170 Sir D. Brewster's Examination of the Phamomena 



Mr. Smith has remarked (page 251, Exp. 1.), "that the 

 same phenomena occur, and are even more vivid, if one of 

 the eyes is exposed to the rays of the sun either direct or re- 

 flected." According to my observations, however, the phe- 

 nomena are quite changed in the latter case. The green image 

 becomes a bright blue, and the reddish image'almost colourless 

 or white. The colours are therefore not complementary, and 

 there is not even the appearance of an influence upon the 

 unexcited eye, whatever tinge there may be of redness in the 

 one image being merely the effect of contrast. 



This result enabled me to make a very decisive experiment. 

 I took two slips of paper, and having illuminated the one with 

 day-light, and the other with candle-light, I excited the right 

 eye with the light of a candle, and doubled the images. The 

 slip illuminated with day-light gave one image blue and the 

 other white ; while the slip illuminated with candle-light gave 

 one image green and the other reddish. Now, in this experi- 

 ment the exciting cause was the same, and yet the colours were 

 different, — obviously proving that the colours depend upon the 

 nature of the light which falls upon the slip of paper, — that 

 they are not complementary, and that when pure white light 

 is used the unexcited eye sees the paper colourless. 



These results may be confirmed by examining the red image 

 when the green image has become a dark purple by bringing 

 it close to the excited spot. In this case the redness instead 

 of being increased is greatly diminished, and the slip appears 

 yellowish or crazw-coioured. This arises from the want of 

 contrast, the green colour which heightened the red having 

 almost wholly disappeared. 



I now took two candles of equal brightness, and having 

 placed them at the distance of about four inches from each 

 other, and about two inches from the eyes, I held a slip of 

 paper between them, and having doubled it by looking at a 

 more distant object, I found that both images were yellowish 

 green, not only when thus seen separately, but also when 

 combined into one. Now if the colours were complementary, 

 and if there was a balance of increased and diminished sensi- 

 bility, the image must have been white. 



Before quitting this part of the subject I may mention the 

 curious fact, that when the eyelid of the excited eye is closed 

 completely, and the eyebrow kept raised, before the exciting 

 light is applied to it, then when the light is applied, there will 

 be only one slip of paper seen, as there is only one eye open ; 

 and the colour of this slip is certainly white as usual : but the 

 moment we open the right eye, withdrawing the exciting light 

 at the same time, the slip seen by the newly opened eye is a 



