the setting sun through a wire gauze blind, protecting the eye with a little tank of dilute ink, a most beautiful coloured cross is apparent. An extremely thin film of a transparent matter, such as a little naphtha or varnish dropped on the surface of warm water or soap bubbles, or a very thin film of glass obtained by blowing out a bulb of red-hot glass till it bursts, or an exquisitely thin plate of talc or mica, all present the phenomena of colour, although they are individually transparent, and in ordinary thicknesses quite colourless.

Fig. 314.Fig. 314.

The two lenses, with the plate or film of air between them, and producing seven coloured rings when the lenses are brought sufficiently close to each other by the screws.

Sir Isaac Newton brought his powerful intellect to bear on these facts, and as a preliminary step invented an instrument for measuring the exact thickness of those transparent substances that afforded colour, and the apparatus displaying Newton's rings is still a favourite optical experiment. It consists of a plano-convex lens,a.(Fig. 314), a slice, as it were, from a globe of glass twenty-eight feet in diameter, or the radius of whose convex surface is fourteen feet. This plano-convex lens is placed on another double convex lens,b., whose convex surfaces have a radius of fifty feet each, consequently the lenses are very shallow, and the space (c c) included between them being filled with air, can of course be accurately measured. (Fig. 314.) It is usual to mount the lenses in brass rings which are brought together with screws, when the most beautiful coloured rings are apparent, and are produced by the extreme thinness of the film or plate of air enclosed between the two lenses; andthe relative thicknesses of the plates of air at which each coloured light is reflected are as follows:—

Red13310 millionths of an inch.Orange120" "Yellow113½" "Green105½" "Blue98" "Indigo92½" "Violet83½" "

By dividing an inch into ten millions of parts, and by taking 133 of such parts, the thickness of the film of air required to reflect the red ray is obtained, and in like manner the other colours require the minute thicknesses of air recorded in the table above. When the thickness of the film of air is about 12/178,000dths of an inch, the colours cease to become visible, owing to the union of all the separate colours forming white light, but if the Newton rings are produced in mono-chromatic light, then a greater number of rings are apparent, but of one colour only, and alternating with black rings,i.e., a dark and a yellow succeeding each other; this fact is of great importance as an illustration of the undulatory theory, and demonstrates the important truth, thattwo rays of light may interfere with each other in such a manner as to produce darkness.

Sir David Brewster remarks that, "From his experiments on the colours of thin and of thick plates, Newton inferred that they were produced by a singular property of the particles of light, in virtue of which they possess, at different joints of their paths,fitsor dispositions to be reflected from or transmitted by transparent bodies. Sir Isaac does not pretend to explain the origin of thesefits, or the cause which produces them, but terms themfits of transmissionandfits of reflexion."

Sir Isaac Newton objected to the theory of undulations because experiments seemed to show that light could not travel through bent tubes, which it ought to do if propagated by undulations like sound; and it was reserved for the late Dr. Young to prove that light could and would turn a corner, in his highly philosophical experiments illustrating the inflection or bending in of the rays of light.

Dr. Young placed before a hole in a shutter a piece of thick paper perforated with a fine needle, and receiving through it the diverging beams on a paper screen, found that when a slip of cardboard one-thirtieth of an inch in breadth was held in such a beam of light, that the shadow of the card was not merely a dark band, but divided into light and dark parallel bands, and instead of the centre of the shadow being the darkest part, it was actually white. Dr. Young ascertained that if he intercepted the light passingon one sideof the slip of card with any opaque body, and allowed the light to pass freely on the other side of the slip of cardboard, that all the bands and the white band in the centre disappeared, and hence he concluded that the bands or fringes within the shadow were producedby the interferenceof the rays bent into the shadow by one side of the card, with the rays bent into the shadow by the other side. (Fig. 315).

Fig. 315.Fig. 315.

In order to show how two waves may interfere so as to exalt or destroy each other, two sets of waves may be propagated on the surface of a still tank or bath of water, from the two pointsa a(Fig. 315), the black lines or circles representing the tops of the waves. It will be seen that along the linesb bthe waves interfere just half way between each other, so that in all these directions there will be a smooth surface, provided each set of waves is produced by precisely the same degree of disturbing force, so as to be perfectly equal and alike in every respect, and the first wave of one set exactly half a wave in advance of the first wave of the other, while at the curve in the direction of all the linec c, the waves coincide, and produce elevations or undulations of double extent; in the intermediate spaces, intermediate effects will, of course, be produced.

Professor Wheatstone has invented some very simple and beautiful acoustic apparatus for the purpose of proving that the same laws of interference exist also in sound, which, as already stated, consists in the vibrations or undulation of the particles of air.

The nature and effects of interference are also admirably illustrated by the following models of Mr. Charles Woodward, President of the Islington Scientific Institution, and to whom we have already alluded.

Fig. 316.Fig. 316.

No. 1. A model of waves with moveable rods.—No. 2. A model of fixed waves.—No. 3. Intensity of waves doubled by the superposition and coincidence of two equal systems.—No. 4. Waves neutralized by the superposition and interference of two equal systems, the raised part of one wave accurately fitting into and making smooth the hollow of the other, illustrating the fact that two waves of light or sound may destroy each other.

Returning again to the coloured rings, we find that Newton discovered that at whatever thickness of the film of air the coloured ring first appeared, there would be found at twice that thickness the dark ring, at three times the coloured, at four times the dark, and so on,the coloured ringsregularly occurring at theodd numbers, and thedark onesat theeven numbers. This discovery is well illustrated by the models (Fig. 316); and it maybe noticed at No. 3 that the highest and the lowest parts of the wavesinterfere, but coincide and produce a wave of double intensity; the little crosses of the upper model are in a straight line with the numbers 1, 3, 5, 7, and are supposed to represent the coloured rings, whilst in No. 4 the upper series of waves is half an undulation in advance of the lower; and if the eye is again directed from the little crosses downward, the figures 2, 4, 6, 8, even numbers, are apparent, and represent the dark rings, when the waves of light destroy each other. The phenomena of thin plates, such as colours from soap bubbles, and the films of varnish, are well explained by the law of interference. The light reflected from the second surface of the film of air (which must of course, however thin, have two surfaces, viz., a upper and a lower one) interferes with the light reflected from the first, and as they come from different points of space, one set of waves is in advance of the other, No. 4, Fig. 316; they reach the eye with different lengths of paths, and by theirinterferenceform alternately the luminous and dark fringes, bands, or circles. Bridge's diffraction apparatus, manufactured only by Elliott Brothers, offers itself specially as a most beautiful drawing-room optical instrument. The purpose of this apparatus is to illustrate in great variety, and in the most convenient and compact form, the phenomena of the diffraction or interference of light. This is attained by the assistance of photography. Transparent apertures in an opaque collodion film are produced on glass, and a point of light is viewed through the apertures.The forms of the apertures are exceedingly various,—triangles, squares, circles, ellipses, parabolas, hyperbolas, and combinations of them, besides many figures of fanciful forms, are included in the set. When an image of the sun is viewed through these apertures, figures of extraordinary beauty, both of form and colour, are produced; and of each of these many variations may be obtained by placing the eye-glass of the telescope at different distances from the object glass. Many of the figures produced, especially when the telescope is out of focus, might suggest very useful hints to those concerned in designing patterns. Although the phenomena are chiefly of interest to the student of science, in consequence of their bearing on theories of light, yet their beauty and variety render them amusing to all. A few words on the mode of using the apparatus may be of service. (Fig. 318.)

Fig. 317.Fig. 317.

Appearance of Newton's rings when produced in yellow light, 1, 3, 5, 7, being the yellow rings, and 2, 4, 6, 8, the dark rings. Light by the odd numbers; darkness by the even numbers. The central spot, where the two surfaces are in contact, is dark.

Fig. 318.Fig. 318.

Elliott Brothers' diffraction apparatus.

Choose a very bright day, for then only can the apparatus be used. Place the mirror in the sun, and let the light be reflected on the back of the blackened screen. The lens which is inserted into this screen will then form an exceedingly bright image of the sun. Then at the distance of not less than twelve feet, clamp the telescope to a table in such a position as to view the image thus formed. Put the eccentric cap on the end of the telescope, clean the glass objects carefully, and attach them to the cap so that they may be turned each in order before the telescope. In this manner, all those which consist of a series of figures may be viewed. Then detach the eccentric cap, and replace it by the other. Into it place any of the single objects. In viewing some of the figures, brightness is advantageous—in others, delicacy; in the former case, let the lens of long focus be inserted in the screen—in the latter case, that of shorter focus. In every case, let the phenomena be observed not only when the telescope is in focus, but also when the eye-glass is pushed in to various distances.

Mr. Warren de la Rue has ingeniously taken advantage of the colours produced by thin films of varnish, and actuallyfixedthe lovely iridescent colour produced in that manner on highly polished paper, which is termed "iridescent paper." A tank of warm water at 80° Fahr., aboutsix inches deep, and two feet six inches square, is provided, and a highly glazed sheet of white or black paper being first wetted on a perforated metallic plate, is then sunk with the plate below its surface, care being taken to avoid air bubbles. A peculiar varnish is then allowed to trickle slowly down a sort of tongue of metal placed in the middle of one of the sides of the tank, and directly the varnish touches the surface of the water it begins to spread out in exquisitely thin films, and by watching the operation close to a window and skimming away all the imperfect films, a perfect one is at last obtained, and at that moment the paper lying on the metal plate is raised from the bottom of the tank, and the delicate film of varnish secured. When dry, the iridescent colours are apparent, and the paper is employed for many ornamental purposes. An extremely simple and pretty method of producing Newton's rings has been invented by Reade, and is called "Reade's iriscope." A plate of glass of any shape (perhaps circular is the best) is painted on one side with some quickly drying black paint or varnish, and after the other side has been cleaned, it is then rubbed over with a piece of wet soap, and this is rubbed off with a clean soft duster. A tube of about half an inch in diameter, and twelve inches long, is provided, and is held about one inch above the centre of the soaped side of the glass plate, and directly the breath is directed down the tube on the glass, an immense number of minute particles of moisture are deposited on the glass, and these by inflection decompose the light, and all the colours of the rainbow are produced. (Fig. 319.)

Fig. 319.Fig. 319.

Reade's iriscope.

The iridescent colours seen upon the surface ofmother-of-pearl, which Mr. Simonds' excellent commercial dictionary tells us is "the name for the iridescent shell of the pearl oyster, and other molluscs," are referrible to fine parallel lines formed by its texture, and are reproducible, according to Brewster's experiments, by taking impressions of them in soft wax. The gorgeous colours of certain shells and fish, the feathers of birds, Barton's steel buttons, are not due to any inherentpigmentor colouring matter that could be extracted from them, but are owing either to the peculiar fibrous, or parallel-lined, or laminated (plate-like) surfaces upon which the light falls, and being reflected in paths of different lengths, interference occurs, and coloured light is produced.

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 atpage 262, andpage 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 ofan 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 circlea b c d(Fig. 322), with two diametersa b, andc d; or a better mechanical notion of a wave of common 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 bandc d.

Fig. 323.Fig. 323.

Model of a wave of common light.

The existence of analternating motion of some kindat minute intervals along a ray is, says Professor Baden Powell, "as real as the motion of translation by which light is propagated through space.Bothmust essentially becombinedin any correct conception we form of light. That this alternating motion must have reference to certain directionstransverseto that of the ray is equally established as a consequence of the phenomena; and thesetwoprinciples 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 lightis merelypolarized light, havingtwo planesof polarization atright anglesto 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.

Fig. 325.Fig. 325.

No. 1.ais the lime light.b.The condenser lenses.c.The beam ofcommonlight. 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´.dis the ray of light polarized by reflection from the glass plates,c c, andeis 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.Oneof 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 passedthroughthe plate of glass, and was likewise polarized in another plane, but bysingle 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 commonlight 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.

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