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 will 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 light 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 Mutuzr’s Physiology, London 1839, nor in 
the Supplement to that work by Baty, London 1848. M. Prarzav also observes: “‘ Personne n’a 
essayé de mésurer le temps nécessaire 4 la production compléte de impression.” Prareav Sur la 
Persistance des Impressions. (Supplément au traité de la Lumiere de Sir J. F.W. Herscner. Par 
A. Queretet, p. 474, 1833.) 
