388 VISION. LECT. XX. 



not been attended to in their construction. We are in- 

 debted to M. Chevreul for a work on this subject, which is 

 remarkable and complete. 



I cannot pass silently over the ingenious theory by means 

 of which Plateau has attempted to explain all the phenomena 

 now alluded to. According to him, when the retina has 

 been impressed and agitated by light, emanating from an 

 object, and the cause of the excitation has ceased, the 

 retina returns to its normal position, after a series of de- 

 creasing oscillations. The conditions through which it 

 successively passes during the continuation of these oscil- 

 lations, produce opposed sensations. There is opposition 

 between the black and the white, and in general between 

 the effects produced by two complementary colours. In 

 fact, two accidental complementary colours produce, by 

 their superposition, black ; that is to say, no effect. During 

 the continuation of the excitation of the retina, the points of 

 the latter, which are near to those upon which the image is 

 formed, likewise suffer oscillations ; which, being identical 

 with those produced upon a tense membrane, ought to be 

 in a direction opposed to the first, just as we know the 

 vibrations of two neighbouring concamerations [arches] of 

 a vibrating plate are in an opposite direction. There is, 

 then, a neighbouring fringe which produces the effect of a 

 complementary colour, or that of an opposite condition. 



In a word, a portion of the retina being disturbed from 

 its normal state, and the cause of the disturbance having 

 ceased, it returns to a state of repose by a series of 

 oscillations, which vary in their directions and in their in- 

 tensity with the time. The movement communicated to 

 it is propagated to all the neighbouring parts by a series 

 of oscillations, which also vary in intensity and direction, 

 according to their distance from the place of the direct im- 

 pression. 



