COMMON THINGS COLOUR. 



be produced by varying the proportion of green, yellow, and 

 orange ; and so on. 



24. If two tints T and T' be produced, the former T by com- 

 bining a certain number of prismatic colours, and the latter T' by 

 combining the remainder together, these two tints T and T' are 

 called complementary, because each of these contains just those 

 colours which the other wants to produce complete whiteness ; and, 

 consequently, if the two be mixed together, whiteness will be the 

 result. Thus, a colour produced by the combination of the red, 

 orange, yellow, and green of the spectrum in their just proportions, 

 will be complementary to another colour produced by the blue, 

 indigo, and violet in their just proportions, and these two colours, 

 if mixed together, would produce whiteness. 



25. Almost all colours, natural or artificial, except those of the 

 prismatic spectrum itself, are more or less compounded, and their 

 combined character belongs to them equally when they have tints 

 identical with the coloured spaces of the spectrum. Thus, a 

 natural object whose colour is indistinguishable from the yellow 

 space of the spectrum, will be found, when subjected to the action 

 of the prism, to refract light in which there is more or less of 

 green or orange ; and an object which appears blue will be found 

 to have in its colour more or less of green and violet. 



26. Instead of receiving the spectrum on a screen, it may be 

 viewed directly by placing the eye behind the prism ABC, fig. 13 



Fig. 13. 



at L, so as to receive the light as it emerges. This mode of 

 observing the prismatic effects is in many cases more convenient 

 than by means of the screen, colours being thus rendered observ- 

 able which would be too feeble to be visible after reflection from 

 the surface of the screen. It is necessary, however, to consider 

 that in this manner of viewing the prismatic phenomena, the 

 colours will be seen in an order the reverse of that which they 

 would hold on the screen ; foi if the eye be placed at L, it will 

 74 



