LUMINOUS IMPRESSIONS ON THE EYE. 583 



on that organ in the millionth part of a second, it by no means follows that this 

 is its full "effect; and thus, while the electric spark renders objects distinctly 

 visible which are seen for less than the millionth part of a second, it may still be 

 true that the apparent brightness of those objects would increase if the duration 

 of the light could be prolonged. 



Having found only very brief and general references, to the gradual action 

 of light on the eye, in any authors to whose works I have had access, I resolved 

 to investigate the subject experimentally; and the object of this paper is to de- 

 scribe a series of experiments undertaken for the purpose of ascertaining the con- 

 nexion between the brightness of the impression produced by light on the retina, 

 and the time during which it acts on the eye.* 



Before entering upon the narrative of my experiments it may be proper to 

 premise, that if, in some cases, I seem to assume that the results obtained by 

 experiments on my own eyes, are to be regarded as universal phenomena, I do 

 so merely to avoid circumlocution; and I believe I may plead the example of 

 most writers who describe experiments on vision in justification of such an ap- 

 parent assumption. 



In several cases, however, some of which wiU be afterwards noticed, the ex- 

 periments have been witnessed by others, whose concurrent testimony has proved 

 that the results were not dependent upon any idiosyncrasy of vision on my part. 

 I may also have made use of expressions which seem to involve the assump- 

 tion that the brightness with which a luminous object is seen at any instant, is 

 the same as the apparent brightness of its image on the retina at that instant ; or, 

 in other words, that the impression of light on the retina is perceived by the mind 

 instantaneously. Such expressions, however, are employed simply for the sake of 

 brevity. The principal object of my investigation is to determine the brightness of 

 an impression made on the retina by a light of a given intensity, acting for a 

 given time ; and it will be found in the sequel that the method I have devised for 

 measuring the time during which the Ught acts, and the intensity of the resulting 

 impression, does not depend for its accuracy on the settlement of the question, 

 whether or not the impressions of light on the retina are instantaneously per- 

 ceived by the mind. 



I. Method of Observation. 



In order to examine the phenomena presented by luminous impressions of 

 short duration, I made use of the following method of observation. If a disc of 



* 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 Mullee's Physiology, London 1839, nor m 

 the Supplement to that work by Baxy, London 1848. M Pi-atkau also observes ;, ' ^^ «-^ ^ ^ 

 essaye de mesurerle temps necessaire ^ la production complete de J-Fe--- ^^^'^Z par 

 Persistance des Impressions. {Supplement an traite de la Lumiere de Sir J. F. W. Herschel. I-a, 

 A. QUETELET, p. 474. 1833.) 



