776 



THE EYE AND VISION. 



[on. LV. 



Colour Sensations. 



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 the coloured 

 rays of the spectrum, 

 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 ther- 

 mometer, 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 either physi- 

 cally, as by a second prism reversing the dispersion produced by 

 the first, or physiologically 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 two or more constituent colour impres- 

 sions blend and give a single compound colour. (Maxwell.) 



White light can be produced by the mixture of the three 

 primary colours, or even of two colours in certain proportions. 

 These pairs of colours, which are roughly red and greenish blue, 

 orange and blue, and violet and yellow, 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. 591). 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. 



Fig. 591. Colour triangle. 



