Sight, and the Eye. i4-> 



further modification on top of the yellow specialization, and still fol- 

 lowing the law of differentiation, we shall have the function of a yellow 

 seeing cone gradually split into two sub-functions, the two together 

 equal to yellow, the one above and the other below it, that is to say 

 green and red. The general law referred to is, that when one part is 

 specialized to the better performance of a function, the parts not in- 

 volved in such specialization and from which the one part is separated, 

 lose in their power to perform such function. That the eye is not 

 exempt from the law is shown in the fact that the retina, which is an 

 expansion of the optic nerve, is the only part sensitive to light while the 

 rest of the nerve is totally blind. 



As the rods and finally the cones are the sensitive parts of the retina 

 we cannot logicall} T exempt them from the operation of the law, and 

 must hold that whenever one of them is altered in its functional nature, 

 it is reduced to two parts whose functions are complementary of each 

 other, one above and the other below the original ; the elevation of one 

 compensated by the degradation of the other. Thus we derive the 

 green and red functions from the differentiation of some of the yellow, 

 and in case of reversion and failure of the derived functions, we may 

 retain in their place their original, the yellow. How many of the 

 color functions have been differentiated directly from the rods by 

 action of particular rays and how many have been specialized as sub- 

 divisions and sub-sub-divisions of the yellow, I shall not undertake to 

 determine. But it appears probable that there is at least one other 

 original center of specialization from the rods ; viz. , the blue. 



At any rate if we suppose two or three such original centers all the 

 other specializations could be derived from these by the repeated sub- 

 division of function. 



The number of sorts of cones we possess, according to this theory, is 

 limited to the number of spectrum colors we can see. Our sensations 

 of color are not however limited to these, because the sensations of these 

 original colors can be so compounded and mingled in the receiving 

 organs of the brain as to give the impression of an almost unlimited 

 number of shades and varieties. 



The number of cones in the yellow spot is said to be about 1,200,- 

 000, and the number outside of that is also vast. 



It should be observed that, other things equal, the power of perceiving 

 colors depends on the length of the cones ; the taller they are the more 

 acute the perception. So that where they are the most numerous they 

 are also the largest. They are longer in the yellow spot than in the 

 rest of the retina. 



" The impression produced by the rays of light on the retina seems 

 to be a kind of photogiaphic action, for Professor Kiihne has found that 



