furnish a fairly smootii curve. Tne peak-to-total curve characteristic of the machine used in tne present experiment is Snown in Fig. 6. (3) Counts per second per square inch in the total absorption peak, C. This number is a constant of the machine and is found by dividing the number of counts per second per inch of deflection in the Y direction by the channel width in chart inches. The channel width used throughout the experiment corresponds to O.1 inch on the chart paper and the total Y deflection is 10 inches. =o. ew Ss (4) Then for the 200 c/s scale, C is 200. Source reductidén factor, h. This factor is the fraction of photons leaving the source which enter the crystal and takes into account effects of collimator penetration, finite extension of source, and solid angle subtended by the crystal from the source, assuming a point source and an opaque collimator. | These effects are discussed in some detail in a theoretical paper by “R, L. Mather of this laboratory.” Derived from informa- tion contained in this paper is Fig. 7, showing K, the source reduction factor for a point source, versus r, the distance of the source from the collimator axis. To find the average source reduction factor, K, to account 15