March 2, 189;;] 



NA TURE 



417 



photography that I have obtained up to the present. 

 In this case the surface of the condenser is one 

 square foot, and the discharge is taken through 

 bands of copper about two inches broad, and not more 

 than about four inches long apiece. Extra good con- 

 tact is made between these copper bands and the tin- 

 foil surface by long radiating tongues of copper-foil 

 soldered to the end of the copper bands. The knobs are 

 platinum, but this seems no better than copper. The 

 whole of the light is extinct in less than one-millionth 

 of a second, while the first blaze, which is practically the 

 whole spark, the tail being in comparison of no conse- 

 quence, does not last so long as a ten-millionth of a 

 second ; in other words, it lasts so short a time that it 

 bears the same relation to one second that one second 

 bears to four months ; or again, a magazine rifle bullet, 

 travelling at the enormous speed that is now attained by 

 the use of this weapon, cannot go more than one four- 

 hundredth of an inch in this time. Other sparks of still 

 less duration were examined, but this was chosen for the 

 purpose of photographing 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 

 done by Prof. E. Mach," of Prague, whose method is 

 illustrated by the diagram Fig. 3. The squares on the 



right-hand side represent certain electrical apparatus by 

 means of which a Leyden jar (J) is charged with elec- 

 tricity 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 through which the dis- 

 charge then takes place. The spark at B, magnified 

 by the lens / in front of it, then fills the field lens L 

 with light, so that the camera K focussed 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 for- 

 warded 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 in its journey, certain curious shades, one in ad- 

 vance of the bullet and one from the tail, while a trail is 

 left behind very like that seen in the wake of a screw 



1 These sparks were made to gooff.it the time th.-»t the mirror was facing 

 towards the photographic plate by the employment of the same device as 

 th.-it described below in connection with Fig. 4. On the axle of the mirror 

 an insulated tail of aluminium vvas secured, so ?s nearly to bridge a gap in 

 the discharge circuit of an 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 preferable. The length of lime that the 

 spark lasted was thus measured without taking the electricity round by the 

 mirror, which would have been quite sufficient 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. 



- See Natlrk, vol. xlii. p. 250. 



NO. T2t8, vol. 4.7! 



steamer. In fact, the whole atmospheric phenomenon 

 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 experiment. 



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

 very subordinate. I have attempted to simplify the 

 means, and the results which may be obtained by the 

 modified method which I have devised, are, I believe, in 

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

 structive than those obtained by the more elaborate 

 device of Prof. Mach. 



Fig. 4 is a diagram of the apparatus that I have used. 

 C is a plate of window- 

 glass with a square foot ,''""'•, 

 of tin-foil on either side. 

 This condenser is 

 charged until its potential 

 is not sufficient to make 

 a spark at each of the 

 gaps E and E', though it 

 would, if either of these 

 were made to conduct, 

 immediately cause a 

 spark to form at the other, 

 r is a Leyden jar of very 

 small capacity connected 

 with C by wire, as shown 

 by the continuous lines, 

 and by string wetted with 

 a solution of chloride of 

 calcium, as shown by the 

 dotted line. So long as 

 the gap at B is open this 

 little condenser, which is 

 kept at the same potential 

 as the large condenser 

 by means of the wire and 

 wet string, is similarly 

 unable to make sparks 

 both at B and E', but it 

 could, if B were closed, 

 at once discharge at E'. 

 Now suppose the bullet 

 to join the wires at B, a 

 minute spark is made at 

 B and at E' by the dis- 

 charge of f, immediately 

 C, finding one of its gaps 

 E' in a conducting state, 

 discharges at E, making , ?.^ 

 a brilliant spark, which 

 casts a shadow of the ^"^' "*■ 

 bullet, «&c.. upon the 



photographic plate P. Though this is simple enough, 

 the ends that are gained by this contrivance are not so 

 obvious. In the first place the discharge circuit of C, 

 via E and E' is made of very short broad bands of 

 copper, a form which favours both the brilliancy and the 

 shortness of duration of the sparks; further, the double 

 gap, of which E' may be the longer, causes the intensity 

 of the light of either spark to be greater than it would be 

 if the other one did not exist— in a particular case the light 

 of the shorter was increased six or eightfold — at the same 

 time the duration is not greatly affected. For this reason 

 the spark at E may be made very short, so that the 

 shadow is almost as sharp as if the light came from a 

 point. The spark formed at B, which is due to the dis- 

 charge of c only, is very feeble, so that it is unable to act 

 on the plate, whereas, had the discharge of C been carried 

 round by B, the light at this point would hopelessly have 

 spoilt the plate, and at the same time the light at E 

 would have been feebler and would have lasted longer. 



