18-2 



ELECTRIC-SPARK PHOTOGRAPHS. 



to liow long" the plate remained whole or rather if the bullet had been 

 caught a little sooner before these faint waves had lost so much of their 

 distinctness they would supply this information with great exactness. 

 jNIeanwhile the figure shows that the i)late is now broken up completely. 

 It is true it is still standing, and th(^, stern air wave is seen reflected 

 from the upper part of it, but this is because the different parts have 

 not 3'et had time to get away; their grinding edges however have cast 

 out from the surface little particles, and these are seen over the whole 

 extent of the plate. After about 15 inches the bullet is quite clear of 

 the cloud of dust (PI. xii, fig. 2); one piece only of the glass, no doul)t 

 the piece that was immediately struck, has been punched out and is 

 travelling- along above the bullet at a speed practically equal to its own. 

 1 am also able to show the plate itself in this and a still later stage, 

 when at last the separate pieces have begun to be visibly moved out of 

 their position and in some cases slightly turned round. 



I have merely given this evening an account of a few experiments 

 which in themselves perhaps are of little interest, but they at any rate 

 show the capability of this method for the examination of subjects 

 which would in the ordinary way be considered beyond the reach of 

 experiment. It is hardly necessary to say that the examples given by 

 no means reach the limit of what may be done. I have examined the 

 explosions produced by 15-grain fulminate of mercury detonators 

 and of heaps of iodide of nitrogen, a material which is rather unman- 

 ageable, as if a fly even walks over it it violently explodes. In these 

 cases the exph>sive flash was used to make the B gap of PI. ii, fig. 5 con- 

 ducting, for which it answered perfectly. One might in the same way 

 examine the form of the out-rush of powder gases past the bullet, and 

 so find at once their velocity with respect to the velocity of the bullet, 

 and I see no great difficulty in tracing, if this should be desired, the 

 wliole course of a single bullet for perhaps as much as 100 yards by 

 means of photogra])hs taken every few inches on its way. Though it 

 may not be evident that these or similar experiments are of any practi- 

 cal importance, there can be no doubt that information may be readily 

 obtained by the aid of the spark photogra])h, as in fact has been shown 

 by Prof. ]Mach, Lord Kayleigh, Mr. F. J. Smith, and others, which with- 

 out its aid can only be surmised, and that if, as in other subjects, the 

 first wish of the experimentalist is to see what he is doing, then in these 

 cases surely, where in general people would not think of attempting to 

 look with their natural eyes, it may be worth while to take advantage 

 of this electro-photographic eye. 



