ASTRONOMY: F. H. SEARES 
221 
of 16.3 is accordingly the Groningen visual magnitude corresponding 
to the Mount Wilson limit, provisionally placed at 17.5 photographic. 
Of the difference between these quantities, perhaps half a magnitude 
is to be attributed to the color of the stars; the remaining 0.7 or 0.8 
mag. is the indicated error of the Groningen scale, and agrees with what 
was to have been anticipated from evidence relating to the photo- 
metric standards of Parkhurst. 
Interpolating now from Kapteyn's tables for magnitude 16.3, we 
obtain the densities in the ninth column, and, finally the differences 
in the last column. These are in nowise affected by the outstanding 
errors in the scales, and, though systematic, are very small. The values 
of the galactic condensation for the limiting magnitude here considered 
are, respectively, Kapteyn, 32; Mount Wilson, 21.4. 
The relation of the Groningen and Mount Wilson scales has been 
derived on the assumption that equal densities correspond to the same 
limit of brightness. Were the scales of the same kind — both photo- 
graphic or both visual — the difference in the limits for equal densities 
would be their relative error; as it is, the numerical difference in the 
limits must be apportioned between scale error and the influence of the 
color of the stars. With the distribution suggested above, the influ- 
ence of relative scale error would cause Kapteyn's total at the 16th 
or 17th magnitude for all the stars to exceed by two or three times that 
indicated by the Mount Wilson counts. His values for the totals are 
probably to be decreased, though not by the amount suggested by the 
figures of Chapman and Melotte. 
In estimating the significance of the small divergence shown by the 
last column of the table, which tests the relative homogeneity of the 
Groningen and Mount Wilson counts, the following should be noted: 
Kapteyn's values are extrapolated a magnitude or more beyond the 
limit for which he had photometric standards; and, for many regions, 
he experienced much difficulty in determining the necessary magnitudes. 
Further, the Mount Wilson counts are determined by the limiting mag- 
nitudes of the photographs, and although well distributed in latitude, 
variations from zone to zone may affect the totals. The agreement in 
distribution — even in the values of the galactic condensation, which 
are unduly influenced by the large deviation near the pole — is there- 
fore very good. 
A comparison of Mount Wilson with Chapman and Melotte for cor- 
responding magnitudes might also have been made, but the conclu- 
sions which would have followed were obvious, for the large divergence 
