LUMINOUS IMPRESSIONS ON THE EYE. 589 
about twenty revolutions in asecond. After this no increase of velocity, up to 128 
revolutions in a second, produced the slightest farther diminution of the apparent 
brightness of the light; and again, as the speed diminished the light continued 
uniformly bright, until the motion became so slow as to allow the eye to perceive 
the impressions separately, after which they gradually increased in intensity until 
the disc stopped. The same experiment was repeated, substituting for the flame 
of the candle an illuminated aperture in a screen, covered with tissue paper. The 
apparent brightness of this aperture, when the disc revolved, was compared, in the 
manner already described at p. 585, with that of another similar aperture seen by 
uninterrupted vision, and the result was perfectly in accordance with that ob- 
tained in the previous experiment. The same phenomena were also observed 
when a disc with a sector of 30° was substituted for that with a sector of 2° 30 .* 
A similar result was obtained by varying the form of the experiment, in the 
following manner :—Two discs, one with a sector of 30°, and the other with two 
sectors of 15°, suchas ABCD, abcd (Fig. 4), at opposite extremities of its dia- 
meter, were placed in the selaometer, and made to revolve simultaneously. In 
both discs the ratio of the duration of the flashes to that of the dark intervals, 
is obviously the same; but when the discs revolve simultaneously, for each flash 
produced by the disc with the sectors of 30°, there are two flashes of half the 
duration produced by the disc with two sectors of 15°. The disc with two sec- 
tors of 15°, revolving at a given velocity, is, therefore, precisely identical in its ac- 
tion to the disc with a single sector of 30° revolving at double the velocity. The 
apertures in the screen being made equally bright before the discs revolved, the 
equality of their brightness remained unaltered when the discs revolved so rapidly 
as to produce a uniform impression upon the eye. In the same manner, the 
brightness of the apertures remained equal when any disc, in the first part of 
the following table, revolved simultaneously with the corresponding disc in the 
second part, at such a velocity as to produce a uniform impression on the eye. 
——<—<$<$< 
Number of Sectors.t Angle of Sectors. Number of Sectors. Angle of Sectors. 











2 15° 1 30° 
4 7° 30’ 1 30° 
2 7° 30 1 15° 
2 30° 1 60° 
3 30° 1 90° 
4 30° 1 120° 



In all these cases, the duration of each flash was inversely as the number of 
* I was enabled to make this experiment by the kindness of Mr ALgxanper Bryson, who, 
along with Mr Joun Turnsutt, W.S., witnessed the results above described. 
+ In all experiments in which the discs had more than one sector, the sectors were arranged 
round the circumference at equal distances from each other. 
VOL. XVI. PART V. 70 
