454 Mr. Mil ward on the Action ofLigJit on Revolving Discs. 



a system of wires with various interruptions, when the sparks 

 must theoretically occur in succession, but which, in conse- 

 quence of the velocity of the fluid, all occur at the same time. 

 This however only proves that the eye cannot estimate the 

 lapse of time between the several sparks, and confounds them 

 together ; or in other words, that the time of passing from one 

 extreme of the wire to the other, is less than the duration of 

 the effect of the first spark upon the retina. But this would 

 not show that the velocity of the fluid so greatly surpasses 

 that of ordinary agents, since the same effect may be produced 

 by a velocity proportionally moderate. 



It seems evident that by combining the two experiments we 

 shall obtain a satisfactory test of the velocity in question ; for 

 if we illuminate the disc by a succession of sparks passing from 

 distant parts of an interrupted wire of considerable extent, we 

 shall be able to ascertain whether the sparks do or do not 

 succeed one another with such extreme rapidity, since, as we 

 have previously observed, the disc acts as a delicate test of 

 the duration of light. If the lapse of time between the first 

 and last spark do not exceed the period in which the wheel 

 describes the ?«th part of a revolution, the whole of the sparks 

 will be effective in reflecting only one of these mth. positions 

 to the eye, and therefore the image will be comparatively 

 clear and unconfused ; but should this prove otherwise, i. e. 

 should the velocity of the wheel be less than the above hypo- 

 thesis premises, a multiple number of ?«th positions would be 

 acting at once, though in different degrees, upon the retina, 

 and confusion would to a greater or less extent arise. 



We may consider also, that by increasing the velocity of the 

 wheel and the distance between the points of interruption of 

 the wire, an attempt might be made to obtain an approximate 

 value of the velocity of the electric fluid, for we may suppose 

 the length of the wii'es to be so greatly increased that the 

 sparks proceeding from them should produce a confused image 

 of the disc, while a less length of wire affords comparatively 

 no confusion ; thus we shall have as a limit a length of wire 

 greater than the space which measures the electric velocity; and 

 since we may estimate within certain limits the space which 

 we term the mth portion of a revolution, we can establish an 

 approximate ratio between the velocities of the disc and the 

 electric fluid ; thus by knowing the velocity of the former, 

 which may be mechanically measured, the approximate value 

 required may be obtained. 



A beautiful experiment somewhat similar to the combina- 

 tions which we have just considered, has been the result of 

 Professor Wheatstone's exertions connected with this subject. 



