THE EYE AND THE PHYSIOLOGY OF VISION. 579 



This theory also explains why looking from one color to 

 a complementary one seems to increase the brightness of 

 the second. An animal grazing in the field and seeing prac- 

 tically nothing all day except the green grass and the green 

 foliage, has a much more vivid impression of a red object 

 when this is suddenly presented, than it would otherwise 

 have, the explanation being that the red nerves have been 

 resting all day, and then suddenly stimulated they react 

 with great energy. The offense which certain animals seem 

 to take at such bright objects flared at them may be to a 

 slight extent at least accounted for in this way. Every 

 stroller through the woods must have been struck with the 

 intense redness of a summer redbird or a cardinal as it 

 darted in among the green leaves. 



In terms of the Hering theory negative after-images are 

 explained as follows: 



Taking, for instance, the supposed red-green substance 

 of the eye which when it is being destroyed produces red 

 sensations, when it is being regenerated produces green, it 

 seems entirely possible that the excessive destruction of it 

 might induce its regeneration. Just like an active destruc- 

 tion of muscle tissue is the occasion for a corresponding 

 active regeneration of it. An increased use of any product 

 of the body very naturally brings about an increased pro- 

 duction. 



Turning now to the first illustration, it is supposed that 

 while the eye is looking intently at a red object this red- 

 green substance is in that part of the retina where the image 

 falls being used up, and if the look continues, is being ex- 

 hausted rapidly. If, now, the eye is turned away there is 

 naturally induced an attempt of the eye to produce some 

 new substance of this kind, and this regeneration acting as 

 a stimulus gives us the sensation of green. So with the 

 other two substances. If the white-black substance had at 

 some point of the retina been almost exhausted by staring 

 an instant at the sun, say, there would occur at that point 

 in the retina as soon as the eye was turned away a regener- 



