442 On the Theory of Binocular Perspective. 



true distance of the stereograph being 50 centim. D D / is the 

 curve whose equation is D = J a cosec J a, the value of a being 

 my interocular distance, 60 millim. As might be expected, 

 the probable error is large, being about + 8 millim. ; and the 

 curve A A! is by no means regular, though its general import 

 is unmistakable. It cuts the theoretic curve near the point 

 corresponding to « = 7° 20', but rather lower than might have 

 been anticipated. 



The stereograph employed was one of the full moon, the 

 two pictures being of course cut apart, and so arranged as to 

 produce conversion of relief, thus destroying any resemblance 

 to known bodies; while objects for comparison were excluded 

 from the binocular field of view. All estimates of distance 

 were for the foreground. For very large positive optic angles 

 the dissociation between axial and focal adjustments becomes 

 extremely difficult ; and distinct vision at the distance of 50 

 centim. is then impossible. For negative values focalization 

 is comparatively easy; but beyond — 6° the unaccustomed 

 strain upon the external rectus muscles produces unsteadiness 

 of vision. As limits, therefore, —5° and +45° were selected. 



Although such employment of the muscles of the eyes is 

 unusual when carried to the extent implied in these experi- 

 ments, and the disturbance of natural coordination is at first 

 confusing, the use of the stereoscope nearly always necessi- 

 tates such disturbance, to a limited extent, and is hence not 

 unfrequently productive of discomfort. Most persons, how- 

 ever, who possess healthy eyes will find it possible to secure 

 binocular combination of the conjugate diagrams in fig. 2, if 

 viewed in the stereoscope. For this purpose they are placed 

 at the bottom of the page, which may be rested at the proper 



Fig. 2. 



