162 COLOUR VISION 



of the terms for the nuances of colour, such as mauve, puce, and so on. 

 The latter are, however, quite unnecessary for colour testing, and we 

 may conclude that, with the exception of the monochromats, the great 

 majority of people know that the terms red, yellow, green, blue, violet, 

 and purple are applied to different colour sensations. Amongst the 

 uneducated, and even amongst those otherwise well educated but un- 

 practised in describing differences of colour, there are many individuals 

 in civilised races who from inaccuracy or carelessness apply generic 

 colour terms erroneously — thus, it is quite common for blue-greens to 

 be described as blue or green, violets and blue-purples as blue, deep 

 orange as red and so on. This is still more common amongst primitive 

 races, who, however, often have the excuse of an actual paucity of 

 colour terms in their vocabularies. It is important that defective 

 terminology shall not be interpreted as the expression of defective colour 

 perception without adequate confirmatory evidence. 



We must conclude then that we can have no accurate idea of the 

 actual physiological sensations or of their psychological counterparts 

 in the colour-blind. We soon discover, however, that he often mis- 

 names colours, and that if we compare the effects of two stimuli they 

 are often dissimilar to him when they match to us, and vice versa. 

 Thus, with many colour-blind we can find a certain intensity of mono- 

 chromatic blue-green light which exactly matches for him a certain 

 intensity of monochromatic yellow ; they are obviously quite dis- 

 similar to the normal. We shall therefore commence by investigating 

 the laws of colour mixture for the colour-blind. 



CHAPTER II 



DICHROMATIC VISION 



The Mixture of Pure-Colour Sfitnidi. We have seen that within 

 a certain range which includes all ordinary conditions of colour vision 

 and subject to certain limitations of stimuli before referred to, every 

 conceivable light or light mixture gives rise to a sensation which can be 

 accurately matched by the sensation produced by a suitable mixture of 

 only three lights (p. 38). The vision of dichromats can similarly be 

 expressed in terms of two Hghts. Thus, two monochromatic lights can 

 be found, such that, by mixing them in various proportions, the mixtures 

 will match every part of the spectrum as it appears to the dichromat, 



