ON ACCIDENTAL OR SUBJECTIVE COLORS. 225 



mixed with that orange color ; aud when afterwards the eyes are directed to a 

 ic/iite surface, the phenomena of succession manifesting themselves, the orange 

 changes into blue, and the violet which was mixed with it also changes into its 

 complementary — that is to say, into yellow ; whence the image must appear com- 

 posed of yelloit) and blue, or green. 



2d. When the object we look at is blach and placed on a colored ground, its 

 accidental image always appears tinged with the color of that ground. Thus, 

 a black object placed on a red ground will give a ^;ai?c red accidental image on 

 a. green ground, &c. The effect is more decided with the eyes closed and cov- 

 ered Here again, Avhile we wei'e looking at the black object, a light comple- 

 mentary tint of the color of the ground manifested itself, a tint which, when 

 we cease to look at the object, changes into its complementary — that is to say, 

 into the color of the ground itself. 



3d. If, on the contrary, we look fixedly and sufficiently long at a small col- 

 ored object on a black ground of adequate extent, and afterwards cover our 

 eyes, the accidental image appears surrounded to a certain distance by an aureola 

 lightly tinged with the color of the object itself. Thus, a red, object produces 

 a green image surrounded by a reddish aureola ; an effect which evidently pro- 

 ceeds from the circumstance that, during the contemplation of the colored ob- 

 ject, the space which surrounds it takes, to a certain distance, a light tint com- 

 plementary of the color of the object, and this tint afterwards, to the closed and 

 covered eyes, changes into its complementary — that is to say, into the color of 

 the object itself. 



Under the name of subjective colors are further comprised certain colored 

 appearances, produced under circumstances entirely different : for example, 

 when the eye is subjected to pre.*sure in the dark, or when a blow is received 

 on the eye, or in certain indispositions of the stomach, &c. Of these we shall 

 say a few words hei'eafter. 



DIFFERENT HYPOTHESES ADVANCED FOR THE EXPLA.NATION OP THESE SINGU- 

 LAR PHENOMENA. 



Colors of the first class. — Theory of Scherffer. — The theory of which we owe 

 the first idea to Scherffer seems now adopted, with a slight simplification, by 

 the generality of physicists. It supposes that the sustained action of the rays 

 of a certain color on a part of the retina momentarily diminishes its sensibility 

 for rays of that color, so that if the eyes be then directed to a white surface the 

 portion of the retina whose sensibility is thus modified can, for some time, only 

 receive a complete impression from the constituent part of this Avhite which is 

 complementary to the color that has fatigued the organ. Thus, on this hy- 

 pothesis, when we contemplate fixedly and sufficiently long a red object, the 

 part of the retina on which the image of tl^ie object is depicted becomes less 

 sensible to the red light; and if the eyes be then directed to a white surface, 

 'since this white may be considered as composed of red and green, an imago 

 will be I erceived in which the green predominates. This simple and ingenious 

 theory completely explains most of the phenomena relative to subjective colors 

 of the first class ; the diminution of brightness of the colored object in propor- 

 tion as the contemplation is prolonged; the increase of intensity of the image, 

 the longer the object has been observed ; the combination of the accidental 

 colors with one another, &c. Still there are important facts for the explanation 

 of which this theory is evidently insufficient. Such, for example, are : 1st, the 

 production of subjective colors in complete darkness ; how should the simple 

 insensibility of the organ for a color produce, in the absence of all light, the 

 sensation of the complementary color 1 2d. The combination of the accidental 

 with the real colors ; if a red accidental image is occasioned by the retina hav 

 ing become less sensible to green, how comes it that violet is seen when this 

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