456 Mr. Milward on the Action of Light on Revolving Discs. 



acts as a test, when the unassisted powers of vision are inca- 

 pable of appreciating the interval between the sparks. 



Let us first consider the case in which the disc appears sta- 

 tionary. By referring to our previous principle, that a spark 

 of very short duration produces a clear image of the disc, as 

 reflecting only one ?«th position, we may consider the effects 

 produced by a succession of such sparks, the intervals be- 

 tween which are definite and equal : and here it is evident that 

 each spark reflects an image to the eye which will be se- 

 parated from that which succeeds it, by the space through 

 which the wheel passes during the elapsed interval*. Now if 

 this interval be exactly equal to the time in which the disc 

 performs an entire revolution, it is evident that each spark as 

 it is produced will reflect the same image to the eye, and al- 

 ways in an identical position, so that the disc must appear sta- 

 tionary. Thus, in the first case, the phaenomenon arises from 

 the interval between the sparks being exactly equal to the 

 time in which the wheel revolves. In the second case, it is 

 clear, that if during the interval between the sparks the disc 

 describe a space exceeding a revolution by a very small quan- 

 tity, each spark will reflect to the eye the image of a position 

 slifhtiy in advance of the preceding, and thus the effect will 

 be that of a disc, which in place of revolving equally, is seen 

 only in successive positions, separated by the small quantity 

 above mentioned, and the interval between which will be 

 equal to the lapse of time between the successive sparks. The 

 cause of its distinctness depends not only upon the principles 

 which influence a disc in slow motion, viz. the small number 

 of ?Kth positions contained in a group, by which the range of 

 confusion is decreased, and also the disturbing effect of the 

 optic muscles, by which the eye spontaneously follows the re- 

 volving image, but also upon the advantage of the length of 

 time elapsing between the sparks (and therefore between the 

 formation of the images), which permits the effect of the last 

 image to decrease in intensity before its place is supplied by 

 another; thus diminishing the confusion. Thus in the se- 

 cond case, the velocity of the wheel being greater than the 

 interval between the sparks, the effect becomes that of a slowly 

 revolving wheel, whose motion is direct. 



By the same reasoning it is evident, that if the time of re- 

 volution be slightly greater than the interval between the 

 sparks, the space described will fall short of a revolution by a 

 very small quantity, and the effect will be the same as in the 



* I speak here of only one revolution, but it is evident that the phae- 

 noraenon will be the same if the wheel perform two or more revolutions 

 in the interval between the sparks : so also in the other cases. 



