440 Sir D. Brewster on llie Theory of Accidental Colours. 



in the first place, they have that in common, that they con- 

 sider the accidental colour as owing to an impression of a 

 peculiar nature which is spontaneously generated in the or- 

 gans, and. not as the result of a relative insensibility to certain 

 rays. On the other hand, according to Sir David Brewster, 

 the accidental colour unfolds itself on the retina during the 

 contemplation of the direct colour, and combines itself with 

 this latter ; and according to me, the opposite effort of the re- 

 tina, whence results the negative sensation, as soon as that 

 effort ceases to be counteracted, likewise unfolds itself during 

 the contemplation of the direct colour, and combines itself, m 

 some respect, with this latter, neutralizing it partially. But 

 only Sir David Brewster maintains that the combination of the 

 iivo sensations produces ixihiteness, whereas I have shown that 

 upon an object insulated from every lateral influence the 

 result is on the contrary blackness'-'''." 



Now it is not the case, that I maintain "that the combina- 

 tion of the two sensations produces 'whitefiess." My words 

 are : " The effect of this vision of the gree7i (the accidental 

 colour seen when looking at a red seal,) is to make the red 

 appear much paler by its admixture mth it. The red awd green 

 tend to produce whiteness ; but as the direct red greatly pre- 

 dominates over the accidental green, the result is almoays pale 

 red." Now as the original seal was a bright red, and as I 

 maintain that the physiological result is a pale red, why does 

 Prof. Plateau allege that I make the combination of the two 

 sensations produce whiteness? Does not he himself maintain 

 the same opinion which I have above expressed, when he saysf, 

 *' that the combination of the two actions" is a gradual falling 

 off in the apparent brightness of the object looked at ? No 

 observer can doubt the correctness of my opinion, or rather 

 of the fact which I observed. 



Upon the supposition that his theory of negative and 2)0sitire 

 oscillations is an incontrovertible law of our visual sensations, 

 M. Plateau, like all theorists who are not satisfied with the 

 plain results of experiment, extends it to all the other senses, 

 when their respective organs, caused to deviate from their 

 ordinary condition, are suddenly withdrawn from the exciting 

 cause. They return, he conceives, to their ordinary condition 

 by an oscillatory movement, which produces negative impres- 

 sions. He endeavours, without any success, to apply this 

 principle to certain pha^nomena of sciind c\im\ touch, and even 

 taste;}; ; and, pushing his theory to the very verge of sobriety, 

 he asks, "if it is not probable that the principle of oscillatory 



* Sec this Journal, vol. xiv. p. 443. f Hid. p. 442, bottom. 



X He considers the arricre gout, or lack iastc, as the negative cscillaticn ! 



