LIGHT: COLOUR 



297 



effect can be coaxed out of an apparent failure, by simply 

 covering up, not only the other cell, but the thicker part of 

 the one employed, so as to diminish the preponderance of red 

 light which passes through, and may drown the much fainter 

 blue and yellow. Success in the experiment depends chiefly 

 upon a brilliant light, exact and clean finish of the cells, and 

 the proper amount of fuchsine. 



169. Subjective nature of Colour Sensation. The most 

 striking proof of this is an experiment shown in fig. 165, 

 which depends upon fatiguing the retina with any given 

 colour-sensation. The nerves when thus fatigued becoming 

 less responsive, and 

 this being equiva- 

 lent to some sup- 

 pression of that par- 

 ticular colour, pro- 

 duces the sensation 

 of its complemen- 

 tary, though the 

 screen may be white. 

 Eemoving all but 

 the condensers, hold 

 over the open nozzle 

 N a black card 

 pierced with a circular aperture which, focussed by the lens F, 

 gives a disc of 18 or 24 inches diameter upon the screen. In 

 ths centre of the bright disc insert a drawing-pin as a mark, 

 and let the spectators fix their eyes on this steadily while 

 30 is slowly counted then suddenly remove the card, 

 lighting up the whole screen. The retina being fatigued over 

 the region of the previously bright disc, that spot will appear 

 darker than the rest of the screen. 



This done, merely to explain the principle of the experi- 

 ment, it is repeated with a coloured disc, holding a piece of 

 red glass together with the card over the nozzle, and bringing 



FIG. 165. Subjective Colours 



