206 RECENT PROGRESS OF THE THEORY OF VISION. 



had to work long and hard before it was understood, and before 

 this doctrine replaced the belief previously held in the constant 

 and exact correspondence between cause and effect. And we 

 can scarcely say that the truth is even yet universally recog- 

 nised, since in our present subject its consequences have been 

 till lately disputed. 



Therefore, as motor nerves, when irritated, produce move- 

 ment, because they are connected with muscles, and glandular 

 nerves secretion, because they lead to glands, so do sensitive 

 nerves, when they are irritated, produce sensation, because they 

 are connected with sensitive organs. But we have very different 

 kinds of sensation. In the first place, the impressions derived 

 from external objects fall into five groups, entirely distinct from 

 each other. These correspond to the five senses, and their 

 difference is so great that it is not possible to compare in quality 

 a sensation of light with one of sound or of smell. We will 

 name this difference, so much deeper than that between com- 

 parable qualities, a difference of the mode, or kind, of sensation, 

 and will describe the differences between impressions belonging 

 to the same sense (for example, the difference between the 

 various sensations of colour) as a difference of quality. 



Whether by the irritation of a nerve we produce a muscular 

 movement, a secretion or a sensation, depends upon whether we 

 are handling a motor, a glandular, or a sensitive nerve, and not 

 at all upon what means of irritation we may use. It may be 

 an electrical shock, or tearing the nerve, or cutting it through, 

 or moistening it with a solution of salt, or touching it with a 

 hot wire. In the same way (and this great step in advance was 

 due to Johannes M tiller) the kind of sensation which will ensue 

 when we irritate a sensitive nerve, whether an impression of 

 light, or of sound, or of feeling, or of smell, or of taste, will be 

 produced, depends entirely upon which sense the excited nerve 

 subserves, and not at all upon the method of excitation we 

 adopt. 



Let us now apply this to the optic nerve, which is the object 

 of our present inquiry. In the first place, we know that no 

 kind of action upon any part of the body, except the eye and 



