LUMINOUS IMPRESSIONS ON THE EYE. 



593 



3. The mean of a similar set of experiments made with two discs, one having 

 a single sector, and the other two sectors of 15°, gave r^ = 2-099. 



4. Two discs, one having a single sector, and the other two sectors of 



7°*30' were compared, and it was found that ■^= 1"851. 



The results of the experiments in the last four sections are shewn in the ac- 

 companying Table. 



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 



it 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. XVI. PART v. 7 P 



