J. LeConte on Binocular Vision. 421 
line. The visual result is represented in 
go. 
As illustrating the singular confusion into 
which M. Pictet has fallen, I would draw 
0 
teal images differ slightly from each other, being taken from 
different points of view, so do their fac-similes the two illusive 
images ; also, necessarily, that the real and illusive images o 
each eye differ precisely, as do the two real images or the two 
illusive images. He believes that the perception of relief is the 
result of comparison by each eye of tts real with its wlusive image. 
‘ow what advantage this theory has over the usual and simpler 
one of Brewster, Prevost and Briicke, considering the fact that 
the real and illusive image of each eye differ precisely as do 
the real images of the two eyes, it is impossible to imagine. 
But M. Pictet regards the existence of the four images not as a 
question of advantage, but as a question of fact. “A very 
Simple geometric construction shows us thus four images iden- 
Heal, two to two.” I reproduce M. Pictet’s figure illustrating 
