356 W. LeConte Stevens—Physiological Optics. 
graph. he relative distances in the drawing being fixed, 
variation in muscular sensation may modify the imagined 
scale of measurement, while ratios are sensibly the same as 
before. The beautiful results attained by Professor W. B. 
Rogers* in determining the form of the binocular resultant are 
applicable to projecting lines that intersect, and not necessarily 
to visual lines, since the perception is attained when these are 
divergent. ‘The same remark is true of Helmholtz’ admirable 
mathematical discussiont of the stereoscope, in which he 
makes no provision for optic divergence, although elsewhere 
e refers to the possibility of stereoscopic vision by this 
method. It is but due him to state that, under the conditions 
assumed, he closes his discussion with the remark, ‘‘ These con- 
ditions are not always fulfilled for the photographic proofs and 
the stereoscopes of commerce.” 
Associated muscular action, to which special prominence 
has been assigned in the present series of pap is 
in like manner incapable of explaining all the phenomena of 
stereoscopy. uch stress has been laid upon it because it 
overs all the facts that have hitherto been referred to visual 
play of the eyes often enables us to become at once sure © 
interpretations that would be enveloped in uncertainty until 
f In 
the attention is apt to be confined to the point fixed, but also 
ness. 
It has been suggestedt that the perception of relief is by 
* This Journal, IT, vol. xxi, pp. 91, 173, et seq. 
Optique Physiologique, p. 842, i 
-LeConte, Sight, p. 151; Appleton, 1881. 
