228 ON ACCIDENTAL OR SUBJECTIVE COLORS. 



state iu regard to space, it will be seen that the state of excitation caused by the 

 light emanating from the object is not limited to the portion of the retina directly 

 struck by that light, but that this state is prolonged without changing its nature to 

 a small distance from the contour of that position, producing the phenomenon of 

 irradiation, which we will presently explain; that beyond this limit is manifested 

 an opposite state of the organ, from which results the accidental color, and that, 

 in certain cases, at a still greater distance, the color of the object is repeated. 

 Here, therefore, we hare oscillations in regard to space instead of oscillations in 

 regard to time, and we discover an equal analogy with natural phenomena ; it 

 is thus, in effect, that in a vibrating surface the parts separated by nodal lines 

 exist in opposite states ; that, in liquid waves, we find as we remove from the 

 centre of excitation, alternate states of elevation and depression becoming less 

 and less distinguishable. 



It is under this general point of view that M. Plateau regards the assemblage 

 of subjective phenomena. This theory, so simple that we had ourselves adopt 

 ed it before knowing that it bad been stated and maintained by the distinguished 

 professor of Liege, does not merely explain all the facts, but almost enables us 

 to discover them a ptiori. Let it be once supposed, in effect; that an organ 

 which has been excited is then suddenly left to itself, and we cannot conceive 

 otherwise than that it will return promptly, but in an interrupted manner, to its 

 normal state, just as, during the excitation, it cannot be .supposed that along the 

 outline of the excited space there should be an abrupt passage from the state of 

 excitation to the state of repose. 



M. Plateau, in order the better to substantiate his ingenious theory, proceeded 

 to submit to direct experiment the undulatory movement of the impression. He 

 found, first, that the number of oscillations is greater in proportion to the 

 greater length of time during which the object has been observed ; second, that 

 this time, on the contrary, does not appear to have any mfluence on the duration 

 of each of the oscillations taken separately. The manner of operating was as 

 follows : One of two physicists regarded fixedly, during a stated number of 

 seconds, a bit of orange-colored paper placed on a black ground in a well-lighted 

 situation, and then turned his eyes quickly on a white wall. He then indicated 

 with the greatest possible precision the moments at which the accidental im- 

 pression attained its successive maxima of intensity, while the other observer, 

 provided with a watch marking the half-seconds, noted the time. As many as 

 eleven reappearances were thus verified by MM. Plateau and Quetelet. 



One word more on the before- cited experiment of Dr. Smith. M. Plateau is 

 of the opinion that the green appearance of the image perceived by the excited 

 eye should be attributed to the circumstance that a part of the light of the candle 

 traverses the envelopes of the eye and diffuses on the retina a reddish hue; the 

 white image, being environed by this rt d light, must then seem tinged with the 

 complementary color, green. What confirms him in this opinion is that the effect 

 is perfectly well produced when the light of the candle is prevented, by means 

 of a small opaque obstacle held near the eye, from penetrating by the pupil, 

 while it is still permitted to illuminate the outer parts of the organ. In this 

 case, the image of the flame is in no manner depicted on the retina, and yet the 

 red and green appearances are equally produced. If it be objected, says M. 

 Plateau, that the red light which traverses the envelopes of the eye is dissemi- 

 nated everywhere, and that it must diffuse itself also on the image of the small 

 strip, which should consequently appear reddish and not greenish, I would answer 

 by citing an analogous fact reported by M. Gergonne. 



Into one of the schools of mathematics in the college of Montpellier fronting 

 the east, the rays of the morning sun in summer only penetrate after having 

 traversed the foliage of the acacias with which the court is planted. On enter- 

 ing this apartment, all the objects within it present a greenish tint, which seems 

 to be gradually effaced ; but then another phenomenon succeeds — all the traces 



