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KETINAL EXCITATION 



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kinds of white light : as the white that results from the mixture 

 of all the colour- rays of the solar spectrum from that produced by 

 a mixture of red and blue-green, or from orange and blue, etc. 

 None of these white lights present qualitative differences to vision, 

 but, at most, simple quantitative differences due to the different 

 intensities of the light, so that some appear to us more or less 

 pure white or more or less grey. The eye thus differs fundament- 

 ally from the ear, since it is unable to analyse light ; it cannot 

 resolve the different mixed colours into the simple spectral 

 colours of which they are made up, nor differentiate the various 

 kinds of white light obtained by mixing different pairs of comple- 

 mentary colours. This, however, is no imperfection, but merely 

 a physiological necessity of the visual sense. Otherwise we should 

 be unable to see objects as white or variously coloured, but should 

 only perceive a confused medley of different colours in every object. 



When two spectral colours that are not complementary are 

 mixed, the result is not white, but a new colour, which is always 

 less saturated than the two components. This may be further 

 analysed : the two colours to be mixed may be separated in the 

 spectrum by a greater or less distance than the two complementary 

 colours. In the first case the mixture produces an intermediate 

 colour, which is paler as the distance in the spectrum between the 

 two component colours is greater more saturated as the distance 

 is less. In the second case the mixture produces a colour which 

 is more saturated in proportion as the distance of the two com- 

 ponents in the spectrum is greater, paler as it is less (Helmholtz). 



The following table gives the results of the different mixtures 

 of spectral colours according to Helmholtz. The intersections of 

 the vertical and the horizontal columns show the compound 

 colours or white that result from the respective mixtures. 



X. Closely related to the theory of colour-mixture are the 

 phenomena of coloured after-images, described by Peiresc (1634), 

 and colour-contrast, already known to Leonardo da Vinci (1519). 



VOL. IV 2 B 



