A CHAPTER ON LIGHT AND COLOUR. 105 



pigments of certain colours are, however, entirely different means 

 of producing colour. 



In the former case a compound sensation is produced indistin- 

 guishable from some other simple sensation. The latter effect is 

 due to absorption. Certain substances possess the property, either 

 when in solution, or when used to tint glass, of stopping a part of 

 the light which falls on them, while leaving the greater part of it 

 unaffected. Their colour, as seen by transmitted light, is the 

 effect of the light not absorbed by them. 



If two such absorbent bodies act successively on white light, 

 they may abstract nearly all the colours from that light — in fact, 

 they may even abstract all, in certain cases. If a yellow and a blue 

 be used, the one colour not absorbed largely by either is green, 

 and hence the combination of yellow and blue pigments produces 

 a green pigment, while the combination of a pure yellow and a 

 pure blue light may produce a pinkish colour. 



9. — The colours of the spectrum are, both as to their nature 

 and as to order, those shewn by the Rainbow. When sunlight 

 falls on the drops of water forming a cloud, it is partly refracted 

 into those drops. If it emerge after reflection at the back of the 

 drop, there is one particular direction in which the light is greatly 

 condensed. This direction varies slightly for different colours, and 

 hence the drops which send light to the eye in this direction give 

 the appearance of a coloured band, red on the upper side, violet 

 on the lower. Outside the bow there is complete absence of light 

 reflected from the drops. 



After two or more internal reflections the light may emerge m 

 sufficient strength to produce other bows. The secondary bow is 

 frequently seen. It is larger than the primary, and the colours 

 are in the reverse order. The tertiary and quaternary bows are on 

 the same side of the observer as the sun and are drowned in his 

 light. The bows of higher orders are too faint to be visible. 

 • Curious eff*ects of Halos, Parhelia, etc., are produced when 

 ice-crystals take the place of raindrops in the cloud. 



ic— The perfecting of the means of obtaining a sj^cctrum, by 

 using a slit as source of light, as Wollaston did in 1802, revealed 

 the fact that the spectrum was not continuous, but crossed by dark 

 bands, indicating a deficiency of certain kinds of light. Fraun- 



