580 STUDIES IN ADVANCED PHYSIOLOGY. 



ation of this substance, and so a sensation of black. It is 

 unnecessary to carry this further with the yellow-blue sub- 

 stance. 



If two substances were simultaneously being exhausted 

 then there would be induced the regeneration of both, and 

 in this way there would arise in the eye various intermedi- 

 ate colors corresponding to the intermediate colors first 

 looked at. It is apparent that the negative after-image 

 will be not far from the complementary color of the object 

 looked at. 



It will thus be seen that both theories account more or 

 less satisfactorily for many of the phenomena of color sen- 

 sation, and each reader will have to decide for himself the 

 relative probability of these two views. A point, very much 

 in favor apparently of the Young-Helmholtz theory lies in 

 the fact that owls and bats, which from their habits would 

 naturally be suspected of being monochromatics, possess 

 rods only, the cones being entirely absent. A second 

 point in its favor is the observation that we are not able to 

 see red towards the periphery of the retina. For instance, 

 if one take a red object, such as a stick of red sealing-wax, 

 and move it from behind the head forwards it does not ap- 

 pear red at first, and must be moved some distance .further 

 even after its outline is recognizable, when suddenly the 

 sensation red flashes into consciousness. Blue and green, 

 however, are more readily recognized at the periphery. 

 Now, in terms of the Hering theory it would be impossible 

 to see green where one cannot see red, since one substance 

 is the cause of both. The explanation that suggests itself 

 is that green and blue nerves occur here only, the red 

 ones being absent. 



DOUBLE VISION. 



So far everything that has been said would apply if we 

 had but one eye, but, as a matter of fact, we use two eyes 

 in looking at objects and the question naturally arises, why 

 vision with two eyes should result in an apparently simple 



