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



the most perfect image will be equal to the duration of tHe 

 effect of a wave of light on the retina. 



3. We will begin by directing our attention to the effect of 

 a continuous light upon revolving wheels. In this case we may 

 consider the successive changes through which the surface 

 passes until it comes again into the situation which it at first 

 occupied, as constituting an infinite number of positions whose 

 differences are indefinitely small, i. e. that a whole revolution 

 comprises an infinite number of such positions. The action 

 of a continuous light, on the contrary, is made up of a series of 

 successive waves of appreciable length, and separated by a 

 definite though exceedingly small interval. Now a small 

 number of waves of light acting successively will affect the 

 same nerves of the retina, or nearly so, and thus produce either 

 the same image, or one differing from it merely by being at- 

 tended with a certain amount of confusion ; and thus will act 

 in the same manner as though the representations on the disc 

 were confused in their outline. During the time of the dura- 

 tion of this small system of waves, the disc will pass through 

 a small portion of a revolution which we will represent by 



.'. — , and consider the number of positions producing images 



on the retina during a revolution of the disc as m. 



The duration of the system of waves above mentioned being 

 very minute, we will consider that one image is impressed on 

 the retina at one definite epoch during the ??nh part of a revo- 

 lution, and neglect the modifications arising from the fact, that 

 the image is the result of severa/ successive waves. If there- 

 fore the images thus impressed upon the retina remained only 

 during an indefinitely short space of time, each position would 

 be appreciated by the eye, and the wheel would be distinctly 

 seen during its motion. We know, however, that such is not 

 the case ; each image impressed upon the retina remains 

 there for an appreciable length of time, during which the wheel 

 moves through a number of the positions above described. 

 Thus where the light is continuous, each system of waves im- 

 presses the image of a position on the retina, and a number 

 of these positions being depicted at once upon the optic nerves, 

 confusion will of course occur. The amount of this confusion, 

 from the nature of the case, depends upon the velocity of the 

 wheel. The effect of the motion is to reflect to the eye a num- 

 ber of successive positions, producing different effects at the 

 same time, and the intensity of each image so produced, di- 

 minishes as the lapse of time, from its first impression, be- 

 comes greater, and vanishes when that time is equal to the 

 retention of the image on the retina (i. e. ?i"). It is evident, 



