CHAP, in.] SIGHT. 89 



to mean highly saturated, slightly mixed with white. The word 

 'tint' might be used to express various degrees of saturation, 

 the word 'hue' being reserved to denote the dominant wave- 

 length. ' Tone ' is frequently employed to express variations of 

 wave-length within a named colour, as for instance different 

 tones of red. The word ' bright ' is often used somewhat loosely, 

 but it is desirable to employ it exclusively as identical with 

 'luminous,' that is to say, as indicating the intensity of the 

 sensation ; a colour is more or less bright according to the amount 

 of luminous energy which is being expended on the retina. We 

 may remark, in passing, that while we can easily compare the 

 brightness or luminosity of two white lights or of the same part 

 of the spectrum under a feeble and under a strong illumination, 

 we may feel some difficulty in comparing the amount of brightness 

 of one colour with that of another, the brightness for instance of a 

 given yellow with that of a, given red. Conversely the word 

 ' dark ' is used to denote feeble intensity, or admixture with black. 

 Lastly, our appreciation of the colours of external objects is 

 modified by the nature of the surface which is coloured, and 

 features so arising receive various names ; but these are in reality 

 outside actual colour sensations. 



759. Admitting that our colour sensations may be con- 

 sidered to be much fewer in number than those which we appear 

 to have when we look on the colours of the spectrum or of 

 nature, admitting that rays of light awake in us certain " primary " 

 colour sensations, which mixed in various proportions reproduce 

 all our colour sensations, we have now to ask the question, What 

 is the nature or what are the characters of these primary colour 

 sensations ? 



In view of the answer to this question we must call attention 

 to certain results which may be obtained by a further study of 

 the mixing of colours, meaning by that the mixing of colour 

 sensations. 



We have seen that all the colours of the spectrum mixed 

 together make white. We have now to add that white may also 

 be produced by mixing two colours only, provided that these are 

 properly chosen. If we take a part of the red of the spectrum, 

 and by any of the methods given in 756, mix it with successive 

 parts of the spectrum, we shall find that the mixture with a par- 

 ticular part of the green or blue green gives white. These two 

 colours are said to be complementary to each other. In order 

 to get a complete white, that is a white free from all colour, 

 a certain proportion between the relative amounts of red 

 and green light, that is to say between the intensities of the two 

 sensations, must be observed. And it will be understood that the 

 white thus produced by two small parts of the spectrum is not 

 equal in intensity to the white which would be produced by the 

 combined effect of the whole of the same spectrum. The following 



