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



m\h positions will be likewise augmented, and confusion will 

 in proportion arise. 



It is on these principles that a popular toy has been con- 

 structed, by which various deceptions are represented in a 

 pleasing manner. It consists of two circular discs, on one of 

 which are represented figures, or other objects, in various po- 

 sitions ; while the other, which is larger, has a number of small 

 holes pierced at equal intervals around its circumference. 

 Both are screwed firmly together (the first disc being on the 

 outside) and held before a plane mirror ; whilst the eye is di- 

 rected towards the reflected image through the apertures in 

 the circumference, the whole being put in rapid revolution, 

 the figures appear in motion as the images ofthe different and 

 slightly varying positions depicted on the disc are successively 

 reflected through the revolving apertures to the eye. The 

 cause is easily explained. The holes perform the office of di- 

 viding the continuous light reflected from the mirror, into a 

 certain number of systems of very short duration. It is evi- 

 dent that we may consider the experiment as resolving itself 

 into that of two discs facing one another, and revolving simul- 

 taneously, the one containing the figures and represented by 

 the image on the glass, the other provided with the apertures. 

 As the second disc revolves, a few images of m\h positions pass 

 through each of the apertures during its transit ; and thus 

 each hole provides the eye with a small group of positions, 

 and these groups are separated by the interval between the 

 openings. Thus a confused image of the wheel is reflected 

 to the eye through each of the openings, and remains there 

 undisturbed, until as its effects upon the eye become very 

 weak, it is replaced by another indistinct image passing through 

 the succeeding aperture and slightly different in position. In 

 this manner images replace one another upon the retina, as 

 succeeding sounds in music do upon the ear. It is evident, 

 that if there be as many distinct forms on the disc as there are 

 openings, each holding the same relation to the apertures as 

 to position, the forms will appear stationary, in consequence 

 of the repetition of the same image upon the retina through 

 each of the apertures. 



Our attention may now be directed to the application of 

 the above principles to the purpose of proving the extreme 

 velocity of electricity, which is often said to be effected by 

 causing a disc to revolve with considerable velocity, and to 

 be then illuminated by a flash from a Leyden jar, when it is 

 seen in an apparent state of rest, with the whole of its surface 

 distinctly visible. Referring to the principle which we have 

 previously considered, viz. that a light of very short duration 



