168 J. LeConte on Binocular Vision. 
take their places successively in front of the observer. And 
this is true whatever the degree of convergence or divergence. 
In all cases, whatever be the position of the eyes, objects in 
the visual lines, and whose retinal itaages therefore fall on the 
central spots of the retin, are seen in front. If therefore the — 
directed straight right and left, objects directly right and left 
would be seen in front, and therefore 90° from their true 
position. 
IX. Comparative Physiology of Binocular Vision. 
For many years past I have reflected much, and attempted 
to make some observations, on the phenomena of vision of 
lower animals, for the purpose of testing the existence or non- 
existence of binocular vision in them. I must frankly confess 
I have accomplished but little. The phenomena of binocular 
vision are so essentially subjective, that it seems almost impos- 
sible to reach any satisfactory results by observations or ex- 
more success. 
In man the axes of the conical eye-sockets diverge about 
25°, or each makes an angle with the median line of about 12°. 
In these slightly diverging conical sockets, the eye-balls are so 
placed, and the muscles so adjusted, that in the waking passive 
state their axes are parallel; and from this passive parallel 
objects. In man, then, though the eye-sockets still diverge 
considerably, the eyes are set in front with axes naturally 
arallel. This is evidently the position most suitable for 
inocular vision. : 
In monkeys the position of the eyes is much the same as 1m 
man. They are placed well in front, near together, their axes 
apparently parallel, and therefore well adapted to binocular 
convergence. ut as we go down the vertebrate scale, the 
eyes are placed no longer in front, but on the sides of the head, 
with their optic axes inclined nearly or quite 180° to ea 
other. It seems quite evident that animals with eyes so placed 
cannot converge the optic axes on a single point, especially 4 
near point. In fact, it is well known that most birds, when 
viewing an object very attentively, turn the head on one side 2 
