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- 



