THE EYE AND THE PHYSIOLOGY OK VISION. 591 



during the night and becomes sensitive when the shadow is 

 suddenly projected against it. 



From this long chapter on the eye it will be noted that 

 while our knowledge is very satisfactory in certain direc- 

 tions there is much that needs further study and investiga- 

 tion. We understand the eye fairly well as a physical m r 

 strument, but the gap in our knowledge is in the physi- 

 ology of the retina. Unfortunately it must be granted that 

 neither the Young-Helmholtz theory nor the Hering theory 

 explain satisfactorily all the phenomena. In conclusion 

 there may be mentioned some observations made upon the 

 eyes of birds which may lead in the. future to important 

 results. 



There occur in the rods and cones of the retina of birds 

 small, fatty globules, some of which are red, others yellow, 

 and still others colorless. These globules are so situated 

 that the light must pass through them before reaching the 

 sensitive endings of the rods and cones in question. One 

 can hardly resist the suggestion that -these colored globules 

 of fat determine to some extent, if not wholly, the sensation 

 which the rod in which it is imbedded shall give. It is 

 evident that the red globule, for instance, will absorb all 

 the other colors and permit only the red to pass through, 

 and so stimulate the end of the rod or cone. Similarly the 

 yellow globules will permit only yellow light to pass through. 

 One feels tempted to believe that there may be globules 

 answering to the entire scale of simple colors and thus the 

 retina become a veritable color sounding-box, its physiology 

 somewhat analagous to the basilar membrane of the ear, 

 with its strings attuned to the various vibrations. The 

 many individual rods and cones might each be provided 

 with some contained colored globule to permit only that 

 light in question to pass through it and affect the retina. 

 In fact, it would be as if each rod and cone had a little 

 colored plate of glass spread before it which permitted only 

 the light of that color to penetrate it, and consequently to 

 stimulate it to a sensation. May it not be possible that in 



