LUMINOUS IMPRESSIONS ON THE EYE. 597 
In fig. 3, the line ae contains the projections of these observations, which are 
denoted by crosses ; and it nearly coincides with the line a 6, containing the projec- 
tions of the observations in the last Table, shewing that the ratio of the brightness 
of an impression of given duration to that of the absolute brightness of the light 
which produces it, is almost exactly the same in both sets of experiments. On 
thus comparing the apparent intensities of the flashes exhibited in the above Table 
with the similar results in the preceding one, it will be seen that although the ab- 
solute intensity of the light is only half as great as formerly, the time required for 
the propagation of the luminous impression on the eye remains unaltered ; while 
both sets of experiments prove that the brightness of a luminous impression caused 
by a light of given intensity is proportional to the time during which the light 
acts on the eye. 
On repeating the experiment with the sector of th of the circumference, re- 
volving once in a second, with the fixed light 24 inches from the screen, the mean 
of ten trials gave 
d, = 22-27; d, = 48-46; 6 = 0-2112. 
The ratio of the apparent brightness of the flashes to that of the light seen 
by continuous vision is, in this case, almost exactly the same as in the preceding 
experiments, as will be seen from the following comparative view :— 


Time during which | Xatio of the bright- 
Distance of | Intensity of aera 7 ness of Flashes 
y Light acted on 
Light. Light. the Hye. to that of the Light 
seen continuously. 

6:0 1:0000 0”:02 0:2061 
8:5 0-5000 0”-02 0:2240 
24:0 0:0625 0”:02 0.2112 






The conclusion to be derived from these results will be distinctly apprehended 
by reference to fig. 3, where the ordinates e/, cf, and df, represent the ap- 
parent intensities of the lights shewn in the above table. In order to pre- 
vent misunderstanding, it is necessary to observe, that although the absolute 
brightness of the lights used in the three experiments given in the table are in 
the ratio of the numbers 1, 2, and 16, they are all represented in the figure by 
the same line ag; and since the lines ef, cf, df, are nearly equal, they may be 
regarded as having the same ratio to ag, the slight differences between them ob- 
viously resulting from errors of observation. It thus appears that after an inter- 
val of ath of a second, the three lights of very different intensity have all pro- 
duced the same portion of their total effect on the eye; the impression in each case 
having nearly 1th of the absolute brightness of the light. 
Lights of different intensity, therefore, produce like portions of their total effect 
VOL. XVI. PART y. iQ 
