266 The Bight Hon. Lord Bayleigh [Feb. 6, 



your own impressions as to the rapidity of the operation. For some 

 time it has been my ambition to photograph a soap bubble in the act 

 of breaking. I was prepared for diiSficulty, believing that the time 

 occupied was less than the twentieth of a second. But it turns out to 

 be a good deal less even than that. Accordingly the subject is far 

 more difficult to deal with than are those jets of water or coloured 

 liquids, which one can photograph at any moment that the spark 

 happens to come. 



There is the film, seen by reflected light. One of the first difficulties 

 we have to contend with is that it is not easy to break the film 

 exactly when we wish. We will drop a shot through it. The shot 

 has gone through, as you see, but it has not broken the film ; and 

 when the film is a thick one, you may drop a shot through almost any 

 number of times from a moderate height without producing any 

 effect. You would suppose that the shot in going through would 

 necessarily make a hole, and end the life of the film. The shot goes 

 through, however, without making a hole. The operation can be 

 traced, not very well with a shot, but with a ball of cork stuck on 

 the end of a pin, and pushed through. A dry shot does not readily 

 break the film ; and as it was necessary for our purpose to effect the 

 rupture in a well defined manner, here was a difficulty which we had 

 to overcome. We found, after a few trials, that we could get over it b^ 

 wetting the shot with alcohol. 



We will try again with dry shot. Three shots have gone through 

 and nothing has happened. Now we will try one wetted with alcohol, 

 and I expect it will break the film at once. There ! It has gone ! 



The apparatus for executing the photography of a breaking soap 

 film will of necessity be more complicated than before, because we 

 have to time the spark exactly with the breaking of the film. The 

 device I have used is to drop two balls simultaneously, so that one 

 should determine the spark and the other rupture the film. The 

 most obvious plan was to hang iron balls to two electro-magnets, 

 and cause them to drop by breaking the circuit, so that both were let 

 go at the same moment. The method was not quite a success, how- 

 ever, because there was apt to be a little hesitation in letting go the 

 balls. So we adopted another plan. The balls were not held by 

 electro-magnetism but by springs (Fig. 8) pressing laterally, and these 

 were pulled ofl' by electro-magnets. The j^roper moment for j)utting 

 down the key and so liberating the balls, is indicated by the tap of 

 the beam of an attracted disc electrometer as it strikes against the 

 upper stop. One falling ball determines the spark, by filling up most 

 of the interval between two fixed ones submitted to the necessary 

 electric pressure. Another ball, or rather shot, wetted with alcohol, is 

 let go at the same moment, and breaks the film on its jDassage through it. 

 By varying the distances dropped through, the occurrence of one event 

 may be adjusted relatively to the other. The spark which passes to 

 the falling ball is, however, not the one which illuminates the photo- 

 graphic plate. The latter occurs within the lantern, and forms part 



