ON ACCIDENTAL OR SUBJECTIVE COLORS. 229 



of chalk upon the black-board become invested with Imo.s more or less rose 

 colored. These traces, though receiving the green light like the rest of the 

 tableau, are tinged nevertheless with the complementary color. M. Trechsel, in 

 his memoir on colored shadows, has adduced, from experiment, an analogous 

 phenomenon : let the sun's light, introduced cautiously by slightly raising the 

 curtain for instance, be made to fall on the back of the camera ohscura when it 

 is colored green by an object-glass of that hue, the place illuminated by the 

 eun's light will assume a tint of pale red, without the presence of any shadow. 

 If the object-glass be red, the light of day will cause the place on which it falls 

 to appear of a greenish blue. In the observation of M. Gergouue, the white 

 chalk re[)laced the white ray. 



M. Plateau's theory of objective colors is for us the precise expression of the 

 facts ; it explains everything, nay, it foresees everything. There is but one 

 class of phenomena which still leaves anything like serious uncertainty Colored 

 shadows bear no resemblance to other accidental colors ; the latter are, in gene- 

 ral, very difficult to perceive and very fugitive; the colored shadows, on the 

 contrary, are vivid, well defined, persistent, and the mind, however convinced, 

 repels with difficulty the idea that they are no illusions, no purely subjective 

 appearance, but in truth real colors The following was said of them by Count 

 Rumford, who produced them by receiving the eun's light through two small 

 apertures perforated at sufficient distances to obtain two distinct shadows of the 

 same opaque body, which apertures he covered with colored glasses : " The 

 shadows," he says, " were tinged with an infinite variety of colors, the most un- 

 expected and often the most beautiful; they varied continually, sometimes with 

 inconceivable rapidity ; the eyes were fascinated, and the attention involuntarily 

 fixed on this magic tableau, equally enchanting and new. The clouds borne by 

 the wind seemed, each in its turn, to bring an endless succession of different 

 colors, with the most harmonious tints." Happily, to the enraptured amateur 

 succeeded the cool philosopher ; he would fain resist tlie testimony of his senses ; 

 he suspects that these brilliant appearances are deceptive, and that they can only 

 be the effect of contrast between neighboring colors; he therefore submits these 

 charming illusions to the control of new experiments, and another light, that of 

 truth, dawns upon him. He places a friend in the position he had occupied in 

 order to be certain that these brilliant phenomena have not ceased, and limiting 

 his own vision by means of a tube blackened within, directs it so as to see the 

 shadows alone; and now, while the friend is in ecstacies over its brilliant color, 

 the rigid investigator sees nothing but an obscure and colorless shadow. The 

 demonstration is exact and complete, and it will be no further necessary to ex- 

 plore these colored shadows with sensitive plates and papers in order to assure 

 ourselves that in no manner do they produce the action indicated by their color. 

 The memoir of M. Fechner on subjective colors, published in 1840 in the An- 

 nals of Poggendorff, is the most recent treatise upon these subtle inquiries. We 

 have studied it with great care, and if we do not here analyze it, it is because, 

 while founded on observations well made and judiciously reasoned, it has seemed 

 to us to contain no fact essentially new. On reading it, we have remained satis 

 fied that, in the phenomenon of colored shadows, contrast alone, and not irradia- 

 tion, is in action, and that the final reason of the problem is to be sought in a 

 fundamental principle analogous to that of relative movement. This last prin- 

 ciple, we know, consists in the essential fact that the relative movement of two 

 or more bodies in motion by no means depends on the common forces or veloci- 

 ties, but solely on the forces in excess, that is to say, the forces which act on 

 some of the bodies without acting on the others. In like maimer, as regards 

 light, the relative sensation that is really perceived by the eye, does not depend 

 on the common tints, but on the excedent or differential tints Thus, the por- . 

 lion of a green ground illuminated by a white ray on the white chalk, as we 

 have seen, appears red in certain circumstances So too, if a white ground has 



