Mr. Milward on the Action of Light on Revolving Discs. 453 



must produce a clear image on the retina, which will of course 

 convey no idea of motion, we shall immediately perceive 

 from the experiment in question, that the duration of the elec- 

 tric spark does not exceed the period of revolution of the disc 

 through an exceedingly small space (that is through the rnih. 

 part of a revolution); and if it can be proved that the electric 

 fluid passes through a definite space during the exceedingly 

 short duration of the light, it follows that it moves with a ve- 

 locity bearing a known relation to that with which the cir- 

 cumference of the disc describes the mth part of a revolution. 

 It will be seen, however, that by this experiment nothing more is 

 proved than that the duration of the light is exceedingly short. 

 The main point depends upon the consideration of the actual 

 and definite motion of the fluid during that short duration; 

 and this I apprehend cannot be proved by the spark from a 

 Leyden jar ; for though that spark evidently arises from the 

 motion of the fluid, we have no proof at all that the fluid has 

 moved during the interval for which the spark is in existence, 

 and it is upon this point that the value of the experiment 

 must depend. 



The case is different when the electric discharge takes place 

 in the form of a stream of light ; for here we have good rea- 

 son to believe that the fluid causes a succession of sparks 

 along the line of its progress ; and consequently that it has de- 

 scribed the space between the first and last spark, or the two 

 extremities of the apparent stream, during the collective dura- 

 tion of the continuous light. If therefore such a lio-ht pro- 

 duces the same effect as the spark of the Leyden jar when 

 tested by the wheel, we have high presumptive evidence 

 that the fluid has moved through the length of the apparent 

 stream of light, while the disc has described only the mih part 

 of a revolution. The same is the case with a flash of lio-ht- 

 ning, which is the agent mentioned by Mrs. Somerville. It 

 is probable, however, that the experiment has never been ac- 

 tually tried. I have termed the evidence of the above-men- 

 tioned motion only highly presumptive, because in our present 

 uncertainty as to the manner in which the electric light is pro- 

 duced (whether by the compression of the medium, through 

 which it passes^ or otherwise) we have no absolute proof that 

 the light and motion are connected in the manner which we 

 have supposed. However this may be, it would appear that 

 the experiment when performed with the Leyden jar proves 

 nothing, the main point being left out of consideration. 



I may here mention another experiment, the aim of which 

 according to some persons is to prove the velocity of the elec- 

 tric fluid, — I refer to that of passing the electric current along 



