440 Mr. P. Coopei' on Accidental Colours, 



colour, which falls upon a part of the eye, not previously 

 brought into action.* 



Mr. Tomlinson, in order to establish his hypothesis that 

 the superposition of complementary colours produce black, 

 appears to have fallen into a similar mistake. He. says, 

 ** if we view red lead through a disk of green glass, the 

 red powder will appear as black as lamp black. "(28) But in 

 this experiment, the only colour reflected by the powder is 

 red, and this colour the green glass refuses to transmit ; so, 

 that instead of a superposition of complementary colours, 

 it is the absence of light of all colours which produces 

 black.* 



If accidental colours are produced by rays reflected by 

 the surface upon which they are seen, the images formed 

 by these colours are to all intents and purposes as com- 

 pletely primary as the images which arise from the colours 

 that produce the first impressions ; and, as we have already 

 observed, if they are of suflRcient intensity, and their action 

 upon the eye be continued for a sufl^icient time, they ought 

 to produce accidental colours complementary to the colours 

 of the images by which they are formed, and, therefore, 

 corresponding with the original impressions. 



This view of the subject is illustrated and supported by 

 the experiments of Newton, Aepinus, Brewster, and others; 

 in which they impressed the eye with the image of the sun, 

 and then, by directing it to a white ground, produced an 

 accidental spectrum, which was invariably surrounded with 

 a border, the colour of which was generally complementary 

 to the centre. t 



I have observed in a former paper, that when we look at 

 any object with a view to impress its im^ige upon the retina, 

 with whatever determination it may be opposed, there is 

 an involuntary motion of the eye. apparently of vibratory 

 character, which brings a part of the retina surrounding 

 the image into the alternate directions in which it sees the 



* These explanations must be modified, if we take into consideration, the white 

 light wliich usually accompanies the prevailing colour reflected by coloured sur- 

 faces ; but this would not effect the general principle. 



t It may be remarked, that these complementary colours, as observed by Sir 

 David Brewster, and given in his Optics, p. 308, exactly correspond with our 

 fheory of primitive colours. 



