220 
ASTRONOMY: F. H. SEARES 
each group includes eleven areas and the number in each area has been 
derived from the better of two photographs, the means per field cannot 
be seriously affected. The fields are small (one ninth of a square de- 
gree), but local variations of density have been well eliminated and the 
mean number of stars increases uniformly with decreasing galactic 
latitude. With the exception of Zones I and II which include the ir- 
regular cloud-forms of the Milky- Way, the deviations from the smooth 
curve of the plotted data are only 2 or 3%. 
For comparison with other results the logarithms of the numbers of 
stars have been reduced to the latitudes in the fifth column and referred 
to the square degree as unit. The resulting logarithms of the star- 
density (number of stars per square degree brighter than the limiting 
magnitude) are in the sixth column. 
The zero point corrections of the Mount Wilson results have not yet 
been found, but since Pickering has determined the visual intensity 
of the central star of each Selected Area, the approximate limiting 
magnitude is known. His data fix the zero point and indicate a mean 
apparent limit of 17.1, but, to obtain the real limit on the photographic 
scale, this must be increased by the average color index of the central 
stars. Adopting 0.6 as the average color, we have 17.7; the counts 
are not complete, however, for the last few tenths of a magnitude, and 
we accept provisionally 17.5 photographic as the limit of the Mount 
Wilson densities. 
Since the results of Chapman and Melotte do not extend below the 
17th magnitude, their data used for the comparison in the seventh 
column of the table are for this limit. The Mount Wilson densities, 
referring to 17.5, should be the larger by approximately 50%. At 80° 
the excess is only 10%, but increases rapidly toward the lower latitudes, 
and at 5° the Mount Wilson value is 5.5 times that of Chapman and 
Melotte. This divergence is remarkable and, in its implications, serious. 
In comparing Mount Wilson and Groningen results. Professor Kap- 
teyn has called my attention to the importance of using the same limit- 
ing magnitude for both series of counts, in order to avoid the disturb- 
ing effect of the rapid change in galactic condensation with magnitude 
previously alluded to. The magnitude on the Groningen scale cor- 
responding to the Mount Wilson limit can be interpolated from Kapteyn's 
tables with the Mount Wilson values of log Nm as argument. The 
results for each zone of latitude are in the eighth column of the above 
table. The agreement, excepting for the last two values is close and 
indicates a very satisfactory homogeneity in the counts. The mean 
