THE LIGHT OF THE STARS. 5 1 



ured at Harvard with the twelve-inch meridian photometer, thus per- 

 mitting 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 magni- 

 tudes. By this method, the range of the photometer may be increased 

 indefinitely. Sirius and stars of the twelfth magnitude have been sat- 

 isfactorily measured in succession. A further modification of the in- 

 strument 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 disc 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 disc to be 

 compared with that of the faintest nebula? visible. By these instru- 

 ments, the scale of photometric magnitudes has been carried as far as 

 the thirteenth magnitude. To provide standards for fainter stars, a 

 small appropriation was made by the Bumford Committee of the Amer- 

 ican Academy. Cooperation was secured among the directors of the 

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

 ilar photometers were constructed for all, in which an artificial star 

 was reduced any desired amount by a photographic wedge. Tele- 

 scopes 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 illus- 

 tration of the advantages of cooperation, and combined organization. 

 When these observations are reduced, we shall have standards of mag- 

 nitude according to a uniform scale, for all stars from the brightest 

 to the faintest visible in the largest telescopes at present in use. The 

 sixty-inch reflector of the late A. A. Common has recently been secured 

 by the Harvard Observatory. It is hoped that still fainter stars may 

 be measured with this instrument. 



We have as yet only considered the total light of a star, so far as it 

 affects the eye. But this light consists of rays of many different wave 

 lengths. In red stars, one color predominates, in blue, another. The 

 true method is to compare the light of a given wave-length in different 

 stars, and then to determine the relative intensity of the rays of differ- 

 ent wave-lengths in different stars, or at least in stars whose spec- 

 tra are of different types. This is the only true method, and fortu- 

 nately spectrum photography permits it to be done. The Draper cata- 

 logue gives the class of spectrum of 10,351 stars, and the relative bright- 

 ness of the light whose wave-length is 430, is determined for each. In 



