608 



Cajytain W. de W. Ahney 



[May 6, 



reason is that the green I Lave selected for mixture is in the part 

 of the spectrum where great absorption takes place, whilst the com- 

 l^arison white contains the green of the whole spectrum, some parts 

 of which are much less absorbed than others. I may remark that 

 just outside the yellow spot the eye is less sensitive to the red than 

 is the centre, and this is one additional cause of the difference. See 



More on this subject I have not time to say on this occasion, but 

 it will be seen that the extinction of light for the centre and the 

 outside of the eye differs on account of this. 



I must take you to a theory of colour vision which, though it may 

 not be explanatory of everything, at all events explains most pheno- 

 mena—that is, the Young-Helmholtz theory. The idea embodied 



Fir. -. 



in it is that we have three sensatiotis stimulated in the eye, and that 

 these three sensations give an impression of a red, a green, and a 

 violet. These three colours 1 have said can be mixed to match any 

 other colour, or, in other words, the three sensations are excited in 

 different degrees, in order to produce the sensation of the inter- 

 mediate spectrum colours, and those of nature as well. 



The diagram Fig. 4 shows the three sensations as derived from 

 colour equations made by Koenig. It will be seen that there are three 

 complete colour sensations, all of which are present in the normal 



