LUMINOUS IMPRESSIONS ON THE EYE. 599 
to that of the uninterrupted light, according as the apparatus was illuminated by 
sun-light or by gas-light. Therefore, since it has been shewn that this was not the 
case, it is evident, that the sun-light and gas-light produced similar portions of 
their complete impressions on the eye with the same rapidity. 
It has thus been proved, that, when light acts on the eye for short intervals 
of time, the rapidity of the development of its impression is independent of its ac- 
tual brightness ; and it seems highly probable that this law extends to the whole 
time required for the complete production of luminous impressions. For, when 
it has been found, that lights of very different intensity acting on the eye during 
zoth of a second, all produce impressions, having almost exactly }th of the ab- 
solute brightness of the lights, it seems natural to conclude, that they will also 
produce their complete effect on the eye in exactly equal times. 
I hoped to have been able in this paper to exhibit the results of some experi- 
ments upon the intensity of impressions of short duration, repeated by different 
individuals, in order to ascertain whether the rapidity of the production of visual 
impressions varies much in different eyes. I have only obtained one comparison 
of this kind, through the kindness of Mr AtexanpER Wattace, of the Royal Ob- 
servatory, Edinburgh, who observed the impression produced on his eye by a disc 
with a sector of 7° 30, revolving 20 times in a second. The following result is 
the mean of three trials, 
d, =4°39 ; d,=42°5 ; 6=0:01067. 
The result of my own experiments gives b=0:0137 : which agrees very well 
with Mr Wattace’s observations. I trust to be able to obtain some more com- 
parisons of this kind, in order to ascertain whether the agreement between Mr 
WALLACE’s result and my own is to be regarded as proving that visual impressions 
in the eyes of different individuals, are propagated with nearly equal rapidity. 
VII. On the time which Light requires to produce a full impression on the Eye. 
I have found, by means of a disc revolving once in a second (see p. 594), that 
impressions produced by a light acting on the eye for 1th of a second, have very 
nearly the same brightness as the light seen by continuous vision ; but that when 
light acts on the eye for a shorter time, its apparent brightness is sensibly di- 
minished. As the brightness of the impression produced by light increases by in- 
sensible degrees, until, at length, it attains its full intensity, it is obviously al- 
most impossible, by direct observation, to assign the exact instant when this 
takes place. The experiments in Sections V. and VI. have proved, that, up to 1th 
of a second, the brightness of a luminous impression is strictly proportional to the 
time during which light has acted on the eye, and also, that the impression produced 
iN zAoth of a second, has almost exactly jth of the brightness of a full impres- 
