^{page 581 note *} “At in visu (cujus actio est pernicissima) liquet etiam requiri ad eum actuandum momenta certa temporis: idque probatur ex iis, quae propter motûs velocitatem non cernuntur; ut ex latione pilae ex sclopeto. Velocior enim est praetervolatio pilae, quam impressio speciei ejus quae deferri poterat ad visum.”—(Novum Organum, lib. ii., Aph. xlvi. Bacon's Works, vol. i., p. 370. Lond. 1711.)

^{page 581 note †} This appears from the following passages, which will be found to contain little more than a repetition of Lord Bacon's statement:—

“ Il est un fait auquel on a généralement accordé peu d'attention, quoiqu'il ait été remarqué (voyez Essai d'un Cours Élémentaire et Général des Sciences Physiques, par M. Beudant: Partie Physique, p. 489 de la 3me edition), c'est que les impressions directes exigent un certain temps pour se développer sur la rétine. Pour se convaincre de la réalité de ce fait, qui devait naturellement se prévoir a priori, il suffit de se rappeler qu'un objet qui passe très rapidement devant l'œil, ne se voit pas, ou s'aperçoit à peine. On peut encore prouver la chose par l'expérience suivante. Si l'on fait mouvoir circulairement, devant un fond noir, un petit morceau de papier blanc, avec une vitesse telle que l'anneau apparente qu'en résulte présente une teinte parfaitement uniform et tranquille, cet anneau ne parâitra pas blanc, mais gris. Or il suit de l'uniformité de la teinte, que pendant le petit intervalle de temps qui sépare deux passages successifs de l'objet au même point, l'impression ne décrôit pas d'une quantité sensible: il faut done nécessairement admettre que cette impression n'est pas blanche, comme celle qui est produite par l'objet en repos, mais qu'elle est grise, e'est-a-dire d'une blancheur imparfaite, ou enfin qu'à raison du temps extrêmement court que l'objet emploie à passer devant l'œil, il ne produit qu'une impression incomplete. Il est inutile d'ajouter qu'on obtiendra des resultats analogues en employant un objet d'une couleur quelconque: toujours l'anneau parâitra plus sombre que l'objet en repos. L'éclat de l'anneau sera d'ailleurs d'autant moins éloigné de celui de l'objet en repos, ou, en d'autres termes, l'impression approchera d'autant plus d'être complète, que cet objet aura plus de largeur, et que par suit il emploiera, dans son mouvement, un temps moins court à passer devant l'œil: ainsi l'expérience, que nous venons de décrire, conduit de plus à cette conséquence facile à prévoir que le développement de l'impression directe est progressive quoique très rapide.”—(Essai d'une Theorie Génerale comprenant l'ensemble des Apparences Visuelles, &c. par J. Plateau, p. 53. Nouveaux Memoires de l'Academie Royale des Sciences et Belles Lettres de Bruxelles, tome viii., 1834.)Google Scholar

A statement almost identical with this will be found in Plateau sur la Persistance des Impressions de la Rétine. Supplément au Traité de la Lumière de Sir J. F. W. Herschel. Par A. Quetelet. p. 474, 1833. See also Muller's, Physics, p. 274. London, 1847Google Scholar.

The following is the passage in Beudant Cours de Physique, to which M. Plateau refers: —“C'est aussi parce que l'impression d'un objet sur notre œil ne se fait pas instantanément, que nous ne pouvons apercevoir un corps qui se meut avec une extrême vitesse. Ainsi par example, un boulet de canon lancé par une bouche à feu, est invisible pendant une grande partie de sou mouvement, parce qu'il ne reste pas assez de temps dans un même lieu, pour qu'on ait celui de l'apercevoir.”

M. Plateau 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 appreciable time to produce visual impressions on the eye.

^{page 582 note *} It may be supposed that a different explanation of this effect might be afforded by the persistence 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 sensibly 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.

^{page 582 note †} Philosophical Transactions, 1834, p. 591.

^{page 583 note *} Additional proof that almost no attention has hitherto been paid to this subject, may be derived from the fact, that no notice is taken of it in Muller's Physiology, London 1839, nor in the Supplement to that work by Baly, London 1848. M. Plateau also observes: “Personne n'a essayé de mésurer le temps nécessaire à la production complète de l'impression.” Plateau Sur la Persistance des Impressions. (Supplément au traité de la Lumière de Sir J. F. W. Herschel. Par A. Quetelet, p. 474. 1833.)

^{page 585 note *} The gas-light used in all the experiments described in this paper, was that of coal-gas burned by a No. 2 swallow-tail jet. It will be seen that the numerical results, afterwards obtained, do not depend on the absolute brightness of this light.

^{page 585 note †} Screens covered with black paper, which are not represented in the figure, were used to protect the eyes from the action of extraneous light, and also to intercept any rays, whose influence might have otherwise affected the accuracy of the experiments.

^{page 586 note *} This is conveniently done by means of a pulley and cord. When the apertures are being made equally bright before the disc is made to revolve, it is necessary that the aperture D should be fully exposed. Where the sector is too narrow to admit of the whole aperture being seen at once, another sector is cut in the disc for this purpose, which admits of being closed by a slider of pasteboard before the disc is made to revolve.

^{page 589 note *} I was enabled to make this experiment by the kindness of Mr Alexander Bryson, who, along with Mr John Turnbull, W.S., witnessed the results above described.

^{page 589 note †} In all experiments in which the discs had more than one sector, the sectors were arranged round the circumference at equal distances from each other.

^{page 596 note *} In an experiment made since this paper was read, I have found that the same law extends to impressions lasting for of a second, of which the observed brightness was 0·6118.

^{page 598 note *} By means of this arrangement, the brightness of the impressions produced during the revolution of the disc, can be compared with the light transmitted through the aperture B. Since the intensity of a ray of polarized light when transmitted through a doubly-refracting crystal, varies as the square of the cosine of the inclination of the principal section of the crystal to the plane of polarization of the ray; by attaching an index to the tube B D, so as to measure the angle through which it has been turned, the intensity of the transmitted light might be estimated, and thus the brightness of the impressions produced by the revolving disc might be determined. (See Supplément au Traité de la Lumière de Sir J. F. W. Herschel. Par A. Quetelet, p. 595.)

^{page 601 note *} As my object here is simply to discuss the possibility of explaining the personal equation by the gradual action of light on the retina, I have intentionally refrained from entering upon any explanation of that phenomenon which may be derived from the supposition that time is required for the transmission of impressions from the organs of sensation to the mind.

^{page 602 note *} The electric spark is a light whose intensity places it undoubtedly within the limits of the experiments on this point, as its brightness is inferior to that of sun-light. According to Sir John Herschel, the lime-ball light appears only as a black spot on the disc of the sun when held between it and the eye.—(See Treatise on Astronomy, Lardner's Cyclopædia, p. 210. London, 1835.

I have observed that, in like manner, the spark produced by a strongly-charged Leyden phial, is absolutely invisible when it passes between the eye and the sun's disc.

^{page 602 note †} See Light. Encyclopædia Metropolitana, Art 58.

^{page 602 note ‡} Experimental Researches in Electricity, vol. i., sec. 333. Lond. 1839.