BULLETS IN FLIGHT, PHOTOGRAPHING The first attempt to photograph a projectile in flight is said to have been made at Woolwich Arsenal in i 86o. Owing to the slow wet-plate process used, the results were unsatisfactory. In 1884 some experiments were made at Prague by Mach and Wentzel which were far more successful ; these were on the lines described below. In 1887 an improvement on the Prague results was made by Drs. Salcher and Riegler, of Fiume.
Probably the most important experiments in the photography of flying bullets were those published in 1892 by Prof. C. V. Boys, a full account of which will be found in the Journal of the Royal Photographic Society (April 3o, i892). His procedure was partly suggested by Lord Rayleigh's methods of obtaining photo graphs of drops, breaking soap bubbles, etc., which were taken by the light of an electric spark. The following description of the two chief methods employed by Prof. Boys are due to the above-mentioned publication.
The first is shown diagrammatically at A. j is a fulminating pane or condenser of small capacity, which may be charged on its two sides respectively, positively + and negatively —, by means of any kind of frictional or induction machine. This pane is allowed to discharge through a very short circuit in which are two gaps s si. The spark at s is allowed to shine on the photographic plate P without the intervention of lenses of any kind. The spark at sl is hidden. A second condenser, the jar j (of very small capacity compared with the pane j), is con nected to j, one coating by means of wire, as shown by the full lines, and the other by means of a string wetted with a solution of calcium chloride, as indicated by the dotted line. Its coatings, therefore, till the time of discharge arrives, are kept at the same potentials as those of the pane j. The discharge circuit of j includes the gaps sl and n. The potential is so chosen that neither condenser is able to discharge across the two gaps s sl or n as the case may be, but that either would go off if either of its gaps were made conducting. This is effected by the passage of the bullet across n, which immediately causes a feeble spark at due to the discharge there of j. The air here, being now conducting,
no longer prevents the pane j from discharging across the gap s, and therefore a spark is pro duced which casts upon the photographic plate a shadow of the bullet, and in effect an image of any atmospheric phenomena accompany ing the bullet. The difficulty in photographing bullets is to obtain a spark which, while it is bright enough to act on the plate, is yet of such short duration that the bullet has not time to move more than a hundredth of an inch or less while it is yet in existence. If glass lenses are used a large proportion of the actinic rays are absorbed (in the case of a spark between magnesium terminals, four fifths of the whole); but in the case of a true image any spark formed by the bullet at where difficulty might arise in closing the gap n. In the first arrangement the gap n must be more perfectly closed than is the small space s, in order to make the action of what may be called the " spark relay " certain. By the introduction of a second pane or jar (see B) in series with j', the potential of the little jar i may be made n I times that of if th. capacity of is that of j'. In the discharge, only j' is affected, so it will be necessary to discharge also before beginning a new experiment.
The above sufficiently indicates the principle of the electrical arrangements. With regard to the practical details, the apparatus mostly used consisted of a box lined with black cloth, in which the photographic plate was placed. The large condenser was a plate of glass about a foot square, and the small condenser was a jar or bottle to act as a starter for the spark. The bullet enters and leaves the box by two holes covered with paper to exclude the light, and in passing in front of the plate it touches the terminals of two thin lead wires, thus completing the circuit and causing two flashes—a small one which does not affect the photographic plate, and a larger one which does affect the plate, and has a duration of less than one-millionth of a second.