Trans. X. V. Ac. Set. 12 QcL 24, 



two points nearer or farther apart cannot fall on corresponding retinal 

 points at the same moment with those of A and B, though small differ- 

 ences are easily neglected. Whatever may be the importance therefore 

 of optic convergence, as a factor ordinarily in determining the binoc- 

 ular judgment of distance, it has no such exclusive and measurable 

 value as that attributed in Brewster's experiments ; and the apparent 

 distance of objects viewed through the stereoscope is obviously not 

 determined by intersection of visual lines, if conditions are such as 10 

 render these parallel or divergent. The visual effects of optic diver- 

 gence can be more conveniently studied by using stereographs than by 

 the method already described, and a modification of Wheatstone's 

 reflecting stereoscope affords the best means of measuring variations of 

 the optic angle. As the lenticular stereoscope, however, is now almost 

 universally. employed, it is important that this instrument, as found in 

 the market, be examined first. 



By diminishing the natural convergence of visual lines, the stereo- 

 scopic effect of binocular relief can be quite easily obtained, while gazing 

 upon a stereograph, without any instrument, when the interval between 

 corresponding points of the two pictures does not exceed that between 

 the observer's optic centres. This distance does not often differ very 

 much from 64 mm., which may be taken as an average value. In Fig. 

 3 the distance betweeathe two central dots is 50 mm. If the reader 

 will fix his gaze upon a point ten feet off, just visible below the edge of 

 the page, and then suddenly raise the visual lines to the figure without 

 changing their convergence, he will see three circles instead of two; 

 the central one moreover will appear as the base of a cone whose 

 vertex is pointed toward him, but capped with a small circle. A little 

 attention then will reveal the fact that when the dots are seen distinctly 

 and singly, the small circle is double and slightly indistinct, and vice 

 versa. 



On stereographs, however, the interval between corresponding points 

 is always greater than 50 mm. As the result of measurement made 

 upon the foreground intervals of 166 cards, European and American, 

 taken at random, the mean value I have found to be 72.9 mm., the 

 maximum being 95 mm. If binocular combination is secured without 



Kiu. 3. 



