# The wonders of optics/The stereoscope

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THE STEREOSCOPE.
Having devoted so much space in the preceding chapters to optical amusements of a purely recreative character, it is only right that we should now say a few words on certain instruments of a less frivolous character than those we have lately been considering, and which deserve at our hands the most serious attention. We shall, therefore, in the present chapter, speak of an ingenious instrument which serves to show in relief the images of objects depicted on a flat surface. We have already seen, that although we have two eyes, provided with lenses and screens by means of which the images of things around us are formed, we only perceive a single object; and the student has no doubt long since wondered why nature has bestowed two eyes upon us, when only one would have apparently served the same purpose. This question was for a long time a complete puzzle to philosophers, and it was not until Professor Wheatstone made his experiments on binocular vision in 1838, that the matter received a satisfactory explanation. He showed that each eye receives a different impression of any object upon the retina, and that it is in consequence of the union of these slightly dissimilar images that the sensation of relief is experienced. A one-eyed man or a Cyclops would only partially perceive relief in the objects presented to his view, in ​consequence of a single image being sent to his brain. He would, no doubt, after examining the things he saw with his hands, know they were solid, and generally see them so; but if a new object were presented to his view he would have some difficulty in knowing whether it had a flat surface or not.
Fig. 67.
The principle of binocular vision may be explained as follows: If a playing die, such as is represented in fig. 67, be held out at arm's length in the position indicated in the figure, and looked at first with the left eye and then with the right, we shall find that in the first case we see a little of the three dots on the left-hand side, and in the second we lose sight of the three dots and see a little of the single one on the right-hand side. The images seen by each eye are, therefore, slightly dissimilar, and it stands to reason that, if by any means we can combine two slightly dissimilar flat pictures of a solid object, we shall see it in relief. This was proved practically by Professor Wheatstone, who constructed an instrument capable of effecting the desired union, and which has since been called the stereoscope, from two Greek words signifying 'to see solid.' The instrument remained for a long time fallow, so to speak, from the difficulty of drawing two pictures that should be identical in size and details, although dissimilar in the arrangement of their perspective. It was, therefore, not until photography enabled us to do this with the greatest ​ease and exactitude that the stereoscope became common. The instrument first devised by Professor Wheatstone, was what is termed a reflecting stereoscope, and was expensive to make and cumbrous to use. It was modified by Sir David Brewster, by the substitution of prisms for reflectors, and was thus made cheaper and more portable. The refracting form of stereoscope is so familiar to most people, that it really needs no description. It will only be necessary to mention that the prisms used in the eye-pieces are made by cutting a double convex lens in two, and reversing the halves. They are so placed that the centre of each prism is just in the centre of each eye; but as the eyes of different people vary in distance, an arrangement is generally added so that the eye-pieces may slide from side to side. Being cut from lenses, the prisms have a magnifying power; consequently other means are provided for sliding them up and down to suit the length of focus in different eyes.
Fig. 68.—Stereoscope.
In fig. 69 we can follow the path of the rays proceeding from each picture, and reach the eyes apparently from a spot exactly between the two.
In the reflecting stereoscope two mirrors are joined ​together at right angles to each other, the two pictures being placed at each side, at a distance corresponding to their size. The reflecting instrument, although not Fig. 69.—The Principle of the Refracting Stereoscope. so portable, is in some sort superior to the other, inasmuch as pictures of any size can be seen by it, whilst in the smaller instrument the size of the photograph is limited by the distance at which the eyes are placed.
It should be mentioned, that no optical instrument of any kind is absolutely necessary to obtain a stereoscopic effect from two suitable drawings or photographs, as it is quite possible by a little management of the eyes to cause the two images to combine with each other. Referring again to fig. 67, it will be perceived that the two figures of the dice are about an inch and a half from each other. Holding the book at about ten inches from the eye, they are viewed by squinting strongly until the right eye looks at the left die, and the left eye at the right. This may be also done by converging the eyes on a point beyond the centre of the figure, which may be easily done by looking at a point midway between the two. In both cases the images at first appear doubled, and we see four dice, but a little practice will soon enable you to cause the two inside images to coalesce, ​and so give the effect of relief. It is true that even then three images are seen, but the eye soon grows accustomed to neglect them altogether. This habit is a very pleasant acquirement for the London flâneur, who can thus see in perfection the numberless stereoscopic views now shown in our shop-windows without the intervention of an instrument of any kind.
The method of photographing subjects for the stereoscope is very simple, and consists in taking two views of the object to be depicted, from two different points. According to the distance of these points from each other, so will the resulting pictures appear in greater or less relief. This is readily seen in some stereoscopic portraits which have been taken at a large angle, and consequently show such increased relief as to produce distortion. Theoretically, the interval of the two points of view ought to be two inches and a half, that being the average distance between the two eyes; but in practice it is better to increase it in the case of portraits or other near objects to about twelve inches, and in that of views to even several feet. Brewster's original rule for taking stereoscopic photographs, was to place the cameras one foot apart for every twenty-five feet of distance. The beautiful stereoscopic pictures of the moon photographed by Mr. Warren de la Rue were taken at more than 1,000 miles' distance, in order to obtain the necessary relief. The principle of the stereoscope has received many useful applications in the way of book illustrations, art teaching, and anatomical demonstration, and has thus gained a position among philosophical instruments that it did not at first possess.
A combination of the principles of the phenakistiscope (fig. 4) and stereoscope, has resulted in the invention of an instrument called the stereotrope. A number of binocular photographs of some object in motion—a steam-engine, for instance—are taken when ​the moving parts are in different positions, and mounted on two revolving discs, the images being combined by means of a pair of semi-lenses, as in the ordinary refracting stereoscope.
We cannot leave this subject without describing the pseudoscope, also the invention of Professor Wheatstone. If a stereoscopic pair of photographs of some solid body—a ball, for instance—are mounted the reverse way, that is to say, if the picture intended to be looked at by the right eye is placed on the left, the relief of the object will be reversed, and the ball will appear as a hollow hemisphere. If, therefore, we can by means of lenses or prisms cause the image of any natural object, as seen by the right eye, to be conveyed to the left, and vice versâ, we shall see the relief reversed. A conical cap will appear in relief as a cone, a globe will look like a hollow sphere, and the human face will take the semblance of the inside of a mask. The same deception may be effected by looking at a seal through a short-focused lens, so that the image shall seem reversed. In this case, the light coming apparently from the wrong side, and shining on the parts in relief, gives them the appearance of being hollow. An intaglio will, of course, appear in relief when so looked at. Photographs of gems and bas-reliefs will also present a pseudoscopic appearance, if looked at in a light coming from the opposite side to that in which they were taken. The same appearance may be seen sometimes in wall papers having patterns painted in strong relief.