of the Eye to Colour. 225 



dows have, by Rumford and many subsequent observers, been 

 ascribed to the effect of contrast. But this appears an inadequate 

 explanation ; for if, with one magic lantern, a half- white and a 

 half-red circle of light be thrown on a screen, a shadow thrown 

 across the two fields is simply dark, without any colour at all. 

 If, again, a red and a white disc of light be thrown from two 

 magic lanterns respectively on a screen, so as partially to over- 

 lap, where the overlapping takes place two complementary 

 shadows of any object are seen, but in the other two parts of 

 the field only one colourless dark shadow is seen. 



The following facts seem to form the basis of the explanation 

 of coloured shadows : — First, the experiment of Rumford*, — 

 that a piece of grey paper placed next to a piece of coloured 

 paper, both on a black ground with the exclusion of extraneous 

 light, appears tinged with the complementary colour. Secondly. 

 I found by my own experiments that if, in a dark room, the screen 

 is illuminated with a red circle of light from a magic lantern, the 

 greenness of the shadow and the redness of the ground on which 

 it appears are inversely proportional to one another. By approxi- 

 mating the red light to the screen this becomes redder, whilst 

 the shadow of the rod placed before it becomes less green and 

 darker, till it becomes an ordinary black shadow ; that, on the 

 other hand, removing the red light till it leaves the white screen 

 but faintly tinged with red, brings out the green shadow very 

 prominently, and on admission of light into the room, a second 

 faint red shadow comes out. 



Meusnier observed " that when the sun shone through a hole 

 a quarter of an inch in diameter on a red curtain, the image of 

 the luminous spot was green." Another observer, Mr. Smith 

 of Fochabers f, states, "If we hold a narrow strip of white paper 

 vertically, about a. foot from the eye, and fix both eyes upon an 

 object at some distance beyond it, so as to see it double, then if 

 we allow the light of the sun, or a light from a candle, to act 

 strongly upon the right eye without affecting the left, which 

 may be easily protected from its influence, the left-hand strip of 

 paper will be seen of a bright green colour, and the right-hand 

 strip of a red colour." 



From all these facts, I think the conclusion arrived at by Sir 

 David Brewster appears highly probable, that "as in acoustics, 

 where every fundamental sound is actually accompanied with its 

 harmonic sound, so in the impressions of light, the sensation of 

 one colour is accompanied by a weaker sensation of its accidental 



* Op. eft. p. 3.36. 



t Brewster's ' Optics,' p. 405. Loiul. and Edinb. Phil. Mag. for October 

 1832, vol. i. p. 249. 



