This diagram represents the outlines of a near object, as seen by each eye separately. The one on the right hand shows it as seen with the right eye, and the other as it looks with the left eye; and if both drawings be combined into one image, it stands out in bold relief. This may be done without any instrument, by squinting at them; but the effect is more readily and far more agreeably produced by the Stereoscope, so named from the Greek words στερος {steros}, solid, and σκοπεω {skopeô}, to see.
Professor Wheatstone claims to be the first who contrived an instrument to illustrate this effect of binocular vision, and he also claims to be the first who brought to notice the different appearances of objects seen with each eye separately. Sir David Brewster, however, disputes, on behalf of Mr. Elliot, of Edinburgh, Professor Wheatstone's claim to the invention of the first stereoscopic instrument; and he has shown that the difference of vision with each eye was remarked by Galen, 1,700 years ago; that it was noticed by Leonardo da Vinci in 1500, and formed the subject of a treatise by a Jesuit, named Francis Aquilonius, in 1613; and that it was a well-known phenomenon of vision long before it was mentioned by Professor Wheatstone.[5] Mr. Elliot, though he conceived the idea, in 1834, of constructing an instrument for uniting two dissimilar pictures, did not carry it into effect until 1839, the year after Mr. Wheatstone had exhibited his reflecting Stereoscope to the Royal Society, and at the meeting of the British Association.
Mr. Elliot's contrivance, to which Sir David Brewster is inclined to give precedence in point of date, was very inferior in its effects to the reflecting Stereoscope. It was without lenses or mirrors, and consisted of a wooden box 18 inches long, 7 inches broad, and 4½ deep, and at the end of it was placed the dissimilar pictures, as seen by each eye, that were to be united into one. The view he drew for the purpose comprised the moon, a cross, and the stump of a tree, at different distances; and when looked at in the box, the cross and the stump of the tree appeared to stand out in relief.
The accompanying woodcut represents the original stereoscopic pictures, copied from Sir David Brewster's book; and by looking towards the picture on the left with the right eye, and on the right-hand picture with the left eye, the two will be seen united, and the cross and the stump of the tree will appear to stand out solidly.
The arrangement of the apparatus, as described by Professor Wheatstone, in his paper read before the Royal Society, consists of two plane mirrors, about 4 inches square, placed at right angles; and the drawings, made on separate pieces of paper, were reflected to the eyes looking into the mirrors at their junction. The diagram is a sketch of this arrangement. In the middle of a narrow slip of wood, d e, about 12 inches long, the two mirrors, a b, are fixed, inclined at the required angle from their line of junction at c. Upright pieces of wood, d h, e f, at each end, are furnished with slides or clips to hold the drawings, which are reflected from the inclined mirrors, and seen in them by each eye separately. Thus, the left eye sees only the picture fixed on d h, and the right eye sees the one placed at e f; and the two images, being combined at the seat of vision, produce the same impression as a solid body.
It is almost unnecessary to describe the external appearance of the lenticular instrument invented by Sir David Brewster, and explained by him at the meeting of the British Association in 1849. In the best kind of instruments the glasses, through which the pictures are seen, are composed of a single large double-convex lens, divided in the middle, the thin edges being set towards each other, about 2½ inches apart. The more improved instruments, indeed, are made from lenses upwards of 3 inches in diameter, which, being cut into two, and the thin parts being ground flat, are set edge to edge, and from an aperture sufficiently large for both eyes to look through. By this means the instrument suits all eyes, without requiring adjustment, and the field of view is increased. A diaphragm, or partition, placed at the junction of the two lenses, confines the vision of each eye to its appropriated picture, and thus tends to prevent the confusion of images that might otherwise arise.
The object of using semi-lenses is to facilitate the union of the two pictures into one, by looking through the lens towards its edge, instead of through the centre, the image being thus refracted to a different position. This may be easily exemplified by looking at an object steadily through different parts of the same lens. After looking at it with the right eye through the centre, and whilst keeping the axis of the eye in the same direction, move the lens slowly towards the right, so as to bring the edge of the lens opposite the pupil. This movement of the lens towards the right hand will be accompanied by an apparent movement of the image towards the left, so as to bring it to a point between the two eyes. If the experiment be repeated with the left eye, the image will be removed towards the right hand; and thus, by looking at the two stereoscopic pictures through the thin parts of two lenses, the images are superposed and form a single one.
Sir David Brewster attached much importance to the semi-lenses, which have the effect of prisms in refracting the rays of light; but that form of lens is not essential to give apparent solidity to the images; and many of the commoner kind of instruments are now made with ordinary double-convex lenses, and without any partition. With the semi-lens, however, there is less difficulty in uniting the two pictures into one than when an ordinary lens is employed.