342 PHYSIOLOGICAL PHYSICS, tchap. xxvi. 



said to be COMPLEMENTARY to one another, white light 

 being the result of their union. It is evident, also, 

 that, suppose red and violet were taken, to produce 

 white, greenish-blue and yellowish green are all that 

 are necessary ; but, as the table shows, green is the 

 result of a mixture of greenish-blue and yellowish- 

 green, so that a mixture of red, green, and violet 

 would produce white. This the colour top or rotating 

 disc shows to be the case. If the three colours are 

 arranged on the disc, in proper proportions, and 

 rapidly rotated, the eye has an impression of white. 

 Further, by varying the proportions of the three colours 

 on the disc all the shades of the spectrum may be 

 produced, not with the brilliancy of the spectrum, 

 because of the admixture of white or the less degree 

 of saturation, as it is phrased. These three colours, 

 red, green, and violet, are for these reasons called 



FUNDAMENTAL Colours. 



Primary colours they are also called ; and, as we 

 have seen, if mixed in various proportions they pro- 

 duce various other colours. Thus the spectral red 

 and green produce yellow, the resultant colour being 

 called a secondary colour. The following table shows 

 similar combinations, the arrows indicating the two 

 colours combined and pointing to the result : 



Primary. Secondary. 



Red-".;-.; -;.::;:.-"->Yellow. 

 G reen-c.-:-.."""' """"""^P urple. 



It is not to be supposed, however, that the three 

 colours exist as such in the spectrum, and that the 

 blending of them in different proportions produces the 

 gradations of colour. Colour is only a subjective 

 thing. The colours of the spectrum are due to vibra- 

 tions of varying rates of rapidity, of different wave 

 lengths, and when these vibrations affect the eye they 



