130 Dr. Nicholl on a peculiar Imperfection [Feb. 



sensation seeittgy it is necessary that it produce a distinct corre- 

 sponding state of the retina, which state is essential to the 

 existence of such corresponding sensation. There is, for 

 instance, one state of the retina which is essential to the exist- 

 ence of tlie sensation seeing red; another state which is essential 

 to the presence of the sensation seeing yellow ; and so on. So 

 that, wnen we speak o{ seeing a certain fixed number of colours, 

 we imply that the retina can have that number of distinct states 

 produced in it. These states which are essential to the presence 

 of sensation, I have termed sensual states. 



Let us call that sensual state which produces the sensation 

 seeing redy A ; that which produces seeing orange, B ; that 

 whicli gives rise to seeing yellow, C ; that which produces seeing 

 green, D ; that which causes seeing blue, E ; and that which 

 produces seeing violet, F. 



It has been already stated, that when the sensation seeing red 

 can no longer be produced by the action of rays of light, the 

 sensation seeing green m^y still be produced, or it may arise spon- 

 taneously ; and vice versa. It follows then that the sensual state 

 of retina, A, is a state different from, or opposite to, the sensual 

 Btate, D. 



It appears then that if a retina be capable of assuming the 

 several states. A, B, C, D, E, F, the individual who pos- 

 sesses that retina may have the several sensations, seeing red, 

 seeing orange, seeing yellow, seeing green, seeing blue, seeing 

 violet, produced. If the retina be incapable of assuming any 

 sensual state, the individual possessing it cannot have vision 

 produced, he will be blind; but if a retina be incapable of 

 assuming the state. A, the possessor of it cannot have the sensa- 

 tion seeing red, he will be blind quoad that sensation, and so 

 with regard to the other sensual states. 



The several sensual states of the retina from which the varie- 

 ties of vision arise are commonly produced by the action of rays 

 of light ; but it has been shown that the sensations seeing red 

 and seeing green may arise in the absence of light. It follows 

 then that the presence of rays of light is not essential to the 

 production of sensual states of the retina, or, in other words, to 

 the presence of various kinds of vision. By whatever means 

 then a sensual state of the retina is produced, by such means 

 will vision be produced ; the particular kind of sensation pre- 

 sent will depend upon the particular state which is induced ; for 

 instance, by whatever means the state. A, is produced, by such 

 means seeing red will be produced, and so on. 



The sensations seeing red, seeing yellow, &c. are then only so 

 far dependent upon the action of hght, in as much as such action 

 produces a corresponding sensual state of the retina ; for seeing 

 a colour is a sensation dependent upon a pecuHar state of the 

 retina which is commonly produced by the action of rays of 

 light, but which may arise in the absence of those rays. 



