1835.] Composition of White Light. 188 



is produced by the red, exists on the retina while we look 

 at the red object, and when we cast our eyes upon a white 

 ground, the green impression, which is then isolated from 

 its combination with the primary impression, is added to 

 the white impression. The following are illustrations of 

 this theory : 



1. " The effect of this vision of green is to cause the red 

 to appear paler, by mixing with it. The red and the green 

 tend to produce white. But, as the direct red predominates 

 greatly over the accidental green, the result is always a pale 

 red." 



2. If, after looking at a stick of red wax, long enough to 

 occasion an intense accidental colour, (still looking at the 

 wax), we approximate the eye to the flame of a candle, 

 which is placed in such a manner that the rays proceeding 

 from the object to the eye pass close to the flame; then 

 the colour of the wax ceases to act upon the retina, and 

 appears black ; the stick, at the same time, ajipears covered 

 with a feeble greenish phosphorescent light. In this in- 

 stance the accidental image appears to exhibit itself during 

 the contemplation of the coloured object. 



3. In an apartment painted with a bright colour, and 

 upon which the sun shines, the parts of the furniture upon 

 which the light does not fall directly, appear tinted with 

 the complementary colour. Again, when the light of the 

 sun penetrates through a small aperture, in a coloured 

 stuff curtain, if the light is received on white paper, its 

 colour is complementary to that of the curtain. 



Plateau objects to the first experiment, because he sup- 

 poses that Sir David Brewster probably placed the coloured 

 object upon a white ground, as when we wish to observe 

 tlie accidental colours. In this case, after looking long at 

 the object, its colour would be diluted by the white. The 

 object ought to be insulated and placed on a black surface. 

 Then the colour, instead of becoming paler, would be darker. 

 This may be illustrated by placing a piece of red paper on a 

 black ground. Look at the paper steadily, keeping the eye 

 upon the same point ; then, without changing the position of 

 the eye, place by the side of this paper a second piece of the 

 same coloured paper. It is obvious that the image of the latter 

 falling upon a different part of the retina, . will serve as a 



