1835.] Accidental and Complementary Colours. 289 



that if a fundamental colour, be combined with a colour 

 complementary to it, that is, an accidental colour equal in 

 intensity to the fundamental, white will result ; whereas, 

 if a similar combination be made between two accidental 

 impressions resulting from two fundamental , colours, the 

 result will be black. I need scarcely remark that the term 

 complementary is derived from the Latin compleo, the com- 

 plementary ^/Zzw^ up that which is wanting in the primitive 

 tint to produce white. So that two colours are said to be 

 complementary to each other which, by actual combination, 

 produce white. (12.) 



7. There is no doubt that, under circumstances in which 

 the eye may be employed, that organ can, to a certain extent, 

 exercise its function in total darkness. I cannot agree that 

 the fact has been, as M. Plateau states, either overlooked 

 or forgotten. The various experiments on the persistance 

 of the impressions of the retina, on the pressure of the eye- 

 ball, and many cases of disease of the organ, prove to the 

 contrary ; and the fact itself is certainly as old as Newton,''*' 

 that the image of a highly luminous object has been, from 

 time to time, revived on the retina, days, months, and even 

 years after the direct impression, even when the eye has 

 been in total darkness. In the case of pressure of the eye- 

 ball, colours may be observed in perfect obscurity ; and the 

 fact is not new, that on gazing on a primitive colour, and 

 then immediately covering up the eye, or placing it in dark- 

 ness, the accidental colour is seen. And even more than 

 this, the case has frequently occurred to myself, where an 

 accidental impression has been produced on the retina a 

 considerable time after the cessation of, or rather, after the 

 removal of the object producing the first impression. Of 

 these cases, I need only detail one of recent occurrence : 



* " Though I looked upon the sun with my right eye only, and not with the 

 left, yet my fancy began to make an impression on my left eye as well as upon my 

 right ; for, if I shut my right eye, or looked upon a book, or the clouds with my 

 left eye, I could see the spectrum of the sun almost as plain as with my right 

 eye." * * * * " Yot some months after, the spectrum of the sun 

 began to return as often as I began to meditate upon the phenomena, even tliough 

 I lay in bed at midnight with my curtains drawn." — Newton's Letter to Locke, 

 quoted in Brewster's Life of Newton, page 315. The whole of this interesting 

 letter applies to my present subject, and is peculiarly valuable, in a philosophical 

 point of view ; it is only to be regretted that Newton should have been so laconic, 

 and not have particularized the colours of the first, as well as of the subsequent 

 impressions, if any such there were. 



VOL. II. U 



