170 ELECTRIC-SPARK PHOTOGRAPHS. 



travelling- at the enormous si)ee(i that is now attained by the use of this 

 weapon, can not go more than one four-hundredth of an inch in this 

 time. Other sjiarks of still less duration were examined, but this was 

 chosen for the purpose of photograpliing bullets.* 



Now, having obtained a suitable flash of light, I must next show how 

 a spark may be used for the purpose of photographing a bullet in its 

 passage. This was first d<me by Prof. E. Mach,t of Prague, whose 

 method is illustrated by the diagram, PI. ii, fig. 5. The squares on the 

 right-hand side represent certain electrical apparatus by means of which 

 a Leyden jar (J) is charged with electricity to such an extent that, 

 while it is unable to make two sparks at B and A, it is nevertheless 

 able to, and at once will, make a spark at B when the second gap at A 

 is closed by a bullet or other conductor. The dotted lines represent 

 wires throngh which the discharge then takes place. The spark at B, 

 magnified by the lens I in front of it, then fills the field lens L with 

 light, so that the camera K locused upon the spark gap A will then 

 receive an image of the bullet as it passes, and thus a photograph is 

 secured. I am able to show two of these which Prof. Mach has kindly 

 forwarded to me, and what I wish to point out is that in each of these 

 photographs — and this is perhaps the most interesting feature which 

 any of these exhibit — there are seen, besides the bullet and the wires 

 which the bullet strikes m its Journey, certain curious shades, one in 

 advance of the bullet and one from the tail, while a trail is left behind 

 very like that seen in the wake of a screw steamer. In fact, the whole 

 atmospheric x>hen()nienon accompanying the bullet is not unlike that 

 seen on the surface of water surrounding and behind a steamship. 

 These were seen for the first time, and their visibility by this method 

 was (I believe) predicted by Prof. Mach before he made his first experi- 

 ment. 



The part that I have played in this matter is after all very subor- 

 dinate. I have attempted to simplify the means, and the results which 

 may be obtained by the modified method which I have devised, are, 1 

 believe, in some respects — I don't say in all — clearer and more instruct- 

 ive than those obtained by the more elaborate device of Prof. Mach. 



PI. II, fig. G, is a diagram of the apparatus that I have used. C is 



* These sparks were made to go off at the time that the mimr was facing toward 

 the photographic plate by the employment of the same device as that described 

 below in connection with fig. 4. On the axle of the mirror an insulated tail of 

 alnmininm was secured, so as nearly to bridge a gap in the discharge circuit of au 

 auxiliary jar of small capacity, there being a gap common to both circuits. A self- 

 induction coil was used instead of the wet string, as being for this purpose prefer- 

 able. The length of time that the spark lasted was thus measured without taking 

 the electricity round by the mirror, which would have been quite sufificient to modify 

 the duration of the discharge, and it was easier than making and adjusting a second 

 reflecting mirror, which would have answered the same purpose. 



t See Nature, vol. xi.ii, p. 250. 



