180 On the Colours that enter into the [Sept 



considerable length, those which have been previously pro- 

 posed. The latter amount to eight. 



1. The earliest theory was that of Jurin, detailed in his 

 Essai sur la Vision JDistincte et Indistincte, inserted in Smith's 

 Optical Treatise. He considers the phenomena of accidental 

 colours to depend on this principle : that when we have 

 been for some time affected with a sensation, immediately 

 on our ceasing to be affected with it, a contrary one is pro- 

 duced, sometimes by the act of cessation, and at other times 

 by causes which on another occasion would scarce produce 

 the same sensation in any degree ; or, at least, not in the same 

 intensity. Thus, after looking at a brilliant object, if we fix 

 the eye upon a wall, an image of the object will be observed, 

 but it will be the reverse of brilliant. Analogous cases 

 occur with regard to the other senses. After great and 

 tedious pain, a pleasurable sensation follows. The intensity 

 of a cold bath is succeeded by great heat. When we pro- 

 ceed from a strong light into a dark chamber, we seem to 

 be enveloped in obscurity, but after remaining for some time 

 the room appears to be deprived of its darkness. 



2. Scherffer published a theory in 1761, [Dissertation sur 

 les Couleurs Accidentales^ Journal de Physique de Rozier, 

 xxvi. 1785.) which is of the following import: If a sense 

 receives a double impression, one of which is strong and 

 lively, and the other weak, we are not sensible of the latter. 

 This ought to happen principally when they are of the same 

 species, or when a strong action of an object upon some 

 sense is followed by another of the same nature, but much 

 milder, and less powerful in degree. Thus, if, after looking 

 attentively at a green object, we direct the eyes upon a 

 white ground, the eye being fatigued by long attention to 

 the green colour, and being then suddenly cast upon the 

 white surface, is not in a condition readily to receive a 

 weaker impression of green rays. Now, all the modifica- 

 tions of light are reflected by the white surface, but the 

 green is less in quantity, compared with what reached the 

 eye from the green ground. If, then, we fix the eye upon 

 white paper, those parts of it which had previously re- 

 ceived a stronger impression of green light than the others, 

 cannot now appreciate all the effect of this light. 



This theory, slightly modified by others who have adopted 



