1 -36 EDWARD C. PICKERING. VARIABLE STARS OF LONG PERIOD. 



A large pièce of work is uo-w in pre])aration here, which, if carried 

 out, will greatly facilitate the réduction of observations of variable 

 stars. In the Catalogues accompanying Hagen's Atlas ail of the stars 

 on each chart are arranged in a séquence, thus giving the relative, al-, 

 though not the absolute, brightness. It is proposed to measure photo- 

 metrically a number of stars on each of thèse charts and thus reduce 

 the magnitudes to the photometric scale. This has already been done for 

 a large number of the charts. If it is completed, useful observations 

 can be made of any of thèse variables, very simply. It will only be 

 necessary to select any stars on the chart which are slightly brighter 

 and slightly fainter than the variable. The record will consist of the 

 date, and the catalogue numbers of thèse stars. The observations will 

 be improved if the intervais are estimated, and the hour and minute 

 also noted. Yot long period variables of large range, however, this is 

 not essential. If such observations, for wliich no apparatus but an ordi- 

 nary télescope is required, are made systematically, results of great 

 value will be obtained. If ail variables of long period could be obser- 

 vée! according to this System, our knowledge of the laws regulating 

 thèse curious and interesting objects would be greatly increased. 



Harvard Collège Observatory, 

 Cambridge, Mass., U. S. A. 

 July 27, 1901. 



