592 



MR WILLIAM SWAN ON THE GRADUAL PRODUCTION OF 



In these experiments it is assumed, that when the light is withdrawn from the 

 screen, so as to diminisla the intensity of its illumination, the brightness of a flash 

 of short duration will be diminished in the same ratio. This will only be true, 

 provided lights of different intensity produce impressions of proportional inten- 

 sity in equal times ; but if it be afterwards proved, independently of these obser- 

 vations, that such is really the case, the conclusion which has now been drawn 

 from the experiments will be perfectly correct. In order to avoid this assumption, 

 the brightness of the impressions produced by the revolution of the different discs 

 was next compared with the impression of uninterrupted light. 



1 . A disc, with a single sector of 30 , was placed in the selaometer, with a gas- 

 tlame at a constant distance of 6 inches from its screen. In order to render the 

 illumination of the screens equal, the other light had to be placed at a distance of 

 5-y inches from the second screen. When the disc revolved 20 times in a second, 

 the latter light was gi-adually withdrawn until the apertures in the screens again 

 appeared equally bright. The distance of that light was now found to be 22'2 inches. 

 This experiment, eight times repeated, gave the following mean values : — 



f/, = 5-78 



d.^ = 22-21 



6, = 0-06773. 



where h^ denotes the brightness of the impression produced during the revolution 

 of the disc. In like manner, a disc, with two sectors of 30 , treated in precisely 

 the same manner, gave the following mean values of eight trials : — 



rf, = 5-61 ; d., = 15-22 ; b.^ = 01359. 



b., denoting the brightness of the impression produced by the revolution of this 

 disc. From this experiment, the ratio of the brightness of the impressions pro- 

 duced by the revolution of the two discs, or -^ "^ill ^^ found to be 2-006. 



2. A disc, with three sectors of 30 , was compared with direct light in the 

 same manner. The mean of eight experiments gave 



rf, = 5-61 ; rf. = 12-11 ; 63= 0-2164. 



From which the ratio of the brightness of the impression produced by means 

 of this disc, with three sectors, to that produced by means of the disc with a single 



sector, or -r, is 3169. 



