magnification factor of the optical system. This factor is equal to the ratio Distance, camera to plane perpendicular to optic axis containing the fireball diameter Lens focal length Since the camera is operated at a slightly tilted angle, 7, the desired opticaxis distance equals the horizontal (ground) distance multiplied by cos r. It is thus necessary to measure the horizontal distance, L, the focal length, f, and the angle of tilt, 7; once measured these quantities will remain con- stant for all frames exposed in a given camera. The USAEC measurements are each good to about 0.2 percent, and facilities will be provided to check them to something like 1.0 percent. The time scale, relating each exposed frameto the time after detonation, will be determined from timing marks placed upon the film while the camera is being exposed. These marks will establish the time after detona- tion to better than 0.2 percent, and the observer-check will also be good to 0.2 percent. . The air density requires measurements of barometric pressure, temperature, and relative humidity, all three as functions of altitude. These quantities will be measured by means of weather balloons, released on Eniwetok Island shortly before detonation, which will telemeter the meteorological data to the ground. An independent measurement of the ground- level data will be made at the Control Point adjacent to the photo tower on Parry Island; this will utilize a Cenco mercury barometer and a standard sling-psychrometer. These instruments will be available for check-reading by the Observers, prior to and at shot time. From these readings the air density at ground level may be determined to a precision of about 0.2 percent; check values at various altitudes may be derived from this number through the use of the standard atmospheric data (such as those adopted by the NACA), to better than 0.5 percent. The ground-reflection factor will be taken as equal to 0.50, and no measurements are contemplated. 8. Precision of Measurements In addition to the individual measurement errors described above, one CF4 expects a reading error during the analysis of the films. For each frame