576 PHYSIOLOGY 



is a mixture of red and yellow, and, as a matter of fact, we find that on 

 mixing rays from the red with others from the yellow part of the spectrum 

 we do obtain a sensation of orange. The stimulus obtained by mixing the 

 red and yellow rays is not the same as a stimulus caused by rays from the 

 orange part of the spectrum. In the former case compound waves made up 

 of the two wave-lengths, 656 JULJUL and 564 JUL/UL, are falling on the retina, in the 

 latter case a simple wave with a length of 608 /ULJU., and yet the sensations 

 produced are identical. Experience shows that there are relations between 

 the physiological effects produced by different parts of the spectrum which 

 have no physical analogue in the stimuli themselves. Such, for instance, are 

 the phenomena known as the coloured after-image. We have seen that, as 

 the result of fatigue, stimulation of any part of the retina by a bright object 

 produces, when the stimulation is removed, a dark after-image, which has 

 its seat in the previously stimulated portion of the retina. If we look stead- 

 fastly for a minute in a bright light at a red disc on a white ground and then 

 look away at a uniform white surface, we see an after-image of the disc on the 

 surface. This after-image is, however, green, and the white background takes 

 on a reddish tinge. If the disc in the first instance has been a greenish blue 

 the after-image is red, and to every colour in the spectrum we find there 

 corresponds another which represents the after-image evoked by stimulation 

 with the first colour. If the first disc has been green the after-image will be 

 purple, and vice versa. We can therefore arrange the spectrum into a series 

 of pairs of colours which are known as complementary. The following is a 

 list of such pairs, with wave-lengths of the rays involved, as determined by 

 Helmholtz : 



Colours Wave-lengths 

 Red greenish blue .... 656 492 



Orange blue 608 490 



Bright yellow blue .... 574 482 



Yellow indigo 567 465 



Greenish yellow violet . . . . 564 433 



If the retina be stimulated by any of these pairs of colours simultaneously 

 the effect is not that of colour, but of white. White light, or the sensation 

 of white, can thus be due either to simultaneous stimulation of the retina by 

 all the rays of the spectrum, or to stimulation of the retina by pairs of colours 

 which are known as complementary. If these pairs be taken rather nearer 

 in the spectrum a colour is obtained representing a part of the spectrum 

 situated between the two constituents of the pair. If the rays are further 

 away than would correspond to the complementary colours we obtain again 

 a coloured sensation, which, however, is unsaturated, being mixed with a 

 certain amount of white light. By taking three colours, such as red, green, 

 and violet, or four colours, such as red, yellow, green, and violet, it is possible 

 by mixing them in various proportions to form either white light or any colour 

 of the spectrum, besides the various purples which do not occur in the spec- 

 trum. 



These experiments of mixing colours can be carried out in various ways : 



In every case we aim at stimulating the retina simultaneously with rays of different 



