528 Royal Society : — 



compensated by the clock movement, or from atmospheric tremor, 

 the spectrum, though tolerably defined at the edges, presented no 

 indications of lines. Our other investigations have hitherto pre- 

 vented us from continuing these experiments further ; but we have 

 not abandoned our intention of pursuing them "*. 



I have recently resumed these experiments by the aid of the 

 18-inch speculum belonging to the Royal Society's telescope in 

 my possession. Considerable delay has arisen from the necessity, 

 for these observations, of a more uniform motion of the driving- 

 clock. For 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 satis- 

 factorily. 



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 been obtained of 

 Sirius, Vega, Venus, the Moon, &c. 



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

 the arrangements of the special apparatus which has been con- 

 structed, nor to offer the results of the experiments in their present 

 incomplete state to the Royal Society. Still I venture to hope that, 

 even in this early stage of the inquiry, the enlarged copy of the 

 spectrum of Vega (a Lyree) which accompanies this note may not 



be regarded as altogether unworthy of attention. 



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

 to remain in the instrument until the following morning, when 

 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 w T hich are least refrangible 

 coincide with two known lines of hydrogen in the solar spectrum. 



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

 construction, 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 con- 

 nexion with which photographic observations of stars may be of 

 value. 



* Phil. Trans. 1864, p. 428. 



