LUMINOUS IMPRESSIONS ON THE EYE. 



589 



about twenty revolutions in a second. 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 imtil 

 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", such as ABCD, abed (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 simultaneouslj^ 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. 



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 Alexander Bryson, who, 

 along with Mr John Ttjrnbull, 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 fi-om each other. 



VOL. XVI. PART V. 7 O 



