188 
ASTRONOMY: K. H. SHARES 
insemination by artificial or natural means, thereby minimizing the 
amount of error introduced in experimental work by the use of females 
in unknown physiological condition. In critical experiments by using 
for an indicator a vasectomized male, homozygous for one color factor, 
and for the later insemination sperm from a male homozygous for an- 
other color factor, one will have a definite breeding test showing whether 
or not, by any accident, the vasectomized male was able to. transfer 
sperm to the female. 
PHOTOGRAPHIC MAGNITUDES OF STARS IN THE SELECTED 
AREAS OF KAPTEYN 
By Frederick H. Scares 
MOUNT WILSON SOLAR OBSERVATORY. CARNEGIE INSTITUTION OF WASHINGTON 
Communicated by G. E. Hale, February I. 1917 
A casual experience with the details of astronomical investigation 
reveals, in the number of the stars, a serious difficulty to be overcome 
in undertaking any discussion of the development and structure of the 
stellar universe. Telescopes of even moderate size bring before the 
observer stars to be counted by tens of millions, while those shown by 
instruments of the highest power are many times more numerous. Since 
the individual examination of all these objects will not be seriously con- 
sidered, the question arises as to a rational limitation of the program 
of observations. Fortunately, the problem is not as hopeless as it 
seems, for very important and illuminating facts are to be derived from 
a minute percentage of the total number of stars seen in our telescopes, 
provided only that the objects chosen for study be representative of 
the collection as a whole. 
Kapteyn, in 1906, showed that by proper restriction and selection 
we might hope to obtain, within a comparatively few years, a fairly 
comprehensive notion of the salient features of the structure of the 
universe. His well-known ^Plan of Selected Areas,' published in that 
year,i formulates in a definite way the investigations to be undertaken 
in order that we may acquire an adequate knowledge of stellar posi- 
tions, distances, proper motions, radial velocities, spectra, and magni- 
tudes, both visual and photographic, which are the data essential for 
a consideration of the questions of structure and development. 
The principle of selection adopted by him involves the detailed exam- 
ination of all objects within the reach of observation, situated in cer- 
tain small areas uniformly distributed over the sky. About two hun- 
dred such selected areas are included in the list; the center in each case 
