Fig. 1. Fig. 2.
The preceding diagrams exhibit the appearance of a bust as seen by each eye successively.
Observe that the second figure, which represents the impression received by the right eye, is more of a full face than fig. 1, which, being viewed from a point removed a little to the left, partakes of the character of a profile.
The human eyes are placed about 2½ inches, or from that to 25/8 inches, asunder; hence it follows that, the points of sight being separated, a dissimilar image of a solid object is formed by each eye. We do not however see two images, but a single one, which is stereoscopic.
In looking at a picture painted on a flat surface the case is different: the eyes, as before, form two images, but these images are in every respect similar; consequently the impression of solidity is wanting. A single picture, therefore, cannot be made to appear stereoscopic. To convey the illusion two pictures must be employed, the one being a right and the other a left perspective projection of the object. The pictures must also be so arranged, that each is presented to its own eye, and that the two appear to proceed from the same spot.
The reflecting stereoscope, employed to effect this, forms luminous images of the binocular pictures, and throws these images together, so that, on looking into the instrument, only a single image is seen, in a central position. It should, however, be understood, that no optical arrangement of any kind is indispensably required, since it is quite possible, with a little effort, to combine the two images by the unaided organs of vision. The following diagram will make this obvious:—
The circles A and B represent two wafers, which are stuck on paper at a distance of about three inches from each other. They are then viewed by squinting strongly, or turning the eyes inwards towards the nose, until the right eye looks at the left wafer, and the left eye at the right wafer. Each wafer will then appear to become double, four images being seen, the two central of which will gradually approach each other until they coalesce. Stereoscopic pictures, properly arranged, may be examined in the same manner; and it will be found that the resultant solid image is formed midway, at a point where two lines, drawn across from the eyes to the pictures, cut one another. The experiment here mentioned is sometimes a painful one, and cannot easily be made if the eyes are not of equal strength; but it will serve to show that the essential principle resides in the binocular representation of the object, and not in the instrument employed to view it.