THE SENSES 943 



tional weights lying at the corresponding points on the curve, 

 the point ' white ' will be the centre of gravity of the system.) 

 (3) The position of a colour produced by the mixture of any pair 

 of spectral colours is found by joining the corresponding points by 

 a straight line. The mixed colour lies on this line at distances 

 from the two points inversely proportional to the stimulation 

 intensity of the two colours i.e., it lies in the centre of gravity 

 of the weights representing the two colours. (4) It is a particular 

 case of (3) that the complementary colours are situated, at the points 

 where straight lines drawn through ' white ' intersect the curve, 

 since the point marked ' white ' is the centre of gravity corresponding 

 to a pair of colours only when it lies on the straight line joining them. 

 Thus the orange and yellow lying between the red and green are 

 mixtures of the red and green sensations in different proportions ; 

 the cyan-blue and indigo-blue are mixtures of the green and violet 

 sensations. The purples, represented by a broken line, are not 

 present in the spectrum, and are mixtures of red and violet. 



It is a point of great theoretical interest that on the Young- 

 Helmholtz theory the pure spectral colours, although physically 

 saturated (i.e., due to ethereal vibrations of a definite wave-length 

 for each colour), ought not to be physiologically saturated, since they 

 all affect the three components, although in different degrees. In 

 other words, the red, let us say, of the spectrum ought not to be the 

 purest or fullest red which it is possible to perceive. Now, it is 

 found that 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 ' component, being 

 fatigued before the eye is turned upon the red, the latter colour no 

 longer affects it, or affects it less than it would otherwise do, and 

 therefore the excitation is almost entirely confined to the red com- 

 ponent 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 observation 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 components. 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 Helmholtz supposed the portion of the retina on which the 



