LUMINOUS IMPRESSIONS ON THE EYE. 593 
3. The mean of a similar set of experiments made with two discs, one having 
asingle sector, and the other two sectors of 15°, gave 2 = 2:099. 
4. Two discs, one having a single sector, and ie other two sectors of 
7 °30' were compared, and it was found that 2 = 1°851. 
1 
The results of the experiments in the last four sections are shewn in the ac- 
companying Table. 

Number of Brightness 
Flashes in a of Impres- 
Second. sion. 
20 1-000 
40 2-006 
40 1-851 
60 3°169 
In this table, as in the last, the corresponding numbers in the opposite columns 
will be seen to be almost exactly proportional ; and both sets of experiments there- 
fore, lead to the following results :-— 
1. The brightness of the impression produced by equal flashes of light, which 
succeed each other so rapidly as to produce a uniform impression on the eye, is 
exactly proportional to the number of flashes in a given time. 
2. Within the limits of the different velocities of the discs in the experiments, 
the effect of the combination of the flashes is not sensibly affected by the length 
of the dark intervals between them. 
3. With the same limitation, the effect is also independent of the time of 
duration of the flashes. 
IV. On the connexion between the apparent Brightness of Light and the time during which 
at continues to act on the Eye. 
It has thus been proved that the brightness of the impression produced by 
rapidly succeeding flashes of light is proportional to the number of flashes in a 
given time, provided the brightness of the flashes remains constant. Hence, if a 
rapidly revolving disc, with a sector of a given angle, has its velocity doubled, 
and, consequently, the number of flashes produced by it in a given time also 
doubled, if the brightness of the flashes remains unaltered, the brightness of the 
impression produced by them will be twice as great as at first. But, instead of 
the brightness of the impression increasing, it has been found to continue un- 
changed, notwithstanding the increased velocity. It is, therefore, evident that 
when the velocity of the disc is doubled, and, consequently, the duration of each 
flash is half as great as at first, its brightness is also half as great as at first. 
Thus, if the disc first revolves 20 times in a second, and then 40 times in a second, 
VOL. XV i. PART V. (EP 
