. St. Nicholas [serial] . second the pictures must be taken at the rateof 15,000 to 20,000 exposures per second! In the ordinary motion picture camera thefilm has a jerky motion, for it can move onlywhile the shutter is closed and must come to adead stop when the shutter opens; but it isimpossible to do this at the rate required to filma bullet in flight. A very fast camera may haveits shutter open for one-ten-thousandth part of asecond, but that would be too slow to snap arifle bullet, because in that time the bullet wouldtravel three or more inches and would make onlya blur on the film. In t
. St. Nicholas [serial] . second the pictures must be taken at the rateof 15,000 to 20,000 exposures per second! In the ordinary motion picture camera thefilm has a jerky motion, for it can move onlywhile the shutter is closed and must come to adead stop when the shutter opens; but it isimpossible to do this at the rate required to filma bullet in flight. A very fast camera may haveits shutter open for one-ten-thousandth part of asecond, but that would be too slow to snap arifle bullet, because in that time the bullet wouldtravel three or more inches and would make onlya blur on the film. In the apparatus invented by M. Bull the ex-posures each last less than a millionth of a second,during which time the fastest projectiles movebut ^2 of nch and thus are practically station-ary. The film travels at the rate of over 300 feetper second or about 200 miles per hour, and it isnot stopped for each picture, because, despite aspeed three times as great as that of an express 1920] NATURE AND SCIENCE FOR YOUNG FOLK 1137. M. LUCIEN BULL AND HIS APPARATUS FOR PHOTOGRAPHING RIFLE AND SHOT-GUN BULLETS train, it is practically stationary during the onemillionth of a second of exposure. There is nomechanical shutter that will open for only onemillionth of a second and repeat the performance20,000 times each second, and so M. Bull getsaround the difficulty by keeping the camera opeitin the dark and making the exposures hy the liglil of a stream of sparks which cast silhouettes of thebullet on the film. The sparks can be regulatedto discharge at a rate as high as 50,000 per sec-ond, but that is faster than necessary. The lightof the sparks is gathered by a reflector (shown atthe right in the general view of the apparatus)and is thiown as a beam against a large lens
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