VISION. 79 
any irritation, and reaching the bacillary elements 
stimulate the nerve-terminations in connection 
with them; and when the impressed conditions 
thus started are continued through the retina and 
optic nerve to the brain the sensations of vision 
take place. “Thus the mechanism of vision, though 
more complex than that of common sensation, is of 
a very similar description. 
There is, however, one striking distinction. In 
the case of touch the sensation is referred to the 
spot where the stimulus is applied: in the case of 
vision it is referred, not to the spot where the ray of 
light falls on the inverted picture in the back of the 
eye, but to a point outside the body and placed in 
the direction from which the ray has come. That 
is certainly the case, however inexplicable; al- 
though it is true that the recognition of distance is 
learned slowly by experience. Were the landscape 
at birth perceived as a pair of minute inverted pic- 
tures lying inside the head, it would be impossible 
for a new born animal to learn that they were 
indices of a world outside: and it is curious that 
physiological writers have not always noticed this, 
but have tried to explain how two inverted pictures, 
spoken of as if they were outside the body, are 
rectified and made one. 
The perfection of vision necessary for the ap- 
