But, owing to the different positions of the two eyes, it is quite apparent that the images seen by them differ slightly from one another, and it is by the union of these two pictures that we see the object and know it to be solid. If, therefore, we draw or photograph this object first as seen by the right eve and then as seep by the left eye, and present each eye to that pict ure which was made from its point of view, we see the re presentation of the object in re lief almost as we see the object itself. Fig. 433 proves this. If the two representations of a truncated pyramid, as seen by the right and left eye, be steadily looked at R with the right eye and L with the left, holding a piece of card between the fingers to separate them, we get .a single picture of the object in relief. To assist the eye in combining such pairs of dissimilar pictures, mirrors and lenses have been used, such instruments being termed stereoscopes.
The first stereoscope invented by Wheatstone has been termed the reflecting stereoscope for the reason that mirrors or reflectors are used to change the apparent position of the pictures, so that they are both seen in the same direction. Two mirrors about three inches square are arranged so that their backs are at right angles to each other as shown in Figs. 434, 435. The two pictures are reflected by the two mirrors into the right and left eye respectively, and appear as only one picture. As, however, the picture was in verted by the mirror, it was necessary to place the drawing for the right eye on the left side, and the one for the left eye on the right side.
To Sir David Brewster is due the credit of the invention of the lenticular stereoscope, so called to distinguish it from Wheatstone's instrument. In Brewster's instrument another
means is made use of to compel each eye to see its own picture. This is done by lenses divided in half and inverted (figs. 436, 437). His method is thus described:* " If an object be viewed through the center, or more properly, along the axis, of a convex lens, it will be seen exactly in front of the eye—i.e., in a line with the eye, the center of the lens, and the actual place of the ob ject. If now the lens be moved slightly to the left, the ob ject will appear to advance to the right ; and conversely as the lens is moved to the right, the object is displaced in the opposite direction. Let the lens be cut in half tranversely, and the two semi-circular pieces reversed as to their former position—that is, placed side by side, and so that their thin edges shall be adjacent, while the two plane edges formed by the section of the lens are kept in mutual parallelism and have their faces turned out wards towards the left and right respectively. The right eye will now look through the left half of the lens, and vice versa, and the two pictures, each placed opposite its appropriate eye, and in the principal focus of the eye-piece, will he seen, not in their actual places, but in a position midway between the two. The subsidiary purposes served by this ar rangement are, that the pictures are magnified as well as caused to coalesce, and that the equality of the magnifying power of the eye-pieces (a result by no other means certainly attainable) is secured by the fact of their being cut from the same lens, the whole of which is thus advantage ously and economically utilized." Various forms of stereoscopes are shown on p. 473.