must appear as an ellipse because viewed obliquely, the illusion is that there is no obliquity of vision, but that a narrow cup is suspended directly in front; while the pictures that are really in front of each separate eye appear, without relief, out at the two sides.
If the attention be carefully directed successively to the foreground and background when binocularly viewing a properly constructed outline stereograph, it will be found that perfectly distinct vision of the whole picture at any given moment is not usually possible. The distance between corresponding background points exceeds that between similar foreground points. This excess we shall call the stereoscopic displacement. If it be considerable, a pair of corresponding background points must be seen double, or imperfectly combined, when the foreground is distinct. In transferring the attention, then, to the background, slight associated contraction of the external rectus muscles is necessary to secure perfect combination of corresponding points, and this instantly suggests the idea of greater distance for these. Thus, as the attention is given to different parts of the picture, the tension in the muscles of the eyes is continually varying, and this is one important element in determining our binocular perception of solidity. Unless the attention be very carefully given to it, we are apt to overlook the successive duplication in different parts of the field of view. If the stereoscopic displacement be small, the perception of such duplication may be quite impossible, while the appearance of solidity, or of perspectiveness, as it has been called, remains distinct. The stereograph, represented in Fig. 16, has been specially constructed to exhibit a variety of different stereoscopic displacements. It may be viewed either with cross-vision, or with the aid of a card placed edgwise upon the triple line at the middle, or by placing the page in front of the semi-lenses of a stereoscope. Supposing the last of these methods to be employed, there will be seen at the top of the field of view a truncated cone, with a dot at the center of its lower base, and a pair of projections from the circumference of the
Fig. 16.—Stereograph illustrating the Binocular Combination or Lines.
