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THE POPULAR SCIENCE MONTHLY. 



tions in apparent size and distance of the binocular image are very 

 striking, while the perspective in the picture remains clear but also 

 variable. If the stereograph be that of a reversed cone, this becomes 

 apparently shallow, small, and near, or deep, large, and remote, ac- 



Fig. 12. The Graduated Reflecting Stereoscope. 



cording as the excess of strain is upon the internal or the external rec- 

 tus muscles. This instrument, indeed, is useful in quite a variety of 

 binocular experiments. The visual effects are the same as those from 

 an adjustable stereoscope with semi-lenses, but the limits of variation 

 are far wider, and it is a decided advantage to obtain direct measure- 

 ment of the optic angle. 



Such experiments show very conclusively that the current theory 

 of visual triangulation, whatever may be its application to normal 

 binocular vision, has to be entirely discarded as an explanation of 

 stereoscopic vision. As a theory it is beautifully simple, and at first 

 glance appears eminently satisfactory ; the only objection to it in rela- 

 tion to the stereoscope consists in the fact that it is untrue. Its ex- 

 pression may be found in most of our text-books of physics, and the 

 diagram usually employed in explaining the stereoscope is that of Fig. 

 13, which is taken from Helmholtz's " Physiological Optics," the only 

 change being in the avoidance of Greek lettering. This slight change 

 also is made in the following translation from the French edition of this 

 work, which received the last corrections of the distinguished author, 

 who is universally recognized as the highest living authority on the 

 subject of which it treats. After describing the arrangement of the 

 prism-like semi-lenses, he writes : " The two drawings are placed, side 

 by side, upon the same sheet. The right eye, R, looks upon the draw- 

 ing, a b, through the prism, p / the left eye, L, looks upon the drawing. 

 a' b\ through the prism, p ; the partition, <7, prevents each eye from 

 seeing the drawing intended for the other. The rays, cp and c' p\ 

 sent forth by" the drawings, are refracted by the prisms, following the 

 directions p R and p L, the prolongations of which cut each other at C. 

 The convexity of the surfaces of the prisms has the effect of diminish- 

 ing at the same time the divergence of the sheafs of rays, so that each 

 eye sees at A B an image of the drawing that is presented to it. The 

 object appears in relief at A B." 



This explanation is distinctly geometric, the locality of each point 

 of the image perceived being determined by intersection of visual lines, 



