between the two readings is the lower-stage distance corresponding to the given circle. The Observers may, if they wish, similarly calibrate other circles to assure themselves of the linearity of the device. It is customary to approach each reading from the same direction, to eliminate any backlash that may be present. Similar calibrations are re- quired for each of the objective lenses that may be used. It will probably only be necessary to use the 30X and 20X lenses. To prevent damage to the irreplaceable films, very careful handling is required at all times. Nylon gloves are worn to prevent fingerprints and body oils, which can penetrate the emulsion and serve to catch dust parti- cles. The glass plate, which holds the film flat on the lower stage, is lifted by means of a locking attachment to permit drawing the film through freely and without scratching. 23. Choice of Frames to be Read Referring to Figure 3 (Section 4.) of this chapter, it is seen that regular (constant-¢) growth useful for scaling purposes may extend from about 70 to more than 300 mil’iseconds. Overthis interval each camera will expose more than 450 pictures, at a rate of about 2,000 per second. This many pictures are needed to establish the time-scale to adequate precision; however, it is clearly unnecessary to measure all of them. Indeed it will be more than adequate to measure only one frame in ten, and in some cases one frame in twenty should suffice. Since the film-speed marks are placed at a rate of 200 cps, it will greatly simplify the analysis procedures if we choose to measure only those pictures that occur beside the speed marks. An auxiliary measurement of the position of the mark relative to the frame then suffices to establish the time after detonation at which the frame was exposed. This procedure is more than adequate for large bombs, and is much simpler to accomplish than the rigorous method of measuring the varying distance between successive marks, drawing the acceleration curve for the camera, and integrating numerically to evaluate the time after detonation for each frame of interest, 24, Evaluation of Image Diameters The measurement of an ideal spherical image, with sharp edges and ° a good contrast, is a simple exercise. pom mA UO it wor i i ‘ Unfortunately such images are - 35 - \