298 W. LeConte Stevens—Notes on Physiological Optics. 
volves, the experiment has probably been rarely tried, and 
soon passed into oblivion. The curvature of the phantom sur- 
face, in a median plane passing vertically between the eyes, 
was rediscovered a short time ago by Professor LeConte, and 
soon afterward I discovered the curvature in all directions. 
The effect is in no way due to intersection of visual lines, 
but to the opposite obliquity of vision with each eye separately, 
combined with the fact that the retinal surface is not plane but 
almost spherical at the points impressed, the center of curvature 
being very near the nodal point of the crystalline lens. The ex- 
periment is therefore far easier and more striking if optic paral- 
lelism or slight divergence be substituted for strong convergence, 
and if, instead of a wall, a pair of cards be employed, on which 
are perfectly similar figures, such as a pair of similar series of 
concentric circles. If there be difficulty in directing the eyes, an 
ordinary stereoscope can be used as an aid. I have devised a 
simple attachment for the adjustable stereoscope described in my 
last paper, by which one can with perfect ease thus secure stereo- 
scopy with similar figures. Dissimilarity between the external 
pictures has hitherto been deemed indispensable for the attain- 
ment of true stereoscopic effects. The present method there- 
fore, in which advantage is taken of the globular form of the 
eye, so far as I can learn, is entirely new. The binocular relief 
moreover can be reversed at will without consciously changing 
the relation between the visual lines, and the same pair of siml- 
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lar pictures can be examined with comfort while in form the 
binocular image changes from an elliptic convex shield to a 
flat circular plate and thence into a deep elliptic cup; the pro- 
cess being reversed at pleasure. 
