128 ELEMENTARY PHYSIOLOGY. XXIII. 



22. Colour wheel. Flicker. On a colour wheel 

 or top put two paper discs, a white and a black, inter- 

 leaved so that the combined disc is half white and half 

 black. When the wheel is rotated slowly, the individual 

 sectors will at first be seen ; increase gradually the ratio 

 of rotation, at first more and more sectors will be seen, 

 and later a marked flickering (coarse flicker). With still 

 greater rapidity this passes into a fine flicker, and then 

 into a continuous sensation. Note the speed at which 

 flicker ceases. When a continuous sensation is produced 

 it will be seen that the white and black discs pass into 

 a grey. 



23. Take the wheel into a brighter light; the rapidity of rotation 

 has to be increased to abolish flicker. Take the wheel into a darker 

 light; flicker ceases at a lower rate. Replace the white by a grey 

 disc, fusion occurs more readily. 



Repeat with yellow and blue discs, differing greatly in brightness, 

 flicker is marked, and only disappears with rapid rotation, though 

 much less rapid than with the black and white. Repeat with red 

 and blue-green discs (of approximately equal brightness), flicker is 

 less marked and ceases sooner. 



These experiments show that a greater rapidity of rotation is re- 

 quired to get rid of flicker, the greater the difference in brightness 

 of the two halves of the disc. 



24. Colour Equation. The yellow and blue discs and also the red 

 and blue-green will fuse so as to give approximately grey surfaces 

 if the amounts of each colour are suitably adjusted by sliding one 

 disc over the other. The greys obtained by mixing two colours in 

 this way are always coloured to some extent owing to the pigment 

 colours not being exactly complementary, and the coloration of the 

 grey will become much more obvious if larger black and white discs 

 are placed on the same wheel to form a grey background. The 

 coloration of the grey may then be neutralised by adding a third 

 disc to the two colours; green if the coloration is inclined to red; 

 red if inclined to green. The relative amounts of the three coloured 





