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 gla.s.s 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 with _one_ eye 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 gla.s.s 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 2 inches. (Fig. 309.)
[Ill.u.s.tration: 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 2 inches apart, with their _thin_ edges towards each other, and marked, A B, Fig. 310. The centres of the two stereoscopic pictures C D correspond with the centres of the lenses, and the rays of light _diverging_ from C D fall upon the semi-lenses, and being refracted nearly _parallel_ are, 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 from E; 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 the _spectral images_ of 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-lapping [Page 323] of the _spectral images_ becomes very apparent, so that the combined _spectral images_, and not the _pictures_ themselves, are seen when we look into a stereoscope. (Fig. 310.)
[Ill.u.s.tration: 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 quant.i.ty by which the image of one of the pictures is displaced, place the stereoscopic pictures at a distance equal to twice that quant.i.ty--that is, place the pictures so that the average distance of similar parts in each is equal to twice that quant.i.ty. 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."
XV. _The Stereomonoscope._
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 gla.s.s.
The [Page 324] object A, Fig. 311, is highly illuminated, and placed in the focus of a double convex lens B, when an image of the object is projected, and will be found suspended in the air in the conjugate focus of the lens at C, and from this point the rays of light will diverge as from a real object, which will be seen by separate spectators at D D and E E; and if the screen of ground gla.s.s is placed at G G, the image will appear with all the effect of length, breadth, and depth, which belong to solid bodies. (Fig. 311.)
[Ill.u.s.tration: Fig. 311. The stereomonoscope.]
An image formed on ground gla.s.s 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.
XVI. _The Stereomoscope._
[Ill.u.s.tration: Fig. 312. The stereomoscope.]
This arrangement is an important modification of the other, and consists of a screen of ground gla.s.s (A B, Fig. 312), and two convex [Page 325]
lenses (C D, and E 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 at K K and L L; so that several persons may look at the stereoscope at one time. (Fig. 312.)
XVII. _The Pseudoscope._
[Ill.u.s.tration: Fig. 313. Horizontal section of the pseudoscope, showing at A B two 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 at D. The eyes are supposed to be looking at the globe, C, in the direction of the arrows. E E. Bra.s.s plates blackened, which shut out the side light, and a.s.sist in keeping the prisms in position.]
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, and [Page 326] the whole of its convexity is turned to concavity; and of course a face seen under these circ.u.mstances 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.
Sir David Brewster, in his "Letters on Natural Magic," remarks that "one of the most curious phenomena is that _false_ perception 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.
[Page 327]
CHAPTER XXIV.
THE ABSORPTION OF LIGHT.
The a.n.a.lysis 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 gla.s.s, 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 a.s.sembly 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 large _tow_ torch 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.
[Page 328]
CHAPTER XXV.
THE INFLECTION OR DIFFRACTION OF LIGHT.
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 gla.s.s obtained by blowing out a bulb of red-hot gla.s.s 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.
[Ill.u.s.tration: 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 gla.s.s 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 s.p.a.ce (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 bra.s.s 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; and [Page 329] the relative thicknesses of the plates of air at which each coloured light is reflected are as follows:--
Red 133 10 millionths of an inch.
Orange 120 " "
Yellow 113 " "
Green 105 " "
Blue 98 " "
Indigo 92 " "
Violet 83 " "
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 ill.u.s.tration of the undulatory theory, and demonstrates the important truth, that _two 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, _fits_ or dispositions to be reflected from or transmitted by transparent bodies. Sir Isaac does not pretend to explain the origin of these _fits_, or the cause which produces them, but terms them _fits of transmission_ and _fits 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 ill.u.s.trating 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 pa.s.sing _on one side_ of the slip of card with any opaque body, and allowed the light to pa.s.s 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 produced _by the interference [Page 330]
of 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).