Accidental and Complementary Colours. 295 



admit to the paper light from a second source, not coloured, 

 the paper will be again in a condition to reflect white light, 

 were it not that the artificial coloured light overpowers the 

 light from the second source. A shadow formed by inter- 

 cepting a portion of the coloured light upon white ground 

 was black, in the absence of light from the second source 

 now becomes white from the presence of the latter ; or 

 rather, that portion of the ground occupied by the shadow, 

 is restored to its original whiteness. Homo-chromatic 

 attraction proceeds between the colours of the same kind 

 in the shadow and the coloured ground which encompasses 

 it; while the colour or colours of an unlike kind are not 

 attracted but repelled or reflected, and constitute the 

 accidental impression. 



26. The conditions, therefore, necessary to the production 

 of coloured shadows are, 



1st. That there be two sources of light. 



2nd. That the first source of light be coloured. 



3rd. That the second source of light be not coloured, 

 and not more than equal in intensity to the first. 



27. Most of the experiments in this and the preceding 

 papers are so decided and are performed so easily and so 

 readily, that it cannot be urged that " the diminished sen- 

 sibility of the eye from fatigue," or from any other cause, 

 produces the effects. I deny that it has any thing to do 

 with the production of these eff'ects, for, I believe, that the 

 performance of any one of the experiments contained in 

 these papers does not occupy more than a second of time. 

 I agree with Mr. Cooper, that *' the colours are produced 

 with a facility which those who are not familiar with the 

 subject are not prepared to expect." {Records, vol. ii, p. 

 178.) Mr. Cooper, also, in making an experiment in order 

 to determine the shortest time in which an accidental 

 colour could be obtained, found it to occupy less than a 

 second. Mr. Cooper also says, *' we cannot attribute the 

 appearance of accidental colours to insensibility of the eye 

 arising from fatigue or exhaustion. Brewster also {Optics, 

 p. 309) admits that a new theory of accidental colours is 

 requisite to explain the phenomena of coloured shadows. 

 There are no less than nine distinct theories of accidental 



