BINOCULAR VISION 699 



tion that there are chemical substances in the retina, photogenic substances, 

 which are stimulated by the colors of the spectrum. It assumes three photo- 

 genic substances which are called the red-green, the yellow-blue, and the 

 white-black substances. By the theory, when the red-green substance is 

 stimulated by red or green light, respectively, the former produces destruc- 

 tive or katabolic changes, the latter constructive or anabolic changes, in the 

 substance. When red light falls upon the retina, it produces katabolism in 

 the red-green substance, which in turn develops a nerve impulse that arouses 

 the sensation of red. When green light, on the other hand, stimulates the 

 retina, it produces anabolism of the red-green substance and the sensation of 

 green. The same rule holds with the other two substances. It will be 

 noticed that this theory is based on the complemental colors. 



When we apply the theories mentioned above to the phenomena of color- 

 contrast and color-blindness, we find that each is defective in some point. 

 By the Young-Helmholtz theory it is difficult to understand the perception 

 of the sensation black, for by the theory black could be perceived only as 

 the absence of all colors, and it is generally granted that there is a distinct 

 black sensation other than and different from mere darkness. This theory 

 explains more satisfactorily those cases of blindness to one color, as red- 

 blindness, for example. The Hering theory, on the other hand, gives us a 

 rational explanation for positive black sensation, and is particularly appli- 

 cable to the observed facts of color- contrast and negative color after-images. 



Color after-images, as for instance the after-images of green following 

 stimulation by red light, are readily explained by Bering's theory, since the 

 strong katabolism in the red-green substance will be followed immediately 

 by anabolism to bring this substance up to its normal in the eye, thus pro- 

 ducing the after-image. This phenomenon can be explained by the Young- 

 Helmholtz theory only by assuming that following the stimulation by red 

 light and the consequent fatigue of red-perceiving elements there is sufficient 

 light entering the eye to stimulate the relatively sensitive green and violet 

 perceiving elements, thus producing an after-image. Strong after-images 

 are perceived in the dark room, so that the Hering theory is most applicable 

 in the explanation of these cases. 



Binocular Vision. When one looks at an object with a single eye, 

 the eye is so adjusted that the axis of vision is directed toward the object 

 investigated. This is called ocular fixation. The ocular fixation is accom- 

 plished by the co-ordinated contractions of the six ocular muscles. Its 

 purpose is to bring the image of the object examined in the external visual 

 field as nearly as possible upon the macula lutea. In binocular vision both 

 eyes are fixed on the same point in the visual field. A projection of the 

 visual axis of each eye will pierce the point of fixation in the external object. 

 It is evident that objects to either side of the point of fixation will give off 

 rays which will enter the eyes, stimulating fields in the retina on the opposite 



