ADDEESS. 



27 



sequence of the inadequacy of the insti-nments then in use for so delicate 

 an inquiry, the amounts of these motions were but approximate. 



The method was shortly afterwards taken up systematically at Green- 

 wich and at the Rugby Observatory. It is to be greatly regretted that, 

 for some reasons, the results have not been sufficiently accordant and 

 accurate for a research of such exceptional delicacy. On this account 

 probably, as well as that the spectroscope at that early time had scarcely 

 become a familiar instrument in the observatory, astronomers were slow 

 in availing themselves of this new and remarkable power of investigation. 

 That this comparative neglect of so truly wonderful a method of ascertain- 

 ing what was otherwise outside our powers of observation has greatly 

 retarded the progress of astronomy during the last fifteen years, is but 

 too clearly shown by the brilliant results which within the last couple of 

 years have followed fast upon the recent masterly application of this 

 method by photography at Potsdam, and by eye with the needful accuracy 

 at the Lick Observatory. At last this use of the spectroscope has taken 

 its true place as one of the most potent methods of astronomical research. 

 It gives us the motions of approach and of recession, not in angular 

 measures, which depend for their translation into actual velocities upon 

 separate determinations of parallactic displacements, but at once in 

 terrestrial units of distance. 



This method of work will doubtless be very prominent in the astro- 

 nomy of the near future, and to it probably we shall have to look for the 

 more important discoveries in sidereal astronomy which will be made 

 during the coming century. 



In his recent application of photography to this method of determining 

 celestial motions, Professor Vogel, assisted by Dr. Scheiner, considering 

 the importance of obtaining the spectrum of as many stars as possible on 

 an extended scale without an exposure inconveniently long, wisely 

 determined to limit the part of the spectrum on the plate to the region 

 for which the ordinary silver-bromide gelatine plates are most sensitive, 

 namely, to a small distance on each side of G, and to employ as the line 

 ot comparison the hydrogen line near G, and recently also certain lines 

 of iron. The most minute and complete mechanical arrangements were 

 provided for the purpose of securing the absolute rigidity of the com- 

 parison spectrum relatively to that of the star, and for permitting tem- 

 perature adjustments and other necessary ones to be made, . 



The perfection of these spectra is shown by the large number of 

 lines, no fewer than 250 in the case of Capella, within the small region 

 of the spectrum on the plate. Already the motions of about fifty stars 

 have been measured with an accuracy, in the case of the larger number 

 of them, of about an English mile per second. 



At the Lick Observatory it has been shown that observations can be 

 made directly by eye with an accuracy equally great. Mr. Keeler's 

 brilliant success has followed in great measure from the use of the third 



