568 STUDIES IN ADVANCED PHYSIOLOGY. 



tapetum. This luminosity of the eyes is most readily ob- 

 servable if such an animal be placed in a darkened room 

 and light admitted through a door in which the observer 

 himself stands. In that case it is frequently possible to see 

 the two eyes luminous enough to shine in the dark, the ex- 

 planation being, as just pointed out, the light which is 

 actually reflected out of the eye by the shining tapetum 

 within. 



THE PHYSIOLOGY OF COLOR SENSATION. 



Up to this point the eye has been treated almost exclu- 

 sively as a purely physical instrument, unless we except the 

 nerves and muscles by means of which it accommodates 

 itself. But all these physical arrangements in the eye are 

 intended to make possible the physiological perception of 

 color in the retina. It has been pointed out how the image 

 is spread upon the retina, and there arises now the question 

 how this physical image is translated into physiological 

 phenomena. In other words, how do we see colors? 



It is well to note that it is color and color alone that we 

 observe with the eyes. Distance, solidity, form, size, etc. , are 

 all inferences which we draw from the arrangement of colors 

 before us. The eye sees color or absence of color, and ar- 

 rangement of colors, and nothing more. 



The physiology of vision pure and simple, therefore, 

 narrows itself down to the perception of these colors. Un- 

 fortunately we are yet far removed from a complete under- 

 standing of this question, and the theories that have been 

 advanced have left so much still unexplained that they have 

 scarcely the dignity of scientific theories. They are, how- 

 ever, the best attempts, in the light of what we now know, 

 towards the explanation of these phenomena. 



Two fairly distinct theories are proposed. These are 

 the Young-Helmholtz theory, as suggested by Young early 

 in this century, and later on materially modified by Helm- 

 holtz. It seems to offer the most plausible explanation and 

 is the one probably most generally accepted. The other 

 theory is that of Ewald Hering, which explains colors upon 



