CH. LVI.] COLOUR SENSATIONS 797 



i e., the amount of light per unit area of surface. These differences 

 are dependent respectively on the length, the purity, and the ampli- 

 tude of the light-wave. Colours also differ (4) in brightness or 

 luminosity, this is a purely physiological quality devoid of any 

 known physical counterpart. The brightness of a colour may be 

 measured by determining the shade of grey to which it appears 

 equivalent. Even the most saturated colours (for instance, yellow 

 and blue) have different degrees of brightness varying with change 

 of illumination (see also p. 803). 



If a ray of sunlight is allowed to pass through a prism, it is 

 decomposed by its passage into rays of different colours, which are 

 called the colours of the spectrum; they are red, orange, yellow, 

 green, blue, indigo, and violet. The red rays are the least turned out 

 of their course by the prism, and the violet the most, whilst the other 

 colours occupy in order places between these two extremes. The 

 differences in the colour of the rays depend upon the rapidity of 

 vibrations producing each, the red rays being the least rapid and the 

 violet the most. In addition to these, there are others which are 

 invisible but which have definite properties ; those to the left of the 

 red are less refrangible, being the calorific rays which act upon the 

 thermometer, and those to the right of the violet, which are called 

 the actinic or chemical rays, have a powerful chemical action. 



White light may be built from its constituents in several ways, 

 for instance, by a second prism reversing the dispersion produced by 

 the first, or by causing the colours of the spectrum to fall on the 

 retina in rapid succession. The best way to study the effects of 

 mixing colour sensations is by means of a rapidly revolving disc 

 to which two or more coloured sectors are fixed. Each colour is 

 viewed in rapid succession, and owing to the persistence of retinal 

 impressions, the constituent colour impressions blend and give a 

 single sensation of colour. (Maxwell.) 



White light can be produced by the mixture of the three primary 

 colours, or even of two colours in certain hues and proportions. 

 These pairs of colours, of which red and greenish-blue, orange and 

 blue, and violet and yellow are examples, are called complementary. 



The colours are not of equal stimulation energy, otherwise they 

 might be arranged around a circle ; they are more properly arranged 

 in a triangle, with red, green, and violet at the angles (fig. 593). 

 The red, green, and violet are selected on the theory of Helmholtz 

 that they constitute the three primary colour sensations; other 

 colour sensations are mixtures of these. 



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

 mixtures of the red and green sensations; the blue a mixture of 

 green and violet ; and the purples (which are not represented in the 

 spectrum) of red and violet. 



