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XXV. Observations on the supposed Vision of the Blood-ves- 

 sels of the Eye. By Sir David Brewster, L.L.D., F.R.S.* 

 TN the Number of this Journal for September 1832, I had 

 •*- occasion to refer to the remarkable experiment described 

 by Dr. Purkinje of Breslau, in which the blood-vessels of the 

 retina are supposed to be exhibited; and though I had in vain 

 tried to see this phaenomenon, yet it had been so accurately 

 described to me by Mr. Potter that I ventured to give an 

 opinion respecting its cause. The paper which contained this 

 explanation was read at the Physical section of the British 

 Association at Oxford in June 1 832, and Mr. Wheatstone, who 

 was present, favoured the Meeting with some excellent obser- 

 vations on the subject. These observations have been printed 

 in the Report of the Association for 1832, in the form of an 

 Appendix to the abstract of my paper; and as they are highly 

 interesting, and will form the groundwork of the following ob- 

 servations, 1 shall give them verbatim. 



"After the reading of Sir David Brewster's paper, Mr. Wheatstone said, 

 that having been the first person to introduce Purkinje's beautiful experi- 

 ment into this country, and having repeated it a great number of times un- 

 der a variety of forms, he would take the opportunity of stating a few par- 

 ticulars respecting it, which appeared not to be generally known. The ex- 

 periment succeeds best in a dark room, when, one eye being excluded from 

 the light, the flame of a candle is placed by the side of the unshaded eye, 

 but so as not to occupy any of the central part of the field of view. So 

 long as the flame of the candle remains stationary, nothing further occurs 

 than a diminution of the sensibility of the retina to light; but after the 

 flame has been moved upwards and downwards, through a small space, for 

 a length of time, varying with the susceptibility of the individual on whom 

 the experiment is tried, the phaenomenon presents itself. The blood-vessels 

 of the retina, with all their ramifications, exactly as represented in the en- 

 gravings of Soemmerring, are distinctly seen, apparently projected on a plane 

 before the eye, and greatly magnified. The image continues only while the 

 flame is in motion : directly, or soon after, the flame becomes stationary, 

 it dissolves into fragments and disappears. 



" Mr. Wheatstone dissented from the ingenious explanation of this ap- 

 pearance offered by Sir David Brewster, and also from that opinion stated 

 to be the generally received one ; and begged to repeat the solution he had 

 published, and which he had not since been induced to relinquish. Mr. W. 

 observed, that there was no difficulty in accounting for the image; it evi- 

 dently was a shadow resulting from the obstruction of light by the blood- 

 vessels spread over the retina; the real difficulty was to explain why this 

 shadow is not always visible. To account for this, Mr. W. adduced several 

 facts, which tended to prove that an object, either more or less luminous 

 than the ground on which it is placed, when continuously presented to the same 

 point of the retina, becomes invisible ; and the rapidity of its disappearance is 

 greater as the difference of luminous intensity between the object and the ground 

 is lets ; but by conlinwilly shifting the place of the image of the object on the 

 retina, or by making it act intermittently on the same point, the object may be 



* See Lond.and Edinb. Phil. Mag., last Number, p. 43, and also vol. i. 

 p. 318. 



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