Brewster's Theory of Binocular Perspective. 441 



of sight than is suggested by the tension of the rectus muscles; 

 and there is partial dissociation between actions generally 

 associated. Finally, the visual angle has remained constant; 

 and this tends to produce the idea of constancy in distance. 

 Of the three elements, however, the first is found most impor- 

 tant ; and the combined effect is that the image appears 18 or 

 20 centim. distant, the judgment being quite uncertain. The 

 apparent area and depth of the external image are diminished 

 approximately in the ratio of the diminished distance. 



Let the arms of the stereoscope be now pushed back till 

 a =—5°. Relaxation of the internal rectus and contraction 

 of the external rectus muscles is habitually associated with 

 recession of the point of sight ; and in this case it is carried 

 beyond the limit of axial parallelism. But, again, the focal 

 adjustment is for a point only 50 centim. in front ; and the 

 visual angle is constant. Of the three elements the last is 

 now most important ; and the combined effect is that the image 

 appears about 60 or 70 centim. distant, with corresponding 

 increase in apparent area and depth. 



In the accompanying figure (fig. 1) T have shown the result 



Fiff. 1. 



of experiments conducted at different times during the last 

 few months. The stereoscope was manipulated by an assistant, 

 who varied the optic angle in the most irregular order, record- 

 ing its values and my corresponding estimates of distance, 

 while I was kept ignorant of these records until the entire list 

 of estimates had been completed. Six independent series, of 

 eighteen estimates each, were made; and the curve A A' ex- 

 presses the mean result, the values of the optic angle being 

 taken as abscissas and estimated distances as ordinates, the 



