ON ACCIDENTAL OR SUBJECTIVE COLORS. 219 



will appear only of a dark gray, as if the sensation were partially destroyed at 

 that point of the surface. This will be the case, for example, if, after having 

 looked for some time at a small piece of red paper placed on a black ground, 

 we cast the eyes on a sheet of this s^me red paper. On the contrary, if we 

 project the accidental image on a surfiice whose color is complementary to that 

 of the object — that is to say, which is identical with that of the image — the lat- 

 ter will appear of a finer and purer color than the rest of the surface on which 

 it is detached. This, for example, will take place with a red object and !i green 

 surface, with a yellow object and violet surface. 



6. With a view to isolate from all extraneous influence the colonnl object 

 looked at, we have assumed thus far that it rested on a black ground ; if this 

 ground be white, we obtain analogous effects as regards the color of the sub- 

 jective image, but different as regards the relative brightness of that image, and 

 the surface on which it is projected. Thus when the object rests on a black 

 ground its accidental image, projected on a white surface, appears darker than 

 the latter, and the contrary takes place if the object rests on a white ground. 



In order to form a complete idea of the principal properties of these subjective 

 . -colors, it remains to add the following facts : 



7. We have said that when the eyelids are covered, after looking at a bright 

 object, the primitive impression is usually found to persist for a considerable 

 time. In such a case, if the covering be removed from the eyelids, or the eyes 

 be opened and directed to a white surface, it generally occurs that this primi- 

 tive impression immediatelj'- changes into an accidental image, which passes 

 anew to the primitive state if the eyes be again covered ; and the impression 

 may thus be made to pass several times in succession from one to the other con- 

 dition. The effect is very well manifested by the image of a window, and it 

 was in this way that Franklin observed it. This image appears, to the eyes 

 closed and covered, to be formed of luminous panes and dark sashes ; but if the 

 light be allowed to reach the eye, the panes become dark and the sashes lumin- 

 ous. If we close the eyes anew the first effect recurs, and so on alternately. 

 Darwin has observed an analogous effect after looking at the setting sun. Mr. 

 Brewster observes that if, after having looked at that orb, a reddish brown 

 spectrum be seen, a certain degree of pressure upon the eye Avill cause this 

 image to change into a green spectrum, and that if the pressure ceases the 

 image will resume its former color. 



Let us remark, in conclusion, that since the primitive impression ahvays sur- 

 vives for some time the cause which has produced it, the accidental or subjective 

 image must be regarded as in reality succeeding the objective image after a 

 measurable space of time If, in the greatest number of experiments relative to 

 subjective colors, we cannot seize the trace of that first impression which is ef- 

 faced, it is because it is generally too fugitive, and the time of its existence is 

 further diminished by the circumstance that, in order to give much intensity 

 and duration to the accidental image, the object should be regarded for quite a 

 long time. In effect, while the prolonged contemplation of the object increases 

 the intensity and duration of the second phenomenon, it shortens, in general, as 

 we have seen, the duration of the first. There are cases, however, in which the 

 succession of the two phenomena is perfectly manifest ; thus, when M. Plateau 

 contemplated the window during a space of time neither too long nor too short, 

 he distinguished perfectly, first, the image of the window with its luminous 

 panes and dark sashes, then the subjective image with the panes dark and the 

 sashes luminous, &c. 



As Sclierffer has pointed out in his interesting memoir, we may convert these 

 phenomena of vision into a source of amusement in the following manner : A 

 bust of a man or woman is painted on a black ground in such colors as shall be 

 accidental or subjective as regards the natural colors of such a figure. The 

 skin would, in the supposed case, be of the color of bronze, the eyebrows and 



