The Senses of Animals. 283 



These observations* need further confirmation and 

 extension. We intend to continue the investigation each 

 session. They are, however, sufficient to show that in 

 some individuals E. undergoes disruptive change on the 

 impact of light- waves which have no noticeable effect on 

 the retina of other individuals. 



It is impossible here to do more than just touch the 

 fringe of the difficult subject of colour-vision. And the 

 only further fact that can here be noticed is that trichro- 

 matic colour-vision is apparently in us limited to the 

 yellow-spot and its immediate neighbourhood. Around 

 this is an area which is said to be bichromatic — all of us 

 being, for this area, more or less green-blind. In the 

 peripheral area around this, colour is indistinguishable, 

 and we are only sensitive to light and shade. So far as 

 the structure of the retina is concerned, we may notice in 

 this connection that in the central region of most complete 

 trichromatic vision there are cones only ; around the 

 yellow spot each cone is surrounded by a circle of rods ; 

 and further out into the peripheral region by two, three, or 

 more circles of rods. 



Concerning the sense of sight in the lower mammals 

 little need be said. In many cases the acuteness of vision 

 is remarkable. Mr. Eomanes's experiments on Sally, the 

 bald-headed chimpanzee at Eegent's Park, led him to 

 conclude that she was colour-blind, but I question whether 

 the experiments described quite justify this conclusion. 

 Sir John Lubbock was unable to teach his intelligent dog 

 Van to distinguish between coloured cards ; but the failure 

 was as complete when the cards were marked respectively 

 with one, two, or three dark bands. We are not justified, 

 therefore, in ascribing the failure to colour-blindness. The 

 real failure, probably, was in each case to make the animal 

 understand what was wanted. Bulls are, at any rate, 



* The variations above indicated throw light on a fact to which Lord Ray- 

 leigh has directed attention. The yellow of the spectrum may be matched by a 

 blending of spectral red and spectral green ; but the proportions in which these 

 spectral colours must be mixed differ for different individuals. The comple- 

 mentary colours for different individuals are also not precisely the same. 



