J. LeConte on Binocular Vision. 173 
that if there be any such difference in our own eyes, it cannot 
be more than 10’ ; in this case the horopteric plane would be 
at least 35-40 feet below the eyes. But Prof. Helmholtz 
takes no account of rotation of the eyes on the optic axes, 
hich greatly affects the form and position of the Horopter 
when the point of sight is near ; and we believe that it is only 
when the point of sight is near, that the form and position of 
the Horopter is of any practical importance in vision, for it is 
only then that the doubling of images lying out of the Horop- 
ter is perceptible. 
Tt has been with much hesitation that I have ventured to 
criticise the conclusions of so distinguished a physicist. My 
ability to do so, if well founded, I attribute entirely to a facil- 
ity in the use of the eyes such as I have never seen equalled in 
the case of any other person. 
Although, I believe, Meissner has arrived at truer results 
than any one who has yet written on this subject ; yet I think 
his method very unsatisfactory. I have wondered at the skill 
_ and patience which could attain such true results by such im- 
perfect methods. I have tried Meissner’s experiments Rhone 
h other or not; and in his second experiment with the 
“nnot be vertical but inclined to the visual plane. hon 
