SCIENCE 



[N. S. Vol. XXXIX. No. 992 



brighter. The Potsdam and Harvard 

 systems agree admirably if a correction is 

 applied for the color or spectrum of the 

 stars. They should never be combined, or 

 compared, unless this correction is ap- 

 plied. 



Stellar photography, originating in the 

 work of George Bond in 1857, has revolu- 

 tionized many departments of astronomy. 

 The great work of a chart of the entire 

 sky, undertaken by the Paris Observatory 

 in cooperation with several others, is a sad 

 example of the danger of undertaking a 

 work on too large a scale. Although sev- 

 eral observatories have been continually at 

 work upon it for a quarter of a century, it 

 has been predicted that at least fifty yeare 

 must elapse before it is completed, and no 

 positions of any southern stars have yet 

 been published. In striking contrast to 

 this is the early completion of the Cape 

 Photographic Durehmusterung which gives 

 the positions and magnitudes of nearly half 

 a million stars south of — 19°. It illus- 

 trates the results of the happy combination 

 of skilful planning with routine organiza- 

 tion, conducted on a very large scale. 

 The extension of this work to the North 

 Pole is now being planned, but with the ad- 

 ditional condition that the color index as 

 well as the photographic magnitude will 

 be determined. The former will be found 

 by photographing the stars by means of 

 their yellow or red, as well as with their 

 blue, light, the difference in the magnitudes 

 giving the color index. Much might be 

 said of the numerous applications of pho- 

 tography to the determination of stellar 

 magnitude. The sixty-inch reflector of the 

 Mount "Wilson Observatory, using ex- 

 posures of several hours, has succeeded in 

 photographing stars as faint as the 

 twentieth magnitude. An international 

 committee, with members from England, 

 France, Germany, Russia, Holland and 



the United States, has adopted a scale of 

 magnitudes based on two investigations 

 made at Harvard. One of these was made 

 with the meridian photometer, and the 

 other is an elaborate investigation by Miss 

 H. S. Leavitt of the photographic magni- 

 tude of seventy-six stars near the North 

 Pole. A standard scale is thus provided 

 from the first to the twentieth magnitude. 

 We may say from the minus twenty-sixth- 

 to the twentieth magnitudes since accord- 

 ant results for the light of the sun have 

 been obtained by Professors W. H. Picker- 

 ing and E. S. King. For many purposes, 

 photography may well replace visual pho- 

 tometric measures, since for stars brighter 

 than the fifteenth magnitude photographs 

 may be taken with yellow light. One of the 

 principal uses of measures of the light of 

 the stars is the study of the variables, or 

 those in which the brightness is not con- 

 stant. A bibliography of these by Miss 

 Cannon is recorded on about forty thous- 

 and cards. The number of known vari- 

 ables is now about forty-five hundred, of 

 which three quarters have been discovered 

 by photography, at the Harvard Observa- 

 tory. There are several kinds of variable 

 stars. Variables of long period undergo- 

 changes which repeat themselves somewhat 

 irregularly in a period of several months, 

 and at maximum are often several thousand 

 times as bright as at minimum. The most 

 useful work that an amateur can do with 

 a small telescope is the observation of 

 these objects. An important work under- 

 taken by the British Astronomical Associa- 

 tion has been the observation of variable 

 stars. During the last thirteen years they 

 have accumulated twenty thousand such 

 observations, all reduced to the same scale, 

 which is that of the Harvard Photometry. 

 Similar work in this country has accumu- 

 lated ten and sixteen thousand observa- 



