VISION . 1015 



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 propor- 

 tional 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 corre- 

 sponding 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 propor- 

 tions; 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-Helm- 

 holtz 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 other- 

 wise do, and therefore the excitation is almost entirely confined to the 

 red component 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 observa- 

 tion 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 

 image of the object is formed 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 



