826 A MANUAL OF PHYSIOLOGY 



this is really the case. If, for example, we look first at the bluish- 

 green and then at the red of the spectrum, the sensation of red is 

 fuller or more saturated than if we had looked at the red directly. 

 Similarly, if we look first at a small bluish-green square on a black 

 ground, and then at a red ground, we see a more fully saturated 

 square in the middle of the latter. The explanation, on the Young- 

 Helmholtz theory, is that the ' green ' fibres being tired before the 

 eye is turned upon the red, the latter colour no longer affects them, 

 or affects them less than it would otherwise do, and therefore the 

 excitation is almost entirely confined to the red fibres in the area 

 fatigued for green. This brings us to the subject of retinal fatigue, 

 and the related phenomena of after-images and contrast. 



After-Images. We have seen that the retinal excitation 

 always takes time to die away after the stimulus is removed. 

 If a white object is looked at, especially when the eye is 

 fresh, for a time not long enough to cause fatigue, and the 

 eye is then closed, an image of the object remains for a short 

 time, diminishing in brightness at first rapidly, then more 

 slowly. This is a positive after-image, and by careful ob- 

 servation it may, under certain conditions, be seen that the 

 positive after-image of a white object, of a slit illuminated 

 by sunlight, for example, undergoes changes of colour as it 

 fades, passing through greenish-blue, indigo, violet, or rose, 

 to dirty orange. On the Young-Helmholtz theory this is 

 explained by the supposition that the excitation does not 

 decline with the same rapidity in the three hypothetical fibre 

 groups. If the object is looked at for a longer time, or if 

 the eye is fatigued, a dark or negative image may be seen 

 upon the faintly-illuminated ground of the closed eyes ; but 

 negative after-images may be more easily obtained when the 

 eye, after being made to fix a small white object on a black 

 ground, is suddenly turned upon a white or neutral tint 

 surface. 



Here the portion of the retina on which the image of the object 

 is formed may be assumed to be more or less fatigued. And this 

 fatigue will extend to all three kinds of fibres ; so that white light of 

 a given intensity will now cause less excitation in this part than in 

 the rest of the retina. It is easy to understand that the negative 

 after-image of a coloured object will be seen, upon a white ground, 

 in the complementary colour, for the fibres chiefly excited by the 

 latter will have been least fatigued. The negative after-images seen 

 when the eye, after receiving the positive impression, is turned upon 

 a coloured ground, vary with the colour of the object and ground in 



