896 COMPLEMENTARY COLOURS. [BOOK in. 



which obviously may exist under various combinations. On 

 the one hand we frequently use wholly unlike names for colours 

 which differ only in degree of saturation, such as carmine and 

 pink ; on the other hand we often use the same adjectives for 

 quite different conditions. It is desirable to employ the word 

 4 pale,' to mean little saturated, largely mixed with white, and 

 the word 4 deep ' or 4 rich ' to mean highly saturated, slightly 

 mixed with white. The word 4 tint ' might be used to express 

 various degrees of saturation, the word 4 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 4 bright ' 

 is often used somewhat loosely, but it is desirable to employ it 

 exclusively as identical with 4 luminous,' that is to say, as indi- 

 cating 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 diffi- 

 culty 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 4 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. 



562. Admitting that our colour sensations may be consid- 

 ered 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 " pri- 

 mary " 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 559, mix it 

 with successive parts of the spectrum, we shall find that the 

 mixture with a particular part of the green or blue green gives 

 white. These two colours are said to be complementary to each 



