A Manual Of Photography

THE STEREOSCOPE. 307 being adjusted to accommodate the differences of sight in dif- ferent individuals. At the bottom of the box, as seen through the opening, are placed the two stereoscopic pictures, which may consist either of diagrams, similar to those already repre- sented, or of images taken by the daguerreotype, talbotype, or collodion processes. These photographic processes enable us to obtain such copies of external nature as are required to pro- duce the magical results with which the stereoscope renders us familiar. It is required to take two pictures of a single object, at such a difference of angle as will produce the solidity which is evident in ordinary binocular vision, as the result of viewing two dissimilar images, under certain conditions, on a plane surface. The two accompanying figures represent a bust as viewed by each of the two eyes singly. If the experiment is tried upon a bust or statue, it will be found that one eye will see surfaces which are invisible to the other. Thus in these examples it will be quite apparent, upon examination, that the line of the cheek is more distant from the line of the nose in one than in the other image, and that a similar inequality exists in several other parts. By a little practice, any reader may, by squinting, resolve these two images into one, and thus produce the stereo- scopic effect. Now the object is to place the camera in the position of the eyes, and thus obtain the representation of two images, as viewed by each eye separately. This may be effected with a single camera, by adjusting it at a certain measured dis- tance from the object to be copied, and having obtained one picture, move it round about twenty degrees, and take the