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188 ASTRONOMY: F. H. SEARES 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 another 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. Seares MOUNT WILSON SOLAR OBSERVATORY, CARNEGIE INSTITUTION OF WASHINGTON Communicated by G. E. Hale, February 1, 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 considered, 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,' formulates in a