176 POPULAR SCIENCE MONTHLY 



tion, thanks to the powerful aid of photographic methods, may hope 

 to bring near completion the first part of the problem, and to make 

 good progress on the two remaining portions. 



The existence of a variable star was probably first recognized by 

 Holwarda of Franeeker, in 1639. The number was slowly increased, 

 and some knowledge of their nature learned during the next two cen- 

 turies. Their observation was placed on a scientific basis through the 

 labors of various astronomers, especially Argelander and Schonfeld. 

 The latter astronomer issued, in 1865, a catalogue of 113 variables, and 

 later, one of 165 variables, which included all stars then known to be 

 variable. The list was enlarged, in 1883, at the Harvard Observatory 

 by the addition of forty-eight variables. In 1888 Dr. S. C. Chandler 

 published his first catalogue of variable stars, 225 in number, which had 

 been discovered by some thirty different observers in various countries, 

 by visual methods. Many of these observers have continued their in- 

 vestigations till the present time — the most successful in the line of 

 discovery being Dr. T. D. Anderson, of Edinburgh, who by visual 

 means alone has found forty new variables, a result truly remarkable. 1 



About the year 1889, however, began a rapid increase in the number 

 of variables through the introduction of photographic methods. The 

 first notable addition was made by Mrs. Fleming, through the examina- 

 tion of the photographic spectra of the stars, while engaged in the 

 work of the Henry Draper Memorial, a research carried on at the 

 Harvard Observatory under the direction of Professor E. C. Pickering. 

 By means of an objective prism, placed in front of the lens of a photo- 

 graphic telescope of large aperture and short focal length, photographs 



1 Although the subject of variable stars is now under investigation at many 

 observatories, there is still a wide field in this line of research for amateur 

 astronomers. It is true the light-curves of many variables are now fairly 

 well known, but new ones are constantly being discovered, the study of which 

 offers an interesting field of investigation. It is necessary, in order to accom- 

 plish results of scientific value, that the observations be made, not only with 

 enthusiasm, but with an intelligent conception of the future use to which they 

 must be put. The observations need to include only two things, a record of the 

 time, and the most precise determination possible of the brightness of the 

 variable. The estimate of magnitude is usually made by referring the light of 

 the variable to that of one or more adjacent stars, whose light is constant. For 

 this purpose a series of adjacent comparison stars is selected, forming a se- 

 quence from bright to faint stars, and their brightness is carefully determined. 

 It is very important that these magnitudes be reduced to the photometric scale. 

 For identification of the stars the star charts of Father Hagen are admirable. 

 Marked photographs are also extremely useful. 



The discovery of new variables offers, perhaps, a line of work even more 

 fascinating than the investigation of the peculiarities of those already found. 

 Brilliant work has been done in this direction by amateurs, but at the present 

 time much more can be accomplished by photographic than by visual means. 

 Among those who have done work of special value, in this country, may be 

 mentioned Chandler, H. M. Parkhurst, J. A. Parkhurst, Sawyer and Yendell. 

 Abroad, the number of amateur observers is large. 



