Discovery of his Theory of Colours. 



113 



" Pure red excites strongly the fibres sensitive to red ; and 

 feebly the two other kinds of fibres ; sensation, red. 









Fig. 



1. 







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^ 



s* 











n _-- 



- 













3 









->< 





\ 



" Pure yellow excites moderately the fibres sensitive to red 

 and to green, feebly those fibres sensitive to violet ; sensation, 

 yellow. 



" Pure green excites strongly the fibres sensitive to green, 

 feebly those sensitive to red and to violet ; sensation, green. 



" Pure blue excites in a moderate degree those fibres sensi- 

 tive to green and to violet, feebly those sensitive to red ; sen- 

 sation, blue. 



" Pure violet excites strongly those fibres specially destined 

 to receive this sensation, and the other fibres are feebly affected 

 by this light ; sensation, violet. 



" The nearly equal excitation of all of the fibres will give 

 the sensation of white, or of whitish colours. 



" The choice of the three fundamental colours is to 



some extent arbitrary. We can choose at will any three colours 

 whose mixture produces white. Young no doubt ivas guided by 

 the consideration that the extreme colours of the spectrum occupied 

 the privileged positions* '. If we do not choose these colours, 

 we must take for one of the colours a purple tint, and the curve 

 which responds to it in the figure will have two maxima — one 

 in the red, the other in the violet. The hypothesis, without 

 being an impossible one, will be more complicated. As far as 

 I know of, there exists no means of determining directly the 

 fundamental colours but the examination of persons affected 

 with colour-blindness. We will subsequently see how far that 

 examination confirms the hypothesis of Young, at least so far 

 as the red is concerned. 



" .... In general, then, light which consists of undulations 

 of different wave-lengths produces different impressions upon 



* The writer has italicized the above for the purpose of a future refer- 

 ence to it. 



