wimns, IV Ver. SCz, 12 Oct. 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 to 
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 between the two central dots is 50mm. _ 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 wzce 
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 
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A, 
