LECT. XX. OCULAR SPECTRA. 387 



we need only observe an image accidentally coloured, not 

 upon a white cardboard but on a coloured one ; the image 

 has no longer the complementary colour, but that which 

 results from the mixture of this colour with that of the 

 cardboard on which it is fixed. 



Finally, I am anxious to say a few words on the acci- 

 dental colours formed around objects at the same instant at 

 which we fix them. If we look for a certain time at a co- 

 loured object, placed in the middle of a white cardboard, 

 we see upon the edges a fringe of complementary colour. 

 Observe a strip of white paper pasted on a coloured leaf, 

 and place it near a window, in order that it may receive the 

 greatest possible amount of light; the strip will soon appear 

 to have acquired the complementary colour to that of the 

 leaf. All white bodies, when powerfully illuminated, seem 

 to be larger than black objects, although in reality their di- 

 mensions are the same. This experiment succeeds as well 

 if we employ two similar discs, the one black, placed upon 

 a white ground, the other white on a black ground, the 

 latter appears to be larger than the other. All these facts 

 evidently prove, that each impression produced upon the 

 retina is accompanied by an accidental fringe, so that the 

 excitation is extended beyond the points of the retina, where 

 the image is formed. 



Important applications of these principles are made in the 

 arts in which coloured objects are employed. If the colours 

 which are neighbouring ones, and which mutually influence 

 each other, are complementary, there follows from their re- 

 ciprocal action a greater brilliancy ; the black and the white 

 become, the one more brilliant, the other more deep. On 

 the contrary, all those which are near each other in the 

 series of the seven colours, weaken and injure each other 

 when placed side by side. Pictures, carpets, tinted papers, 

 and decorations in general, sometimes present bad effects, 

 when the reciprocal influences of neighbouring colours have 



