Appearances from the Contemplation of Coloured Objects. 335 



" An accidental colour," says lie in continuation, " is some- 

 thing essentially distinct from a colour produced by the action 

 of direct rays. The rays which produce ordinary colours, 

 can be combined in any proportion we please; and the re- 

 sulting effect is the sum of the actions of each separate ray 

 upon the retina. Hence all the different colours of the spec- 

 trum produce a purely white beam of light; and perfect 

 whiteness may be also produced by two compound colours, 

 one of which is complementary to the other. An accidental 

 colour, however, cannot be added to, or combined with an- 

 other. When the eye sees an accidental colour, suppose red, 

 the excited part of the retina is insensible to all other rays 

 but those of the accidental colour. If we instantly excite the 

 same portion of the retina with another light, which is an ac- 

 cidental gree7i, and thus render it insensible to red, then the 

 eye will see blackness, not because the accidental red and the 

 accidental green compose blachiess, but because the eye has 

 been in succession rendered insensible to the two colours 

 which compose white light itself." 



Thus the author founds himself upon the hypothesis which 

 attributes accidental colours to a diminution in the sensibility 

 of the retina, as if that were a demonstrated truth. But I have 

 shown at the beginning of the article against which the ob- 

 jections are directed, that this hypothesis is insufficient to ex- 

 plain the phaenomena. I have stated in proof of it this fact: 

 " that accidental colours are perfectly seen in the most com- 

 plete obscurity." In fact, in the hypothesis adopted by the 

 anonymous author, the accidental colour, for instance, red, 

 which succeeds the prolonged contemplation of a green object, 

 is explained by saying that the retina fatigued by this con- 

 tinued excitation of green light, loses its sensibility to this 

 light, and is then only affected by the rays which form the 

 complementary red tint. This ingenious theory perfectly 

 explains the phasnomenon, when, wishing to see the accidental 

 colour, we cast our eye on a white surface, as is usually prac- 

 tised. For then we can admit that the red rays, which enter 

 into the composition of whiteness, alone act on the retina. 

 But what does that explanation amount to, when applied to 

 the accidental colour seen in a complete obscurity, and when, 

 in consequence, no ray reaches the eye that can give the sen- 

 sation of the complementary tint ? Now it is by placing my 

 eyes in that condition, as above indicated, that I have observed 

 the result of the combination of two accidental colours com- 

 plementary to each other ; and it is precisely in order to ren- 

 der impossible the explanation of the fact by the diminution 



