The Sense of Sight: Color Vision 177 



it probably discriminates the colors of the red end of the 

 spectrum from those of other regions by difference in the 

 stimulating value of light of different wave lengths, that 

 such specific stimulating value is radically different in nature 

 from the value of different wave lengths for the human eye, 

 and that the red of the spectrum has a very low stimulating 

 value for the dancer. In the light of these experiments we 

 may safely conclude that many, if not most, of the tests of 

 color vision in animals which have been made heretofore by 

 other investigators have failed to touch the real problem 

 because the possibility of brightness discrimination was not 

 excluded. 



Under the direction of Professor G. H. Parker, Doctor 

 Karl Waugh has examined the structure of the retina of the 

 dancing mouse for me, with the result that only a single 

 type of retinal element was discovered. Apparently the 

 animals possess rod-like cells, but nothing closely similar to 

 the cones of the typical mammalian retina. This is of 

 peculiar interest and importance in connection with the re- 

 sults which I have reported in the foregoing pages, because 

 the rods are supposed to have to do with brightness or 

 luminosity vision and the cones with color vision. In fact, 

 it is usually supposed that the absence of cones in the mam- 

 malian retina indicates the lack of color vision. That this 

 inference of functional facts from structural conditions is 

 correct I am by no means certain, but at any rate all of the 

 experiments which I have made to determine the visual 

 ability of the dancer go to show that color vision, if it exists 

 at all, is extremely poor. It is gratifying indeed to learn, 

 after such a study of behavior as has just been described, 

 that the structural conditions, so far as we are able to judge 

 at present, justify the conclusions which have been drawn. 



