100 Mr. Cooper on the Colours that enter into the 



A little consideration will convince us, of the necessity of 

 such an arrangement ; for, if the atoms of light of differen t 

 colours mixed indiscriminately with each other, which 

 would be a necessary consequence of their being trans- 

 mitted by a path common to the whole, it would be im- 

 possible, after the numerous intersections which perpetually 

 occur, that light should re-appear, when proper means are 

 taken for its separation, precisely in the state it was at first 

 produced. Hence it is, that when the sensibility of the eye 

 has been impaired by the action of light of any particular 

 colour, its complementary colour, when presented to it, 

 produces a vivid sensation ; and when both colours are 

 presented together, in the form of white light, the effect of 

 the latter, communicated through channels hitherto unem- 

 ployed, gives such a decided preponderance to the sensation 



image ; if the light to which the eye is exposed be of equal intensity, the image 

 is gray, or black diluted with white. 



The lessened sensibility of the eye, when impressed to light of equal intensity, 

 and its total insensibility to light of a lower intensity, is an important distinction, 

 in strict analogy with our other senses, and verified in a great number of instances ; 

 it is upon the latter principle, that weak light, such as shadows, the reflexion 

 from the first surface of transparent media in Mr. Tomlinson's experiments, &c, 

 so readily assumes the complementary tint of the colour to which the eye has been 

 previously exposed. 



Accidental colours of impressions destroy direct corresponding impressions, [5] 

 by converting the white light, which generally accompanies the distinguishing 

 colour, into the complementary colour of the object ; by which means, the whole 

 is reduced to weak white light, or assumes an appearance between black and 

 white. In some cases, an instance of which, is- mentioned in page 178, the acci- 

 dental colour, thus produced, is more than sufficient to neutralise the direct light, 

 and the object then assumes the complementary tint. (See introductory remarks 

 on the Effect of Contrast, p. 177. See, also, the conclusion of the inferences 

 drawn from the subject to which this note has reference, which presents the same 

 principle in a different form.) 



The same principle, I apprehend, applies to the explanations of Sir David 

 Brewster's experiment, with the red wax, described in page 183. The lessened 

 sensibility of the eye to the colour of the wax, by its previous action, is rendered 

 total by the stronger light of the candle ; and the rays of the other colours, which, in 

 this case, as well as almost every other, accompany it in the state of white light, 

 are thus rendered visible under the appearance of a weak phosphorescent colour. 



The simple theory which I have advocated, accounts so readily for a large class 

 of phenomena, extended by including the effects of contrast ; and it bears such a 

 striking analogy in its application, to the known operations of our other senses, 

 that I cannot surrender it without much stronger proofs of its inadequacy, than any 

 which I am yet acquainted with. 



I am glad the subject is about to undergo a thorough investigation, by a philo- 

 sopher, whose recent discoveries connected with it, lead us to anticipate much 

 valuable information. 



