218 ON ACCIDENTAL OR SUBJECTIVE COLORS 



sion is seen to recur once or even several times. The following experiment of 

 M. Plateau shows this effect in a very remarkable manner : One of his eyes 

 being closed and covered with a handkerchief, he adapted to the other a black- 

 ened tube about fifty centimetres in length and three in width, and regarded 

 with fixed attention, for a minute at least, a red paper placed in a strong light, 

 and so large that its borders were hidden by the tube ; then, without uncovering 

 the closed eye, he suddenly directed the tube to the ceiling of the apartment. A 

 green circular image was now seen to form, but this was very soon replaced by 

 a red image of feeble intensity; after which the green image reappeared, to be 

 succeeded anew by a reddish image, and so on ; the red image reappearing as 

 often as four times, though with a progressive diminution of intensity of both 

 the red and green. 



2. The same phenomena are still manifested, when, in place of turning the 

 eyes on a white surface, after having regarded the object a sufficient time, they 

 are suddenly closed, and at the same time completely covered with a handker- 

 chief on which the hands are applied ; in this absolute darkness the image of 

 the object will be seen perfectly delineated and colored with the complementary 

 tint. 



3. The accidental image appears of more or less size, according to the dis- 

 tance of the surface on which it is projected. When that surface is at the same 

 distance. from the eye as the object, the image presents the same magnitude; if 

 the surface be more or less remote, the image appears proportionably augmented 

 or diminished. 



4. The subjective colors combine with one another in the same manner as 

 the real colors ; that is to say, from the accidental yellow and blue is formed 

 green ; from the accidental red and blue, violet. Of this we may satisfy our- 

 selves by the following experiment, Avhich we owe to P. Scherffer : Place be- 

 side one another, on a black ground, two small squares of paper, colored, the one 

 violet, the other orange, (colors Avhose accidentals are yellow and blue,) and- 

 mark with a black point the middle of each of these squares. Then let the 

 eyes be alternately directed to one and the other of the points, keeping them 

 fixed upon each for about a second, and, after having repeated this operation a 

 considerable number of times, close the eyes, or direct them towards a white 

 surface. We shall now distinguish three squares in juxtaposition, of which the 

 formation is easily conceived, the middle square being evidently formed from 

 the superposition of the accidental colors produced by the two colors employed. 

 In the supposed case the middle square will be green ; it would be violet if the 

 two colors employed were green and orange, the accidentals of which are the 

 red or blue, &c. One case, however, forms an exception ; that, namely, in 

 which the two accidental colors which we combine are complementary the one 

 to the other; then, instead of producing white, like two complementary real 

 colors, they seem, on the contrary, only to give place to an effect of obscurity. 

 Thus, when the two small squares of paper are, the one green, the other red, 

 colors whose accidentals are red and green, the square which occupies the mid- 

 dle in the accidental image appears blackish if projected on a white ground, and 

 completely black if the eyes be covered. 



5. The accidental subjective colors combine with the real colors in the same 

 manner as these last with one another ; that is to say, from accidental red and 

 real blue results violet ; from accidental blue and real yellow, green. It is easy 

 to be satisfied of the fact by projecting the accidental image on a surface painted 

 of the color with which we wish to combine it; thus, by projecting on a leatf of 

 blue paper the accidental image red proceeding from a green object, the result- 

 ing appearance is a fine violet. But it is necessary to bear in mind the follow- 

 ing remarks : The object being always placed on a black ground, if we project 

 its accidental image on a surface of the same color with itself, or, in other terms, 

 on a surface whose color is complementary with that of the image, the latter 



