450 W. LeConte Stevens—The Stereoscope, ete. 
dom resorted to at present. For taking stereographs of statu- 
ary, etc., the lenses of the binocular camera are not often more 
than 80™ or 100% apart. 
That muscular tension is more important than mere intersec- 
tion of axes in affecting the judgment of distance and size 
may be shown by aid of Wheatstone’s reflecting stereoscope. 
Having placed the two outline drawings, each 20™ from its 
mirror so that a distinct combination is attained by axial 
parallelism, the judgment of distance is as definite as could be 
esired. Upon converging the axes strongly and giving atten- 
tion successively to the two monocular images thus obtained, 
each appears greatly ara in comparison with the binoe- 
ular image just seen. Moreover, at the moment one of them 
is made an object of special Etetaba. the other grows ican + 
larger. We have thus images of three apparent sizes, accor 
ing to the degree of muscular tension with which they are 
separately regarded, while the visual angle remains constant. 
visual axes are converged until their intersection is not 
more than 5™ or 6™ off, and the illusive impression is that each 
image is in the direction of its own axis much beyond the inter- 
section. But in fact, being monocular images, the direction of 
the center of each is that of a secondary axis, the right eye 
perceiving that on the right, instead of the left. Since the optic 
center and center of rotation are about 6-6" apart, the former 
being displaced toward the nasal side during the experiment, 
the two secondary axes meet at a very distant point in the rear. 
While the distance of the monocular image is indeterminate, 
it is judged easily enough to be not at the vertex of either the 
apparent or real angle determined by the meeting of axes. 
he experiment is very striking and is not difficult. We have 
a binocular image, of little more than natural size, with clear 
judgment of distance, as the result of axial parallelism ; two 
monocular images, of diminished and separately variable size, 
with very uncertain judgment of distance, as the result of 
axial convergence, the principal and secondary axes being sub- 
jectively interchanged. The apparent diminution in size of us 
monocular images may be easily observed by crossing the e 
while holding in front a card on which a sharply defined oH ha 
is drawn. I may discuss this still further in a future paper. 
No theory of the stereoscope that includes axial divergence 
is possible, unless we recognize the subjective combination of 
the two eyes into a single central binocular eye as the point of 
origin in all perceptions of direction, distance and form. What 
is essential for binocular vision is not any particular relation 
between visual axes but rather superposition of retinal images 
in the binocular eye. What seemed uppermost in the minds of 
slater ne and Brewster * was superposition of external vir- 
Wheatstone, Physiology of Vision, Phil. Mag., 1852, pp. 243 and 246. 
es on New Stereoscopes, Phil. Mag., 1852, pp. 17-26. 
