Comparative Physiology of Binocular Vision. 169 
and look with one eye. It seems —— that the law of 
corresponding points can exist for these animals ; for if it did, 
as we have already seen (p. 167), it woula only lead to constant 
stereoscopic effects, and the complex but accurate visual ro 
ments founded on these effects. They see indeed with two 
owever much their tee may be multiplied, each organ 
pinay peculiar to the eye. Nothing coors exists in 
the other senses. Binocular vision in its perfection, as it exists 
in man, is the last result of the gradual improvement of that 
most refined and wonderful instrument, the eye; specially adapt- 
ing it to meet the wants of the higher faculties of the mind. 
elow the vertebrates, of course, binocular vision does not 
and cannot par ee 
called the central spot. It is the most highly organized and 
most sensitive spot of theretina. It differs from other portions, 
Ist, in the fact that here the layer of rods and cones, the true 
than elsewhere. 
Now it is a familiar fact that while gazing steadily at a cer- 
tain point we see very clearly only a very small area about the 
point of sight. This small area corresponds, point for point, 
to the central spot of the retina. If now, while gazing steadily, 
we observe the relative distinctness of vision in other portions 
of the field of view, we shall find that it becomes less and less 
distinct in proportion as the point observed is more distant from 
the line of sight. In other words, there is a perfect graduation 
of distinctness from the point of sight, where it is greatest, to the 
