leW M. Plateau's Defe7ice of his Theory of the Visual 



periment of Dr. Smith of Fochabers *, and I have mentioned 

 in a former paper f that a stick of red seaHng-wax may be 

 thus made to appear of a dark liver-brown colour. 



" If we apply the strong light to the'eye when the sensibility 

 of the retina has been locally diminished by looking at a red 

 object, a totally insensibility to red light will be produced. 

 In order to observe this curious result in perfection, let the 

 eye be steadily fixed for some time upon a seal of red wax 

 which reflects white light from all its elevated parts. When 

 the eye has been so fatigued that it would see a bright acci- 

 dental green, bring a candle close to the excited eye;}:, and 

 so near its axis that the red seal will be seen by rays which 

 pass near the flame of the candle. When this is done, the red 

 wax seal will be converted apparently into a seal of black 

 wax, the lights reflected from its elevations being still di- 

 stinctly seen, ^^his experiment, when successfully made, af- 

 fords one of the most remarkable optical deceptions with 

 which I am acquainted. 



" The method now described of eliminating the impression 

 of the primitive or exciting colour leads us to a very import- 

 ant determination in the theory of accidental colours. I en- 

 deavoured long ago to show from analogy, as well as from 

 the evidence of experiment, that the vision of the primitive 

 and the accidental colour is contemporaneous, in the same 

 manner as the fundamental and the harmonic sound are heard 

 contemporaneously by the ear. That this is the case may be 

 shown in the following manner. When the eye is fatigued 

 with the excitation of the red seal, a faint green phosphores- 

 cent-looking light covers for a while the surface of the red 

 seal, occasionally overpassing its margin, showing, in the 

 clearest manner, that the accidental green is seen at the same 

 time with the exciting red. The effect of this vision of the 

 green is to make the red appear much paler by its admixture 

 with it. The red and green tend to produce whiteness; but 

 as the direct red greatly predominates over the accidental 

 green, the result is always a pale red. But when a brilliant 

 light is brought near the excited eye so as to extinguish com- 

 pletely the red rays, the phosphorescent green appears alone; 

 and thus we have ocular demonstration that the accidental 

 green is not the light of a white ground deprived of the red 

 j-ays to which the eye has been rendered insensible, but is a 

 colorific impression generated in the retina itself, and super- 

 added to the whiteness of the ground in the case when the 



» Phil. Mag., vol. i. p. 250. f- Ibid. p. 172. 



X In this experiment, the other eye must be coloured. 



