COMPLEMENTARY COLOURS. 



319 



Fig. 160. Zoetrope. 



level with the eye, the painted side being away from the observer 

 and facing a vertical mirror. On looking through the uppermost 

 slits as the disc re- 

 volves (spun round 

 by the other hand) a 

 series of glimpses is 

 obtained of the re- 

 flections in the mirror 

 of the various stages 

 of the motion, pro- 

 ducing the ultimate 

 effect of apparently 

 viewing a moving 

 object, the leaping boy 

 seeming to rise and 

 jump over the head of 

 the other one, repeat- 

 ing the action for each 

 revolution of the disc. 

 Fig. 161 represents a 

 metal holder for the 

 revolving disc. 



Expt. 354. Complementary Colours. For any given colour 

 (i.e., character of ray producing a certain kind of sensation) 

 another colour can be found which is so related to the first that if 

 the two colours be blended together the total effect on the eye 

 is that of whiteness ; such pairs of related colours are said to be 

 complementary. Let the colours in a Newton's disc (Expt. 351), 

 or a chromatrope, or colour top (Expt. 352), be so arranged as to 

 produce on rotation as nearly as possible a sensation of whiteness 

 (i.e., no particular colour or shade being noticeable, but only a 

 slight amount of greyness). Now, cover up with black any por- 

 tion of the coloured disc, &c., so as to obliterate entirely that 

 particular colour; on rotating the disc again a colour will be 

 visible, complementary to the one covered up. 



A peculiar illusion is readily producible by gazing fixedly for a 

 minute or more at a device in some bright colour on a white (or 

 black) ground, and then suddenly looking elsewhere against a 

 uniform background, such as the sky, a cloth, &c. The device 

 will be still apparent for a short time, but the tint seen will be 

 complementary to the original one ; thus, if large red letters on a 

 white ground be viewed intently, on removing the eyes there will 

 be seen the illusion of the same letters in green on a black ground. 

 Similarly, orange letters on a black ground, in the first instance, will 



