of the Eye to Colour. 223 



periment, I invariably perceived the same spectrum with this 

 eye as I did with the left one. 



This experiment shows that the presence of white light is not 

 necessary for the perception of complementary ocular spectra, 

 and further, would appear to indicate that for a sympathetic spec- 

 trum to be excited in the eye which has not been exposed to the 

 colour-stimulus, the excitation of some light is necessary. 



M. Plateau painted one half of a piece of paper red, the other 

 green ; and after alternately directing the eyes to each half, 

 covered them with a handkerchief, and observed a black image, 

 having on each side a complementary-coloured image*. He 

 hence inferred that " the combination of accidental colours pro- 

 duces black/' Sir D. Brewster very properly objects to this 

 conclusion, "because the eye has been in succession rendered 

 insensible to the two colours which compose white light itselff." 

 Elsewhere the same author says, "If we take the two comple- 

 mentary colours, namely, the red and the green tints forming the 

 ordinary and extraordinary pencils in the polarized ring, which, 

 by overlapping, form white light, then it is manifest that the 

 accidental colour of the overlapping part is black, and hence the 

 sum of the action of the red and green acting separately must also 

 be black J." 



Notwithstanding, however, the authority of Sir D. Brewster, 

 the following experiment which I have performed appears to me 

 rather to corroborate Plateau's view. If the two halves of a card 

 painted red and green respectively be illuminated by a green or 

 red light, they appear black. In the same way, but depending 

 on a different cause, the two halves of the card, if viewed through 

 green or red glass, appear black. 



Another set of observations, connected in a degree with the 

 preceding, may be here noticed. Chevreul§ distinguishes two 

 chief species of contrast of colours, simultaneous and successive 

 contrasts. But an examination of these distinctions shows them 

 in my judgment to be more apparent than real, and but the 

 expression of one fundamental fact, viz. that the eye on per- 

 ceiving any one colour acquires a tendency to see its complemen- 

 tary. Thus, to take an example of Chevreul's simultaneous 

 contrast : — If a slip of red and one of yellow paper be viewed 

 side by side, near the line of contact the red paper inclines to 

 violet, the yellow to green. The rationale of this is at once 

 obvious : the red mingling with the complementary of yellow, 



* Annates de Chimie for 1833. 



t Load, and Edinb. Phil. Mag. for May 1839, p. 335. 

 X Op. cit. for December 1839, p. 437. 



§ Tlie Principles of Harmony and Contrasts of Colour, by M. E. 

 Chevreul. 



