582 MR WILLIAM SWAN ON THE GRADUAL PRODUCTION OF 



Before I was aware that any one had noticed the gradual action of light on 

 the eye, my attention was accidentally directed to that subject about eighteen 

 months ago, by observing that the light of the sky seen immediately over a ball 

 in its descent through the air, seemed less 1 iright than at those parts of the retina 

 where the action of the light had not been interrupted by the passage of the dark 

 body. It immediately occurred to me, that this appearance was caused by the por- 

 tion of the retina over which the image of the ball had passed, not having had time 

 to be fully impressed with the light of the sky at the instant when the passage of 

 the ball again exposed it to the action of that light* Such an observation as this 

 does not admit of easy repetition, but a more convenient method of exhibiting the 

 gradual production of luminous impressions wiU be afterwards described. 



It may be necessary here to anticipate an objection to the supposition, that 

 light requires a sensible time to produce its full effect on the retina, founded on 

 the observations of Professor Wheatstone, whose experiments prove, that " the 

 light of electricity of high tension has a less duration than the miUionth part of a 

 second;" and that "the eye is capable of perceiving objects distinctly which are 

 presented to it during the same small interval of time."f 



It is obvious, however, that these statements are perfectly consistent with the 

 gradual action of light on the eye. For, although light may produce a certain effect 



repos, ou, en d'autres termes, rimpression approchera d'autant plus d'etre complete, que cet objet 

 aura plus de largeur, et que par suit il emploiera, dans son mouvement, un temps moins court a passer 

 devant I'oeil : ainsi Texperience, que nous venons de decrii'e, conduit de plus a cette consequence facile 

 a, prevoir que Ic developpemeni de I' impression directe est progressive guoigtie tres rapide." — (Essai d'une 

 Theorie Generale comprenant I'ensemble des Appareuces Visuelles, Sfc. par J. Plateau, p. 53. No-u- 

 veaux Memoires de V Academie Roy ale des Sciences et Belles Lettres de Bruxelles, tome viii., 1834.) 



A statement almost identical with this will be found in Plateau snr la Persistance des Impres- 

 sions de la Retine. Sxtpplement au Traite de la Lumicrc de Sir J. F. W. Herschel. Par A. Quetelet. 

 p. 474, 1833. See also Muller's Pki/sics, p. 274. London, 1847. 



The following is the passage in Beudant Cours do Physique, to which M. Plateau refers :— " C'est 

 aussi parce que I'impression d'un objet sur notre oeil ne se fait pas instantanement, que nous ne 

 pouvons aperccvoir un corps qui se meut avec une extreme vitesse. Ainsi par example, un boulet de 

 canon lance par une bouche a feu, est invisible pendant une grande partie de sou mouvement, parce 

 qu'il ne reste pas assez de temps dans un meme lieu, pour qu'on ait celui de I'apercevoir." 



M. Plateau observes, in the passage wliich has just been quoted, that it was easy to foresee « 

 ^iriori that the development of the impression of light on the eye is progi-essive, although very rapid. 

 With reference to this opinion, while it may be admitted that it is quite natural to suppose tliat the 

 action of light on the eye is not absolutely instantaneous ; yet, certainly, no one would be entitled to 

 conclude a priori that a sensible time is required to produce impressions on the eye. I have, therefore, 

 much satisfaction in availing myself of the present opportunity of directing attention to Lord Bacon's 

 prior claim to the merit of pointing out the curious and interesting fact, that light requires an ap- 

 preciable time to produce visual impressions on the eye. 



* It may be supposed that a diifcrent explanation of this effect might be aftbrded by the persist- 

 ence of the impression of the image of the ball on the eye. That this explanation is identical with 

 that given above, is evident from the image of the ball when seen projected upon the sky, being sen- 

 sibly black. For, since blackness is the negation of light, the persistence of a black impression is 

 but a want of light on that portion of the retina where the impression is perceived ; and the existence 

 of such an impression, or a want of luminosity after the eye is fully exposed to light, clearly proves 

 that its action on that organ is not instantaneous. 

 t Philosophical Transactions, 1834, p. 591. 



