ASTRONOMY: F. H. SEARES 
189 
is definitely marked by a star of more than average brilliance, but the 
boundaries are elastic and may be adapted to the special requirements 
of any given problem. In order that the accidental irregularities of 
distribution may not too greatly influence the results, each region, in 
deahng with stars of moderate brightness, will cover one or two square 
degrees; but for the very numerous fainter objects, sufficiently compre- 
hensive data can be obtained from much smaller areas, including, in 
some cases, only a small fraction of a degree. 
Kapteyn thus avoids the task impossible of execution and sets one 
that is more appropriate to our limited capacities. We occupy our- 
selves with what in effect is a sampling of the contents of the heavens, 
and, because of that underlying regularity presupposed by all our science, 
accept with some confidence its inferences and deductions as to the 
totality of the universe of stars. 
But with even this much restricted plan, the actual labor still is very 
great, beyond the resources of any single institution, and subdivision 
and cooperation have therefore been necessary. Various observatories 
and numerous individuals have devoted themselves to special programs 
of observation, with the result that data are now rapidly accumulating. 
In accordance with this plan several investigations, for which the 
equipment of the Solar Observatory is peculiarly adapted, have been 
undertaken at Mount Wilson. One of these, with which this note is 
particularly concerned, relates to the determination of photographic 
magnitudes. 
In any scheme of sidereal research, measurements of stellar bright- 
ness are an important element; for a knowledge of stellar distribution 
and of the concentration of stars toward the galactic plane, they are 
essential; for the statistical discussion of stellar distances, they furnish 
criteria of the greatest value; and when both photographic and visual 
(or photovisual) magnitudes are known, we have immediately available 
values of the color which, for the fainter stars at least, are of much im- 
portance, since they afford, for objects inaccessible to spectroscopic 
observations, a hint as to physical condition not to be obtained by any 
other means. 
In every photometric research a precise knowledge of the scale of 
magnitudes is an essential feature, and for the Mount Wilson investiga- 
tion this primarily was the requirement. Professor Kapteyn had 
previously been furnished with photographs of an hour's exposure, 
with the 60-inch reflector, on each of the selected areas on and north 
of the parallel at — 15° declination; and for the reduction of these photo- 
graphs sequences of standard magnitudes were required. Although 
