596 MR WILLIAM SWAN ON THE GRADUAL PRODUCTION OF 
within the limits of the observations the brightness of the light increases in exact 
arithmetical proportion with the time during which it acts on the eye. Since the 
observed intensities of the lights when projected, as in the figure, are all nearly 
included in a straight line passing through the origin, it may naturally be in- 
ferred, that the impression of light commences at the instant of its incidence 
on the retina. This conclusion is strengthened, when it is recollected that the 
preceding experiments prove that light, which is incident on the eye only IP 
of a second, produces a distinct impression, while, according to Professor WHEat- 
STONE, less than the millionth part of a second is necessary for this effect. It has 
also been proved (see p. 594), that up to ;,1,, of a second, the impression produced 
by light is proportional to its duration. It seems, therefore, highly probable, that 
from 0” up to 0:05, the brightness of a luminous impression is exactly proportional 
to the teme during which the light has acted on the eye.* 
These experiments, therefore, confirm, in a very satisfactory manner, the 
inference which has already been drawn from the previous investigation, as the 
observed intensities of the flashes are very nearly proportional to their duration ; 
while, at the same time, they exhibit the actual numerical ratio of the apparent 
brightness of a flash of a certain duration, to that of the light which preduces it 
acting continuously on the eye. 


VI. The time required for the complete production of Luminous Impressions is independent 
of the apparent intrinsic brightness of the light. 
The following series of experiments was made partly to confirm the result al- 
ready obtained ; but more especially in order to ascertain whether the time re- 
quired for the complete development of luminous impressions varies with the 
brightness of the light by which they are produced. In this set of experiments, the 
same sectors were used as in the last ; and the circumstances were identical, except 
that the fixed light was placed 8°5 inches (64/ 2) from the screen, so that the 
brightness of the incident light was reduced to half its former intensity. The fol- 
lowing Table exhibits the mean of ten observations with each sector; the bright- 
ness of the light seen by continuous vision, being expressed by unity. 
Duration of Brightness of 
Flash. Flash. 
0-00125 0-0130 
0:00250 0:0275 
0-00500 0:0508 
0:01000 0:0991 
0:02000 0:2240 

* In an experiment made since this paper was read, I have found that the same law extends to 
impressions lasting for ~,th of a second, of which the observed brightness was 06118. 

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