446 



On the Photographic Spectra of Stars. 



[Dec. 14, 



speculum belonging to the Eojal Society's telescope in my possession. 

 Considerable delay has arisen from the necessity, for these obserrations, 

 of a more uniform motion of the dri^dng-clock. Tor this purpose, Mr. 

 Howard Grubb has successfully applied to the clock the control of a 

 seconds pendulum in electric connexion with a sidereal clock. This 

 system works quite satisfactorily. 



The prisms employed are made of Iceland spar, and the lenses of 

 quartz. After an extensive trial of different photographic processes, 

 preference has been given to dry plates. 



The apparatus is so arranged that a solar or electric spectrum can be 

 taken on the same plate, for the purpose of comparison, with the spectrum 

 of the star. Spectra have heen obtained of Sirius, Yega, Yenus, the 

 Moon, &c. 



I do not purpose in this preliminary notice to describe in detail the 

 arrangements of the special apparatus which has been constructed, nor 

 to offer the results of the experiments in their present incomplete state to 

 the E-oyal Society. Still I venture to hope that, even in this early stage 

 of the inquiry, the enlarged copy of the spectrum of Yega (a Lyrse) 













1 a, LYR/EL 1 



which accompanies this note may not be regarded as altogether unworthy 

 of attention. 



After exposure to the light of Yega, the dry plate was allowed to 

 remain in the instrument until the following morning, w^hen a solar 

 spectrum was taken upon it, through the half of the slit which had 

 remained closed when the instrument was directed to the star. 



The photograph shows seven strong lines, all of them slightly shaded 

 at the sides. The two lines which are least refrangible coincide with two 

 known lines of hydrogen iu the solar spectrum. 



It is expected, by means of an apparatus now in the course of con- 

 struction, to obtain also any finer lines which may be present in the 

 spectrum of this star, as well as to extend the photographic method to 

 stars which are less bright. 



I need not now refer to the many important questions in connexion 

 with which photographic observations of stars may be of value. 



Si 



