68 PHENOMENA OF LIGHT AND COLOUR. 



merely surface phenomena, and that colour, such as the 

 green colour of a leaf or an emerald or the yellow colour of 

 an ingot of gold, is due to a selective action effected on 

 light which is composite. 



Another interesting question with which Aristotle deals 

 is that of the mixture of colours, but here again his state- 

 ments are sometimes unsatisfactory, mainly because it is not 

 clear whether he is dealing with the mixture of pigments or 

 of coloured lights. 



It has been stated that, according to Aristotle, light 

 results from the presence in the Diaphanous of something 

 of the nature of fire, and darkness ensues when this is 

 absent. In a similar way, he says, white and black are 

 produced, in solid bodies, i.e., they are respectively caused 

 by the presence or absence of something of the nature of 

 fire in the Diaphanous of those bodies.* He says that one 

 way of producing various colours is by mixing black and 

 white in various proportions, colours pleasing to the eye, 

 such as light red or dark blue, being produced when the 

 proportions of black to white are in simple ratio, just as in 

 harmonies (a-u/x(puviat) , and other colours, less pleasing to the 

 eye, when the proportions are not in simple ratio. The 

 black and white are supposed to be so arranged relatively to 

 each other that each is invisible because of the smallness of 

 its parts, but the colour of the resulting mixture is visible.! 



Aristotle seems to be referring to a mixture of coloured 

 lights, but his conclusions were probably based almost 

 entirely on a process of abstract reasoning. In his expla- 

 nation of the colours of the rainbow, discussed in Chapter iii., 

 he attempts to show that, when the visual rays are directed 

 to a distant bright object, this appears to be white, black, 

 or some colour intermediate between these, according to the 

 weakness or strength of the visual rays. This is both un- 

 satisfactory and difficult to understand, but in another part 

 of his explanation of rainbows there is a passage which 

 clearly refers to a mixture of coloured lights. He says that 

 an orange colour is seen between the light red and the 

 greenish yellow, such colour resulting from an overlapping 

 of the two colours mentioned. I It is true that an orange 

 colour results from a mixture of greenish yellow and light 

 red lights. 



* De Sensu, Sc, iii. 439&. f Ibid. iii. 4396 and 440«. 



I Meteorol. iii. c. 4, s. 26. 



