ASTRONOMY: PEASE AND SHAPLEY 
97 
cluster, Messier 13, and more generally for the other clusters for which 
plates of sufficiently long exposure are available. 
The data have been arranged in a system of twelve equal sectors, 
and these subdivided by a series of concentric circles, according to the 
scheme shown in figure 1. Except for the shortest exposures, the 
counts within a minute or two of the center are uncertain or impossible, 
and for the inner ring are generally ignored in the discussion. In gen- 
eral, results for all such crowded regions are less reliable because of 
possible influence of the Eberhard effect or of similar photographic 
phenomena. The tedious and difficult task of counting and tabulat- 
ing more than 500,000 star images was performed by Miss Van Deusen 
NORTH 
FIG. 1 
of the Computing Division. Miss Richmond, also of the Computing 
Division, has assisted greatly in the arrangement and tabulation of the 
counts for the study of distribution with respect to direction from the 
center. 
Table I and figure 2 show for Messier 13 the number of stars for suc- 
cessive sectors on several plates with increasing exposure time. The 
limits of distance from the center are 2' and 10', except for Plate 133, 
for which they are 3' and 9'. The totals in the third column include 
the count or estimate of stars in the center, and also those farther from 
the center than the limiting distance used in the tabulation for the 
sectors in columns 4 to 15. 
The two maxima and the two minima in each curve show immediately 
the presence of a well defined axis of symmetry. For plates contain- 
ing only one or two thousand stars the elliptical form is little if at all 
