LUMINOUS IMPRESSIONS ON THE EYE. 591 
sectors. The distance of this light was noted, and, when the discs revolved ra- 
pidly, the light was withdrawn until the apertures seemed again equally bright. 
The degree in which the brightness of the light required to be diminished, by with- 
drawing the flame from the screen, in order to equalise the brightness of the im- 
pressions produced by the two discs, was taken as a measure of the ratio of the 
brightness of those impressions when the screens were equally illuminated. Four 
experiments were made, and, as formerly, putting ¢,, and d, to denote the dis- 
tances of the light from the screen before and after the disc had been made to 
revolve, and ¢ to denote the ratio of the apparent brightness of the apertures seen 
during the revolution of the discs when their illumination was actually equal, the 
mean values of those quantities was found to be 
d, =13°82; d,=2013; @=2-122. 
In a second set of four experiments, 
d,=4:075 ; d,=5'95; 9=2°132. 
4, When a disc with three equidistant sectors of 7° 30’ was compared with 
a disc having a single sector of 7° 30’, the mean of four experiments gave 
d,=14:15; d,=24175; 9 =2:920. 
In other four experiments, 
d,=41; d,=715; o=3-041. 
5. A disc with two sectors of 30°, compared four times with a disc having a 
single sector of 30°, gave 
d,=4:05; d,=61; 9=2-269. 
and a second set of four experiments, 
d,=13-95; d,=20; 0=2°056. 
6. A disc with four sectors of 30°, compared four times with a disc having a 
single sector of the same angle, gave 
d,=3-95 ; dy=7-925; 0 = 4-026, 
and a second trial of four experiments, 
@,=141; ¢d,=29°-4; Q =4:348. 
In all these experiments, the discs revolved so rapidly, as to produce a uni- 
form impression on the eye, but the equality of illumination, when once obtained, 
was not affected by increasing the velocity; and from the variation of the quan- 
tity d, in the different experiments, it will be seen that the results are inde- 
pendent of the intrinsic brightness of the light incident on the screen. 
The following table exhibits the mean values deduced from experiments 
No. 3 to No. 6 inclusive, and shews that the brightness of the impression pro- 
duced by rapidly succeeding flashes of light of a given intensity, is sensibly pro- 
portional to the number of flashes in a given time. 
