ON ACCIDENTAL OR SUBJECTIVE COLORS. 239 



by a white object on a black ground is due to this cause, it would seem that the 

 object must appear colored on its borders. Now, among all the observers who 

 have occupied themselves with oculai* irradiation, not one has made mention of 

 colored appearances, and in the numerous experiments which 1 have made ou 

 irradiation, under a multitude of different circumstances, I have never perceived 

 anything of the kind. This absence of visible colors can scarcely be attributed 

 to the small angular breadth of the irradiation ; those in whom the phenomenon 

 has much development will be easily able to satisfy themselves by repeating 

 some of my experiments, or by observing the well-known appearance of the 

 crescent moon, that the band of irradiation ia of quite suiiicient breadth to 

 enable them to see these colors, if any existed. 



" In the second place, I see not how it would be easy to explain by this 

 aberration of refrangibility that singular law to whicb irradiation is subjected, 

 namely, that when two objects of equal brightness are separated only by a small 

 interval, each of them diminishes the irradiation of the other in the parts oppo- 

 site, and this the more strongly in proportion as the two objects arc nearer to 

 one another, so that when at last they touch, irradiation is null for each of 

 them at the point of contact. How shall we admit an action exerted by a 

 luminous image on the aberration produced around another image ? 



" But we can easily decide by direct experiments whether ocular irradiation 

 is or is not due to chromatic abei*ration. For this it will be suificient to ascer- 

 tain whether irradiation is still produced when the object is illuminated by a 

 homogeneous light. If, in this case, irradiation is no longer perceived, we shall 

 he bound to admit as true the hypothesis which attributes the phenomenon to 

 the chromatic aberration of the eye ; but if, on the contrary, irradiation still 

 manifests itself, and to the same degree as with a compound light equal in bright- 

 ness to the homogeneous light employed, it becomes impossible to seek in the 

 aberration in question the cause of the phenomenon. Now, I have executed 

 these experiments by the process which I here indicate. 



" The homogeneous light of which I have made use is that yielded by the 

 flame of a mixture of alcohol, water, and salt. With this mixture I saturated a 

 parcel of cotton wick, which I placed behind a roughened glass arranged verti- 

 cally. The mixture, kindled in the dark, yielded me a voluminous flame, and 

 the glass observed from the other side formed a luminous field of sufficient bright- 

 ness. To render the light still more homogeneous, I interposed between the 

 flame and the roughened glass a yellow glass of an intense color. Everything 

 being thus prepared, I placed successively before the roughened glass the open- 

 worked apparatus described in § 28 of my memoir, and that which has served 

 for my experiments of measurement, after having brought, in the latter, the ver- 

 tical edge of the movable plate into prolongation with that of the fixed plate. 

 The two pieces of apparatus were thus projected on a field having considerable 

 brightness and receiving a light so closely approaching homogeneity that on ob- 

 serving them by refraction through a prism placed vertically at a distance of five 

 metres, their images not only preserved great distinctness, but presented laterally 

 only a greenish shade so slight that much attention was needed to perceive it. 

 I should not forget to say that, in order to give to the eyes more sensibility, the 

 experiments were not made by day in a darkened chamber, but at night. 



"Now, under the circumstances just described, and which would necessarily 

 exclude the eflects which might have depended on the aberration of refrangi- 

 bility, the above apparatus exhibited to me a highly developed irradiation. The 

 same result manifested itself to MM. Burggraeve and Le Francois, who had as- 

 sisted me in the experiments of measurement reported in my memoir, and who 

 are consequently accustomed to judge of the phenomena of irradiation. In order 

 afterwards to compare the effects produced with those to which a compound light 

 of similar brightness would give rise, I placed at the side of the same roughened 

 glass another like it, behind which I so arranged several lighted caudles as to 



