250 Prof. C. V. Boys on Photographs of Rapidly Moving 



exposure was more than enough to make a perfectly black 

 and white negative, and the outlines were clear and sharp. 

 Another series taken at the rate of twenty a second with an 

 exposure of one eight hundredth of a second each, in which 

 forty-three separate views were taken in the complete cycle, 

 was cut up into strips and mounted on a large disk of card- 

 board with forty-three equidistant and narrow slits near the 

 edge. When the cardboard is made to rotate in front of a 

 looking-glass, after the fashion of the thaumatrope, the 

 original experiment is again perfectly presented, and all the 

 features are evident without the necessity for using these 

 objectionable liquids. The drop is seen to slowly enlarge until 

 it is too heavy to be supported by the surface-tension, the 

 form becomes unstable, a neck begins to develop, which 

 gradually gets narrower until the drop separates. The free 

 drop being suddenly released from the one-sided pull, vibrates 

 as it falls, not in the manner figured in the text-books, be- 

 tween a prolate and oblate spheroid, but rather in a triangular 

 fashion, with the base and vertex alternately uppermost, as 

 recently described by Ph. Lenard*. The falling drop reaches 

 the water at the bottom of the box, where it vanishes for a 

 time only, for it has not mixed with this water, and so it 

 presently bounces out, as shown in fig. 1, after which it again 

 falls into the water. Meanwhile the end of the neck from 

 which the drop broke away has gathered itself together into 

 the little drop or spherule so ably described by Plateau, 

 which has an upward velocity imparted to it, since it separated 

 from the upper or pendant drop after the falling drop had 

 broken away. Moreover, just as the falling drop vibrates 

 after it is released, so the pendant drop vibrates also and grows 

 as it vibrates, coming into collision with the upward moving 

 spherule, which it drives away by a blow. Fig. 2 is a reduced 

 copy of the thaumatrope. The method described could be 

 .made more perfect by using films on rollers driven by the 

 same motor that makes the card rotate, so that the separate 

 photographs should neither overlap nor leave space un- 

 occupied ; but I did not make use of this refinement, as 

 -my object was simply to get the true outline in a particular 

 experiment, and not to construct a piece of mechanism. The 

 apparatus described was simply extemporized out of the 

 materials at hand, and it answered my purpose perfectly. 



2. As I wished in the lectures referred to above to show 

 that exactly the same process goes on even when the finest 

 jet of water breaks up into beads, though of course the pro- 

 cess is hundreds or thousands of times more rapid, I was 

 * Annalen der Physik und Chwiie, vol. xxx. 1887. 



