THE LIGHT OF THE STARS 379 



duced to a uniform photometric scale. An investigation of great 

 value has been carried out successfully at the Georgetown College 

 Observatory, by the Rev. J. G. Hagen, S. J. All the stars of the 

 thirteenth magnitude and brighter have been catalogued and char- 

 tered, in a series of regions, each one degree square, surrounding 

 variable stars of long period. Besides measuring the positions, he 

 has determined the relative brightness of these stars. A sequence 

 has then been selected from each of these regions, and measured at 

 Harvard with the 12-inch meridian photometer, thus permitting 

 all to be reduced to a uniform scale. As the photometer was first 

 constructed, stars brighter than the seventh magnitude could not 

 be measured, since they were brighter than the artificial star, and 

 could not be rendered equal to it. This difficulty was remedied by 

 inserting a series of shades, the densest of which reduced the light 

 by ten magnitudes. By this method, the range of the photometer 

 may be increased indefinitely. Sirius and stars of the twelfth mag- 

 nitude have been satisfactorily measured in succession. A further 

 modification of the instrument permitted surfaces to be compared. 

 The light of the sky at night and in the daytime, during twilight, 

 at different distances from the moon, and different portions of the 

 disk of the latter, have thus been compared. Measures extending 

 over seventeen magnitudes, with an average deviation of about 

 three hundredths of a magnitude, were obtained in this way. One 

 light was thus compared with another six million times as bright 

 as itself. A slight modification would permit the intrinsic brightness 

 of the different portions of the sun's disk to be compared with that 

 of the faintest nebulae visible. By these instruments, the scale of 

 photometric magnitudes has been carried as far as the thirteenth 

 magnitude. To provide standards for fainter stars, a small appro- 

 priation was made by the Rumford Committee of the American 

 Academy. Cooperation was secured among the Directors of the 

 Yerkes, Lick, McCormick, Halsted, and Harvard Observatories. 

 Similar photometers were constructed for all, in which an artificial 

 star was reduced any desired amount by a photographic wedge. 

 Telescopes of 40, 36, 26, 23, and 15 inches aperture, including the 

 two largest refractors in the world, were thus used in the same way 

 on the same research. The standards have all been selected, and 

 nearly all of the measurements have been made. This furnishes 

 a striking illustration of the advantages of cooperation, and com- 

 bined organization. When these observations are reduced, we shall 

 have standards of magnitude according to a uniform scale for all 

 stars from the brightest to the faintest visible in the largest tele- 

 scopes at present in use. The 60-inch reflector of the late A. A. Com- 

 mon has recently been secured by the Harvard Observatory. It is 

 hoped that still fainter stars may be measured with this instrument. 



