after detonation at which each frame was exposed, andfinally the calcula- tion of the yield and of statistical confidence limits. The basic measurements from the films are accomplished by means of a Profile Measuring Microscope, Model P219, manufactured by Henri Hauser S. A., Bienne, Switzerland. This instrument is illustrated in Fig- ure 12, and is most frequently termed the Comparator. The film is placed upon the lower stage, which may be translated in orthogonal directions by means of two micrometers. Each micrometer may be read, by means of a vernier, to 0.0001 inch; motion of the stage is limited to 2 inches ineach direction and it can also be rotated. The stage may be raised or lowered to attain critical focus by means of the large hand-wheel at the lower right. The film is observed through the eyepiece, by means of an objective lens. Several interchangeable objective lenses are available, ranging from 6X to 50X. A special jig has been built to hold the film reels; the portion of film under investigation is held flat against the stage by a glass plate. The film is illuminated from below by means of a light bulb whose brightness may be adjusted with a rheostut. This adjustment is particularly useful since the films vary tremendously in density. An additional feature of this instrument is the second stage, located at the top of the comparator. By means of a knob one can superimpose images from the two stages, that from the upper stage being reduced in size to one-tenth. Thus, in the toolmaker's use of the instrument, a piece of work may be compared directly with a ten-times drawing, to observe minute deviations in the work. For the purposes of fireball reading a grid of concentric circles is placed in the upper stage; the diameters of these circles are calibrated through the optical system in terms of the micrometers of the lower stage; and the fireball diameters are estimated in terms of the calibrated circles. Figure 13 shows the circular grid which is provided as a transparency of . glass. In order to calibratethe instrumentit is only necessary to place a xnife edge or razor blade on the lower stage. The edge is aligned with one of the larger circles, and the micrometer reading is noted. Then, using CFI the micrometer to move along a diameter, the edge is aligned with the other side of the same circle, and a second reading is taken. - 32 ~ he toee Le at | oe vé The differen