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 centuries. Their observation was placed on a scientific basis through the labors of various astronomers, especially Argelander and Schönfeld. 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 investigations 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 examination 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 photographic telescope of large aperture and short focal length, photographs
- ↑ 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 accomplish 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 sequence 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.
