LUMINOUS IMPRESSIONS ON THE EYE. 595 
1. The fixed light was placed six inches behind the screen before which the 
disc revolved. The sector had an angle of 0° 27’, or 34, of the circumference of 
the disc; and the disc revolved once in a second. To equalise the brightness of 
the apertures in the screens, when both were seen by continuous vision, the light 
behind the second screen was placed at a distance (d,) of 5:3 inches. But, 
when the disc revolved, this light had to be withdrawn to a distance (d,) of 46:3 
inches. This experiment was repeated ten times, with the following mean 
results: d,=5:11; d,=50'29; and the brightness of the flashes 6 =0:0103, the 
brightness of the light seen by continuous vision, being unity. 
2. A sector of 0°54’, or 4. of the circumference, was next used, and ten 
400 
experiments made as before, from which 
d, = 5:23; d,= 35°24 and 6=0:022. 
3. With a sector of 1° 48’, or ,3, of the circumference, 
d,=5:19; d,=23'52; 6=0-0487. 
4. With a sector of 3° 36’, or ;3, of the circumference, 
d,=4:9; d,=15-08; 6=0-1056. 
. With a sector of 7° 12’, or 2, of the circumference, 
d,=5'13; d,=11-3; 6=0-2061. 
6. With a sector of 15°, or 3, of the circumference, 
d,=5:02; d,=764; 6=0-4317. 
or 
With each sector, the mean of ten results was taken; and at each successive 
trial, the flame was alternately drawn from the screen, or pulled towards it in 
equalising the apparent brightness of the apertures in the screens. The following 
Table contains the results of these experiments, the brightness of the light seen 
by continuous vision being expressed by unity :— 


Duration of Flash} Brightness of 
in Seconds. Impression. 





0:00125 0:0103 
0:00250 0:0220 
0:00500 0:0487 
0:01000 0-1056 
0:02000 0:2061 
0:04167 0:4317 




The results of these experiments are shewn in fig. 3, where the observed 
intensities of the light, denoted by small circles, are projected with the corre- 
sponding times during which it acted on the eye; and it will be observed, that the 
line a ¢ 6, shewing the increase of the apparent brightness of the object, with the 
time during which it remains visible, is very nearly straight; which proves that 
