624 A TEXTBOOK OF PHYSIOLOGY 



a sensation of white. It has been shown that various pairs of colours, 

 when mixed, will give the sensation of " white." The mixing can 

 be done by placing the colours upon a wheel or top which is quickly 

 rotated (Fig. 354). It is better, however, to superimpose on a white 

 surface the different colours from two spectra. The chief pairs of 

 colours which give white are red-green, blue-yellow. Such colours 

 are termed " complementary." If in a good light one of these 

 colours in the form of a disc or letter be viewed steadily for a time 

 on a white surface, and the gaze then turned to another white 

 surface, the disc or letter will appear after a time in the comple- 

 mentary colour. This is known as the negative after-image. After 

 beholding a red letter, a green letter will appear as the after-image, 

 and so on. 



The different colours of the spectrum vary in luminosity. Nor- 

 mally, the yellow is the brightest part of a spectrum. But the lumin- 

 osity of a colour may vary. Thus, any of the colours of one spectrum 

 may be made of equal luminosity with the yellow of another spectrum 

 by increasing the intensity of the white light used to form the spectrum. 

 With feeble light, the maximum luminosity shifts to the green, and 

 the colours of the red end of the spectrum become less easily perceived 

 than those of the blue end. This accounts for the order of the changing 

 colours of a sunset or the change of colours in a flower garden as 

 twilight passes into night. 



Saturation. Besides luminosity, a colour possesses a degree of 

 saturation. By this is meant the extent to which it is mixed with 

 white light. A fully saturated colour is entirely free from white 

 light, such as the sodium flame in a dark room. 



When colours are mixed which are not complementary, we get 

 *' shades." As many as 160 shades have been observed in the 

 spectrum ; some shades, such as purple, are not present in the 

 spectrum. When two colours are mixed which are nearer in the 

 spectrum than the complementary colours, a colour is obtained of 

 the part of the spectrum between the two. If the colours are 

 farther apart in the spectrum than the complementary colours, then 

 a colour mixed with white light is obtained an unsaturated colour. 

 By taking the three colours red, green, violet, or, according to 

 other authorities, four colours red, yellow, green, blue all the 

 colours of the spectrum may be obtained by mixing in varying 

 proportions. These are known as the fundamental colours. 



In the mixing of pigments, the nature of the pigment substance 

 has to be taken into account. A blue and a yellow pigment, when 

 mixed, give green, not white. This is because the blue pigment 

 absorbs the red and yellow rays from white light, and reflects the blue 

 and green rays. The yellow pigment reflects red, yellow, and green, 

 and retains the blue. When the pigments are mixed, the green 

 rays are the only ones not absorbed. By examination of the light 

 reflected from any pigment with the pocket spectroscope, it can be 

 jseen what rays are reflected and what are absorbed. 



