76 



♦ KNOWLEDGE ♦ 



[February 1, 1887. 



PHOTOGRAPHS OF DOUBLE STARS 

 STAR CLUSTERS.* 



By mm. Henry. 



AND 



E have obtained a certain number of good 

 photographs of double stars and star 

 clusters, by regulating the duration of the 

 exposure according to the photographic 

 intensity of the components. The magni- 

 tudes of the discs of stars as photogi'aphed 

 in our charts of the Pleiades and of [a por- 

 tion of] the Swan (see Knowledge for May 

 and June, pp. 213, 243), which might have led to the idea 

 that very close pairs could not be recorded photographically, 



magnitude which has been on the plate but the two- 

 hundredth part of a second, shows only a point, not a large 

 disc. It is the same with a star of the second magnitude 

 which has not been photographed for more than the time 

 (0'013 sec.) necessarj' for recording it, and so forth. It 

 has been found that the durations of exposure for the 

 different orders of star magnitude are as presented in the 

 following table : — 



Fig. 1.— Photograph of the Geeat Cluster in Gemini. (35 Messier.) 



were due to the long exposure necessary for stars of the 

 lowest magnitudes ; the discs grow larger with the exposure, 

 but always begin by a minute point. A star of the first 



* From Jj'Asiroiwmie. 



If the exposures are not exaggerated, as they must be to 

 obtain on the same plate stars included between the 1st 

 or 2nd magnitude and the 16tb, the stars only record 

 themselves as minute points, and thus close doubles are 

 self-recorded (s^enregistrent d'eti.>:-memes). 



