180 STELLAR PHOTOGRAPHY. 



History. 



Stellar photography originated in an experiment made at the Harvard College 

 Observatory on July 17, 1850. Under the direction of Professor W. C. Bond, 

 Mr. J. A. Whipple placed a sensitive daguerreotype plate in the focus of the fifteen- 

 inch equatorial, which by means of its driving-clock was kept pointed upon the 

 star a Lyrce. A satisfactory image of the star was thus obtained. Subsequently, the 

 double star a Geminorum gave an elongated image, evidently due to its two compo- 

 nents. Objects as bright as these gave but faint images, and no impression was 

 obtained from the Pole-star, however long the exposure continued. The experiment 

 was repeated with various stars and clusters, but the work was finally abandoned 

 owing to the imperfections of the driving-clock and the lack of sensitiveness of the 

 plates (Harvard Observatory Annals, I. cxlix, clvii, clxv). Both of these difiiculties 

 were partially remedied in 1857, and the research was resumed by Professor G. P. 

 Bond. The driving-clock, regulated l)y a conical pendulum, was replaced by a 

 larger clock, controlled by a Bond's spring-governor. The introduction of the 

 collodion process had greatly reduced the photographic difficulties, and furnished 

 plates of much greater sensitiveness. The results of this investigation are contained 

 in three important papers, which were published in the Astronomische Nachrichten, 

 XLVII. ], XLYIII. 1, and XLIX. 81, and have now become classical. The first of 

 these papers states that on April 27, 1857, an impression of the double star I, Ursce 

 3Iajoris was obtained in eight seconds. An exposure of two or three seconds was 

 afterwards found to be sufficient to produce an impression of the brighter star ; and 

 when the telescope was at rest, a trail was obtained as the image of the star a Lyrce 

 traversed the plate. A series of measures was made of the position angle and dis- 

 tance of the companion of l, Ursce Majoris. The probable error of a single photo- 

 graphic distance was found to be ±0".12. The faintest star photographed was the 

 companion of e Lyrce, which has a magnitude of 6.0. The second j^aper contains a 

 careful study of the distance of the components of I Ursce Majoris. Tlie result was 

 14".21 ± 0".013 from measures of sixty-two images taken on eight nights. The 

 extreme variations in the results of these eight nights was only 0".08, and the 

 probable error of a single measure was ±0".05. The third paper is devoted to a 

 discussion of the relative brightness of the stars as indicated by the diameters of 

 the photogi-aphic images. The advantages of photography as a means of locating 

 the stars in a cluster are clearly stated. In fact, nearly all the arguments now offered 

 in favor of photography will be foimd in these admirable papers. 



