COLOUR PERCEPTION IN FISH 21 



I have prepared some sections of the retina 

 of the trout, and examined them carefully 

 in comparison with the retinae of several 

 other fish. A short account of what is 

 known at present of colour-vision is, I 

 think, advisable to make my meaning clear 

 to those of my readers who may not be 

 sufficiently well versed in this particular 

 subject. 



The sensation of an individual colour is 

 produced by rays of light of a particular 

 wave-length falling upon the retina. A 

 sensation of " white " is produced by rays 

 containing all the wave-lengths which are 

 able to affect it. When, on looking at an 

 object, we find that neither a colour nor 

 white sensation is produced, this sensation 

 is called " black." 



The white sensation may be mixed with 

 the sensation of any colour of the spectrum, 

 as also may the sensation of black, and 

 when these two are mixed they produce a 

 sensation of " grey." Some colours of the 

 spectrum are probably produced by a mix- 

 ture of various wave-lengths of different 

 primary colours, and many colours in nature 

 do not exist in the spectrum. 



