^^ On the Colours that enter into the [Sept. 



same set of rays combined the idea of contrary sensations, 

 advanced by Jurin. Thus, the green image seen when the 

 eyes are cast upon a white ground, after having been fixed 

 upon a red object, proceeds from two causes : 



1. The retina, fatigued by the red, has become insensible 

 to the rays' of this colour, and is now only affected by the 

 complementary green. 



2. This part of the retina takes on, spontaneously, an 

 opposite mode of action, which produces the sensation of 

 the complementary green colour. 



Hence, we see that if the accidental image, (or inverse 

 spectrum, as Darwin calls it, in opposition to direct spectra, 

 that is, to the images which preserve the colour of objects), 

 is observed in a dark place, or on a coloured surface, its 

 production may be explained by attributing it to the second 

 cause alone ; for, from the nature of this cause, the acci- 

 dental colours may be produced without the participation 

 iof external light. 



7. The theory of contrast was developed in the year 13, 

 by Prieur of Cote d'Or, in a memoir read to the Institute. 

 (See an analysis of it in Annates de Chimie, vol. liv.) He 

 employs the word contrast to characterize the effect of the 

 simultaneous prospect of two differently coloured objects. 

 In other words, it is a comparison from which results the 

 perception of some difference, either great or small ; and 

 further, the new colours developed by contrast, are always 

 conformable to the shade from which they were obtained, 

 by extracting from the peculiar colour of the one body, the 

 rays analogous to the colour of the other body. Thus, in 

 certain circumstances, a small stripe of orange paper, placed 

 upon a leaf of red paper, appears yellow, that is, a colour 

 which may be considered as orange deprived of its red. 

 Upon yellow paper it appears red. 



8. Sir David Brewster (Edinh. JSncyclop.i., Philosophic. 

 Mag. iv. 354., Letters on Natural Magic, p. 22.) compares 

 the state of the eye during the contemplation of a coloured 

 object, to that of the ear during the perception of a sound ; 

 and admits that the vision of the primary and accidental 

 colours are simultaneous, in the same manner that the 

 fundamental and harmonic sounds are perceived by the ear 

 at the same time. Thus, the green accidental colour which 



