206 



Mr. G. J. Burch. On the Belation of 



He does not, however, deal with the positive after-image and after- 

 effects. This was done by Eobert Waring Darwin,* in his paper on 

 the " Ocular Spectra of Light and Colours," in which he treats of the 

 "direct and reverse spectra" of brightly illuminated pieces of silk 

 of various colours. He describes very clearly the series of changes of 

 these after-images from negative to positive and back again, observable 

 under certain conditions, and points out that in order to see the direct 

 spectrum (positive after-image) all extraneous light must be excluded, 

 whereas "it is difficult to gain the reverse spectrum (negative after- 

 image) where there is no lateral light to contribute to its formation." 

 " The reverse spectrum is instantaneously converted into the direct 

 spectrum by excluding lateral light, and the direct into the reverse by 



admitting it " "The green spectrum which is perceived 



on removing the eye from a piece of red silk to a sheet of white paper, 

 may either be called the reverse spectrum of the red silk, or the direct 

 spectrum of all the rays from the white paper except the red, for in 

 truth it is both." Thus the " direct spectrum " is the sensation of 

 each colour persisting after the cause that produced it has ceased to 

 act, and the " reverse spectrum " is the effect of compound colours 

 upon the retina, which still remains liable to be excited " by any other 

 colours except the colour with which it has been fatigued." He proves 

 this by showing that the colour of the " reverse spectrum " depends 

 upon that of the " lateral light," i.e., the light which reaches the eye 

 after the retina has been fatigued.! He compares these phenomena 

 with those of taste, touch, and hearing, and shows that each of these 

 senses undergoes a partial temporary paralysis after being strongly 

 excited. Although, therefore, the unaided evidence of the senses 

 might have suggested that each pair of complementary colour-sensa- 

 tions, such as red and green, or blue and yellow, were conjugate 

 functions of some nerve structure, it is plain that he desired to empha- 

 sise the fact that they must be regarded as due to separate nerve 

 structures. One nerve, one sensation. The sensation might be weak 

 or strong, according to the physiological condition of the organ at the 

 time, but its character could nut be changed. 



Darwin, following Newton, refers to seven colours as primaries. In 

 1792 Wiinsch,^ whose method consisted in the superposition of spectra 

 projected upon a screen, stated that a mixture of three colours, 

 namely, red, green, and violet-blue, could be made to match any given 

 tint. 



For the basis of Young's theory there existed, therefore, experimental 

 evidence of the small number of the primary colour-sensations, and 

 of their being functions each of some nerve structure specially 

 * 'Phil. Trans./ vol. 76 (1786), p. 313. 



t Compare this paper, Section II (1), p. 208, and Section III, pp. 212, 216. 

 X * Ueber die Farben des Liehtes.' Leip2ig, 1792. 



