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 bright than at those parts of the retina 
where the action of the light had not been interrupted by the passage of the dark 
body. Itimmediately 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 theaction 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 will 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 millionth 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.” + 
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, l’impression approchera d’autant plus d’étre compléte, que cet objet 
aura plus de largeur, et que par suit il emploiera, dans son mouvement, un temps moins court 4 passer 
devant l’wil : ainsi l’expérience, que nous venons de décrire, conduit de plus a cette conséquence facile 
i prévoir que le développement de V' impression directe est progressive quoique trés rapide.’ —( Essai dune 
Theorie Génerale comprenant Vensemble des Apparences Visuelles, &c. par J. PuatEau, p. 53. Nou- 
veauz Memoires de ’ Academie Royale des Sciences et Belles Lettres de Brumelles, tome vui., 1834.) 
A statement almost identical with this will be found in Piatgeau sur la Persistance des Impres- 
sions de la Rétine. Supplement au Traité de la Lumiére de Sir J. F. W. Herscner. Par A. QuETELET. 
p. 474, 1833. See also Mutusr’s Physics, p. 274. London, 1847. 
The following is the passage in Brupant Cours de Physique, to which M. Puatzav refers :—“ C’est 
aussi parce que l’impression d’un objet sur notre ceil ne se fait pas instantanément, que nous ne 
pouvons apercevoir un corps qui se meut avec une extréme vitesse. Ainsi par example, un boulet de 
canon laneé par une bouche a feu, est invisible pendant une grande partie de sou mouvement, parce 
qwil ne reste pas assez de temps dans un méme lieu, pour qu’on ait celui de l’apercevoir.” 
M. Prareav observes, in the passage which has just been quoted, that it was easy to foresee a 
priori that the development of the impression of light on the eye is progressive, although very rapid. 
With reference to this opinion, while it may be admitted that it is quite natural to suppose that 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 different explanation of this effect might be afforded 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. 
+ Philosophical Transactions, 1834, p. 591. 


