ASTRONOMY: F. H. SEARES 
191 
magnitude of a given star, though the average number is only five or 
six; for the faintest stars there are naturally but two — those derived 
from the full- aperture images of the two plates of long exposure. 
It was planned originally to include only the 115 Selected Areas on 
and north of the Equator, and the 460 plates necessary for the deriva- 
tion of relative magnitudes in these regions have nearly all been taken. 
The measures are also well advanced, and for 8.0 regions the reductions, 
up to and including the relative magnitudes, are complete. It now 
seems desirable to include the 24 areas at — 15°, and these accordingly 
will be added to the program. 
To reduce the relative magnitudes to absolute values on the inter- 
national system, it is convenient to rely upon the North Polar Standards, 
for which precise results are known-. To determine the various zero- 
point corrections, two series of intercomparison photographs have been 
made: first, comparisons of each area with the adjacent areas of the 
same zone of declination, and second, duplicate comparisons of six 
equidistant areas in each zone with the region of the Pole. By means 
of the zonal comparisons, the magnitudes of each zone can be brought 
to a common zero-point, which, through the polar comparisons, can 
finally be referred to the international standard. Since the inter- 
comparison exposures are of only two minutes duration, the photo- 
graphs are rapidly accumulated. All but 50 of the 302 plates required 
for the original program have been obtained. 
Since these photographs have not yet been reduced, the lower limit 
for the magnitudes is not accurately known, but probably it is not far 
from 17.5 on the photographic scale. The range of brightness is always 
such as to include the central star of the area and usually is eight mag- 
nitudes or more. The field is that of good definition, with the full 
aperture of the 60-inch mirror; its diameter is 23' and its area 
about one-ninth of a square degree. The number of stars shown in 
such a field on the plates of fifteen minutes exposure varies from 40 
or 50 in high galactic latitudes to 2000 or 3000 in the star clouds of the 
Milky Way. The total for the Areas on and north of the Equator 
should be 55,000 or 60,000. 
1 Published by the Astronomical Laboratory at Groningen. 
2 Seares, F. H., ML Wilson Contrib., No. 80; Astroph. /., Chicago, 39, 1914, (307-340). 
