590 MR WILLIAM SWAN ON THE GRADUAL PRODUCTION OF 
flashes in a given time; and these experiments, therefore, confirm the result ob- 
tained by varying the velocity of a disc with a sector of a given angle. We may.. 
therefore, infer, | 
lst, That if the number of flashes, in a given time, succeeding each other so 
rapidly as to produce a uniform impression on the eye is inversely as the duration 
of each flash, their aggregate effect on the eye will be constant. 
2dly, This compensation of the diminished intensity, by the increased fre- 
quency of the flashes, is independent of the interval of time betweeen each im- 
pression, within the limits of the observations; that is, with intervals varying from 
goth to zgth of a second. 
3dly, The effect is also independent of the ratio of the duration of the lumi- 
nous to that of the dark intervals. 
It is thus shewn that, after a uniform impression is produced, increasing the 
number of flashes in a given time, compensates for their diminished intensity. 
This naturally leads to the inquiry, at what rate would the brightness of the 
resulting impression increase with the number of flashes in a given time, sup- 
posing the intensity of the flashes to remain constant? In order to ascertain the 
connexion between the number of flashes of a given intensity in a given time, and 
the intensity of their combined effect on the eye, I made the following experi- 
ments :— 
1. Two discs, one with a sector of 15°, and the other with two equidistant 
sectors of 15°, as ABCD, abcd, Fig. 4, were placed in the selaometer, and the 
screens were equally illuminated by carefully adjusting the distances of two simi- 
lar spermaceti candles placed behind them. When the discs were made to re- 
volve so rapidly as to produce a uniform impression, a second candle placed be~ 
hind the screen, whose disc had a single sector of 15°, restored the equality of the 
apparent brightness of the apertures, and a similar rest was obtained when 
discs with single sectors of 30° and 7° 30’, were compared with discs having two 
sectors of the same angles. 
2. To vary the experiment, the flame of a gas burner was placed 10 inches 
behind the screen, whose disc had two sectors of 30,° and a similar flame was ad- 
justed behind the screen, whose disc had a single sector of 30°, so as to illuminate 
the apertures in the screens equally. When the discs revolved rapidly, the flame 
behind the disc with two sectors was withdrawn to 14:1 inches (10 / 2): so as 
to halve the intensity of the light incident on the screen, and the illumination of 
the apertures appeared to be perfectly equal. In like manner, when the light was 
first placed at 20 inches from the screen, after the discs revolved, the screens 
seemed equally illuminated when it was withdrawn to 28-2 inches. 
3. A disc with a single sector of 7 30, and another with two sectors of the 
same angle, were placed in the selaometer. The illumination of the screens was 
then made equal by adjusting the distance of the light behind the disc with two 

