544 COMPLEMENTARY COLOURS. 



prism, their colours will always be found to be com- 

 pounds, that is, a sum of various colours. It follows 

 from this that if one colour represents a compound of 

 simple colours which forms one part of the complete 

 spectrum, while another colour is a compound contain- 

 ing the remaining part of the spectrum, these two 

 colours will together produce white light. It is there- 

 fore not requisite to blend all colours in order to pro- 

 duce White. Certain mixtures of only two colours will 

 give white light, and any pairs of colours which com- 

 bined produce white light are called complementary 

 colours. To find the colour complementary to any 

 given colour is not always easy, but it is more easy to 

 find the complementary colour for any of those repre- 

 sented in our Newton's disc. Let a piece of thin black- 

 ened cardboard be cut of such a shape that, when placed 

 upon the coloured disc, two equally coloured opposite 

 sectors, and also the small black circle in the middle, 

 may be covered by it. Let a hole be made in its centre, 

 and the cardboard be screwed, with the disc beneath it, 

 upon the whirling-table. The visible remainder of the 

 disc when turned no longer appears white, but coloured 

 in the following manner: When the yellow is covered, 

 the remainder appears violet; when the green is covered, 

 the remainder appears bluish-red ; when the blue is 

 covered, the remainder appears orange. The colours 

 which thus appear successively during the experiment, 

 that is, Yiolet, Bluish-red, Orange, are hence proved to 

 be complementary to Yellow, Green, and Blue respec- 

 tively, because we know that White would be produced 

 if in each case the whole of the disc had been un- 

 covered. 



