ON LIGHT. 255 



relation to each other of these complementary colours 

 is curiously and strikingly illustrated by the spontaneous 

 production within the eye itself of the tint complement- 

 ary to any vivid colour, which takes place when, after 

 gazing steadfastly on an area so coloured, on a white 

 ground, and strongly illuminated, the gaze is suddenly 

 transferred to a uniformly white surface. There is seen 

 on it, though only for a few moments, a picture or opti- 

 cal image of an area similar in form and size, but tinted 

 with the complementary hue, which fades quickly away. 

 This curious and beautiful experiment, which requires no 

 apparatus to exhibit, and which any one may try in a 

 moment, is exceedingly illustrative of the mode in which 

 the sensation of colour is produced. It proves that, in 

 the nervous tissue which receives and feels the picture 

 within the eye, there are nerves individually and exclu- 

 sively sensitive to each of the coloured rays, or at all 

 events to each of those primary colours (if such there 

 be) by whose mixture all colours are compounded.^ 

 When white light falls on a portion of the retina wholly 

 or partially deadened or fatigued by the excitement of 

 the nerves appropriate to one set of rays, the sensibility 

 of the others being left unexhausted ; that other portion 

 will be for a time proportionably more sensitive to the 

 remaining rays : so that under the stimulus of white light 

 an undue preponderance is temporarily given to their 

 influence, and the sensation of the complementary tint 

 is conveyed to the mind. This is only one of innumer- 

 able instances of the wonderful adaptation of that most 

 astonishing organ to the performance of its office of con- 



