Photography and Chronographic Measurements1

Abstract

IN chronographic measurements in physiological experiments, photography has been in constant use for several years, and the methods are well known. I have extended recently the method of what may be called photographic chronography to measuring the velocity of projectiles. On former occasions I have shown that to obtain the best chronographic results, magnetic and solenoidal arrangements should be avoided, since by their use a time lag is introduced. The following chronographic method depends entirely on light. Two sources of light at a suitable distance apart throw two beams of light on to a sensitive plate, carried on the carriage of a tram chronograph. By means of lenses, the Deams of light are caused to form two sharp images on the plate in a vertical line, one above the other; a tuning-fork trace is also made on the plate; if the plate traverses, when the beams of light are not interrupted, on development, two black parallel lines appear on the plate; but if, during the passage of the plate, the beams of light are cut by any solid object which shuts off the light, then on development two gaps are seen to exist. The distance between these markings when interpreted in terms of the fork trace, give the velocity of the object which cuts through the beam of light. The method was illustrated by allowing a Drojectile to pass through the focus where the convergent beams of light from two sources of light cross.