578 STUDIES IN ADVANCED PHYSIOLOGY. 



which only gradually fades and disappears. Here the entire 

 retina having been stimulated with red, everything later pic- 

 tured upon it is clothed with bluish tints the complementary 

 color of red. We have now to consider how these negative 

 after-images are accounted for by Young's theory and by 

 Hering's. 



EXPLANATION OF NEGATIVE AFTER-IMAGES. 



These negative after-images are explained by the Young- 

 Helmholtz theory as due to the fatigue of the retina. If 

 one looks intently for some time at a red light evidently the 

 red nerves of that part of the retina on which this image 

 falls will be stimulated, while the green and the blue nerves 

 are resting. If this stimulation continues it is evident that 

 these red nerves will become fatigued. There will be a 

 tendency for the red object to become paler and paler as this 

 effect proceeds. If, now, the eye be turned upon a white 

 wall the entire retina is flooded with white light. In that 

 part of the retina where the red image was the red nerves are 

 fatigued and so do not respond; at least not fully. The con- 

 sequence is that the green and blue nerves react alone and 

 so that area appears a greenish-blue or a bluish-green. As 

 the red nerves gradually recover from their fatigue their 

 stimulation is mixed with the blue and green, which of 

 course produces white. This explains very satisfactorily the 

 complementary color of the negative after-image and how it 

 is gradually transformed back into white. 



But not only may one nerve be thus fatigued; two or 

 even three may be so affected. If one, for instance, gazes 

 at the sun an instant the image of the sun falling on the 

 retina and strongly stimulating that portion, fatigues it. 

 When, now, the view is turned in another direction this por- 

 tion of the retina which the sun's image had fatigued fails 

 to be fully excited, and so there arises the sensation of 

 black, or, if the excitation is only a partial one, shades of 

 gray. 



