AN'O MODKKX PHYSICS. 97 



mathematically the theory of three elements of colour ; 

 but the disposition of the colours on the circle was 

 only a provisional arrangement; the true relations 

 of the colours could only be determined by direct 

 experiment 



Thomas Young 1 appears to have been the next, after 

 Newton, to work at the theory of colour sensation. He 

 made observations by spinning coloured discs much 

 in the same way as that which was afterwards adopted 

 by Maxwell, and ho developed the theory that three 

 different primary sensations may be excited in the eye 

 by light, while the colour of any beam depends on 

 the proportions in which these three sensations are 

 excited. He supposes the three primary sensations to 

 correspond to red, green, and violet. A blue ray is 

 capable of exciting both the green and the violet ; a 

 yellow ray excites the red and the green. Any colour, 

 according to Young's theory, may be matched by a 

 mixture of these three primary colours taken in proper 

 proportion ; the quality of the colour depends on 

 the proportion of the intensities of the compon- 

 ents; its brightness depends on the sum of these 

 intensities. 



Maxwell's experiments were undertaken with the 

 object of proving or disproving the physical part of 

 Young's theory. He does not consider the question 

 whether there are three distinct sensations corre- 

 sponding to the three primary colours; that is a 

 physiological inquiry, and one to which no completely 

 satisfactory answer has yet been given. He does show 

 that by a proper mixture of any three arbitrarily 

 chosen standard colours it is possible to match any 



