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THE POPULAR SCIENCE MONTHLY. 



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 di- 

 rectly in front ; while the pictures that are really in front of each sepa- 

 rate 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 out- 

 line stereograph, it will be found that perfectly distinct vision of the 

 whole picture at any given moment is not usually possible. The dis- 

 tance 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 back- 

 ground 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 mus- 

 cles 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 con- 

 structed 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. 



