My set up for photographing the arrow at any desired point comprised two cameras with F3.5 lenses, 3¼"x 4¼". All earlier work, principally on bullet photography, had been done by the silhouette method (See Scientific Papers of the Bureau of Standards, No. 508: "Spark Photography and Its Application to Some Problems in Ballistics" by Philip P. Quayle), i.e., by casting a direct shadow of the projectile on the plate, with a relatively small spark as the source of light. The dimensions of the bow and arrow are such as to rule out this method. This leaves but one alternative, namely, to secure enough light to make possible a direct exposure with a camera in a time interval of such short duration that the arrow cannot move perceptibly during this time. I calculated that, for a sharply defined picture, five one-millionths of a second would sufficiently stop the motion in the image of the highest speed arrow. An area about 5 x 7 feet had to be illuminated in such a way that proper lighting would be secured from the side and from directly above. Without going into details, it may be mentioned that the "lightning flash" illumination is about 200,000 times as intense as is the light from a 40-watt lamp, and that it persists for about five one-millionths of a second.
This intense light is obtained by discharging a 9 microfarad condenser at about 5600 volts d.c., across a gap made of two large blocks of magnesium metal. The magnesium discharge is highly actinic. "Triggering" the discharge at the right instant of time was quite simple. It was solved by causing the arrow to puncture a special paper and tinfoil screen—a device which I had used in velocity measurements during the war—thereby closing a trigger circuit. This circuit causes a small spark to jump between small auxiliary electrodes of silver, placed in the magnesium gap. The latter is of such width that the 5600 volts will not jump under normal conditions, but when the trigger spark passes, it breaks down the insulation in the gap; there is a blinding crash, and the images of the bow and arrow are faithfully caught in both cameras. The work is done in a room lighted only with red light, and the camera shutters are left open just as they would be if it were desired to obtain a picture of a flash of lightning. A suitable system of reflectors and reflecting screens is used for distributing the light. My first test photographs only were taken with the arrow projected from the bow in a shooting machine. All other pictures were taken of the bow being shot normally because our chief interest lies in what happens in hand shooting. The series of pictures shown in Fig. 1 is one selected from the collection so taken.
|Fig. 1. A series of "stills", taken of the same bow and the same arrow, from directly above, giving a detailed story of how the arrow passes the bow. Compare with "summary", appearing on pages 143 and 144.