536 Contributions to the Physiology of Vision. 



If a red or pink card be substituted for the white card, the decomposi- 

 tion of the ray appears still more distinctly. 



If, on the contrary, a blue tinted card be employed, this decomposition 

 is less distinct than with the white. 



Besides, all these colours are subject to vary according to different 

 circumstances, such as the velocity of the motion, the obliquity of the 

 axis to the card which cuts it, the distance from the section to the origin, 

 or to the base of the luminous ray, the different shades or tints of the 

 card, the intensity of the light, &c. But there is always an apparent 

 decomposition. 



With a yellow card, a circular areola, of a more brilliant yellow than 

 that of the card itself, is seen externally, when there is no motion. 



With a black card there is no coloration, unless a nebulous shade in 

 the centre may be so considered. Besides, it is probable that this shade 

 arises from the black of the card being far from perfect. A card covered 

 with black velvet did not present the slightest appearance of decompo- 

 sition. 



The phenomenon is observed if the card passes the ray but once ; this 

 proves that it is independent of the fatigue of the eye. 



Neither does it depend immediately on the agitation or motion of the 

 card, but doubtlessly only on some effect of this motion, most probably 

 because the illuminated space strikes the eye during a short time only: 

 for if the card be so large that the illuminated space always remains upon 

 it, and that, notwithstanding the motion, the eye continues always to see 

 it, it appears white, as if it were at rest, and there is no appearance of the 

 decomposition of the light. 



In the preceding experiments the coloured rings evidently 

 result from the diffraction of the light at the edges of the aper- 

 ture which admits the ray ; the colours thus produced, being 

 of very feeble intensity, become almost immediately invisible 

 when constantly presented to the same part of the retina; but 

 the intermittent action of the luminous object produces an 

 analogous effect to the shifting of its place on the retina in the 

 previously mentioned experiments. The explanation of these 

 phenomena given by Prevost himself, is founded on Cuvier's 

 theory, which supposes that the visual sensation is occasioned 

 by the chemical action of material light on the nervous sub- 

 stance of the retina ; and that each colour, having a different 

 affinity for this substance, requires a different time to exert its 

 energy upon it ; but admitting for a moment this totally unsup- 

 ported hypothesis, the attempted explanation does not accord 

 with the facts of the case. 



