THE SENSES 



^hen 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 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 applicable 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 catabolism 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 coordinated contractions of the six pairs of 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 pofnt 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 

 side of the visual axis. An examination of figure 476 will show that each 

 point in the visual field, A, B, C, D, stimulates corresponding points, a, b, c, d, 

 a', b', c', d', in the retinas of the two eyes, a, b, c, d, and a', V, c', d', are corre- 

 sponding points in the two retinas. When a and a' are stimulated at one 

 and the same time, the resulting sensation is attributed to one object in the 

 visual field, A, and these are corresponding points. This can be shown 

 by pressing one eye out of its normal fixation so that the axes of the two eyes 

 are not directed toward the same point. If one eye is pressed lightly by the 

 thumb while examining a given object, as soon as, the pressure is applied 

 two objects will appear. This phenomenon is known as diplopia. Diplopia 

 is due to the fact that the images of visual objects do not fall on correspond- 

 ing points in the two retinae. 



