160 J. LeConte on Binocular Vision. 
words, the verticals should “ deviate much from a right angle,” 
IZ: have tried many eyes and I have yet found 
none in which the coincidence of the verticals of Prof. Helm- 
holtz’s diagram was perfect when combined by means of a 
stereoscope, i. ¢., beyond the diagram; but I have found one 
person to whom the coincidence seemed to be perfect when the 
combination was made by squinting. io 
It is evident, then, that Prof. Helmholtz’s principle cannot 
explain the stereoscopic coincidence by squinting in his own 
experiment. I myself believe that if the coincidence takes 
place only by squinting (as in the case mentioned above), it 
can only be explained by rotation of the eyes inward. It 
true that in this case the horizontals ought to cross also; but 
Prof, Helmholtz himself admits that such is sometimes the 
fact, but attributes it to fatigue. “After keeping the eyes,” 
e says, ‘‘a long time looking at a near object, as in reading 
or writing, I have found that the horizontal lines crossed each 
other ; but they became parallel again when I had looked for 
some time at a distant object.” 
angle of 11 degrees with the true vertical and the true hori 
together, the coincidence of both verticals and horizont 
seemed to be perfect. When the plane of the diagram was t00 
near or too far, all the lines crossed, in the one case in on 
direction and in the other case in the other direction. I the? 
constructed still other diagrams, in which the inclination of 
