438 Mr. P. Cooper on Accidental Colours, 



which are complementary to green, viz., violet and red; 

 the crimson image, then, formed by these colours, is as 

 much an original or primary image, as the image formed 

 by the green light transmitted by the glass, and, therefore, 

 if this image be of sufficient intensity, it ought to produce 

 in the eye a lessened sensibility to the colours by which it 

 is formed; but the eye was previously insensible to the 

 complementary colour, green, and, consequently, when the 

 glass is instantly re-placed, the eye, under the influence of 

 both impressions, insensible to the three colours, or to the 

 whole of the light reflected by the white paper. 



This brings us to an explanation of the experiment of 

 M. Plateau; [6]^ which is introduced by the rather para- 

 doxical statement, that while two real complementary 

 colours produce, together, white, two accidental comple- 

 mentary colours produce, together, black. — In this experi- 

 ment the eyes, by viewing the complementary colours in 

 succession in the manner he has directed, become insensible 

 to both these colours, which, together, constitute white 

 light, when it is presented to them in the weak state to 

 which it must be reduced in penetrating to the eyes closed 

 and covered ; and it produces a black accidental image, 

 precisely as it would have done if the colours had been 

 viewed together upon a white surface. The complementary 

 colours are seen on the right and left of the black image, 

 because these parts of the eyes have been exposed during 

 the progress of the experiment to one colour only. 



I have discovered only one difficulty in the theory which 

 I have been endeavouring to support, and that difficulty is 

 found in the present experiment. When the light is ex- 

 cluded from the eyes in the manner described by M. 

 Plateau, it appears difficult to conceive, that it can be 

 present to form the accidental colours which he afterwards 

 observed ; but the following experiment will at least render 

 it probable, that by some means, which we cannot explain, 

 it still penetrates to the eyes, though in a state of very low 

 intensity. If, after looking at a coloured surface, we close 

 the eyes, we see the complementary colour of the previous 

 impression : if we then pass the hand before the eyes, so as 

 to intercept the light which falls upon the eye lids, the 



• Records of General Science, vol. ii. p. 282. 



