Mr. Mil ward on the Action of Light on Revolving Discs. 4-55 



The light is afforded by means of an extended wire of consi- 

 derable length, from interruptions in which sparks are emitted. 

 Immediately opposite this is placed a circular disc obliquely 

 inclined to its axis, so that no two of its positions are in the 

 same plane, and two successive I'ays of light will be reflected 

 in different lines. Hence it is evident, that unless the sjjai'ks 

 emitted from the parts of the wire be separated by so indefi- 

 nitely small an interval of time that the wheel cannot in that 

 time move through an appreciable space, the light thus falling 

 on the disc will be reflected in different directions, and thus 

 we have a test of the utmost delicacy. 



It has been suggested that this experiment may be applied 

 to the purpose of measuring the velocity, and with much greater 

 chance of success than that which I have previously mentioned ; 

 for if the light be reflected from the disc upon a screen, the 

 space between the projection of the rays upon it may be mea- 

 sured ; and since the position of the screen with relation to the 

 disc is known, we may calculate the angle of revolution ac- 

 tually passed through by the latter. If therefore the velocity 

 of the wheel be known, the exceedingly short period in which 

 it passes through the angle just determined, may be likewise 

 calculated ; and it is evident that this is the period required 

 by the electric fluid for its passage along the wire, by which 

 means the velocity will be determined. 



Let us now turn our attention to an experiment of a very 

 beautiful description, which is more immediately connected 

 with our subject. Of the name of the author I am not aware, 

 but I once saw it performed by Mr. Janson of Pennsylvania, 

 near this city, and was particularly struck with it ; for from 

 the concealed nature of the cause it appears at first sight al- 

 most inexplicable. It consists in illuminating a rapidly re- 

 volving disc by means of the light emitted in the circuit of 

 Clarke's magnetic electrical machine, when a singular effect 

 is produced which varies according to the velocity with which 

 the machine is turned. The two velocities may be so adjusted, 

 that the disc though in rapid revolution appears to be ab- 

 solutely at rest, and its surface distinctly visible. By decrea- 

 sing the velocity the disc appears to move slov/ly Jbrwardy 

 while by increasing it a retrograde movement appears to take 

 place. 



The cause of this depends entirely upon the nature of the 

 light. When the machine moves slowly we perceive that a suc- 

 cession of sparks is emitted at the surface of the mercury; but 

 as the velocity becomes greater, tiie light appears continuous, 

 in consequence of the duration of the efl'ect upon the retina ; 

 and here, as in an experiment previously mentioned, the disc 



