THE PRESIDENT'S ADDRESS. 321 



recession was observed to be decreasing gradually from year to 

 year, till in 1882 the star was found to be approaching the earth. 

 In the present year Sirius continues its approach, but there are 

 distinct signs that the maximum motion in that direction has 

 been attained, so that we may expect that a period of recession 

 will probably return after an interval of a few years. This 

 periodic change in the direction of the motion of Sirius may be 

 accounted for by assuming that the star is moving in an elliptic 

 orbit, as then the part of the star's motion, viewed in the 

 direction of the line of sight, would naturally vary periodically 

 as the star changes its position in its orbit. Periodic fluctuations 

 have also been observed in the line-displacements in the spectra 

 of several other stars. Those of Spica Virginis and Pigel seem 

 to indicate an orbital motion, and that the systems of these two 

 bright stars are each compounded of a brilliant and a com- 

 paratively obscure body. 



The limited nature of this address will not allow me to enter 

 more fully into this branch of my subject. To do full justice to 

 it, much more time would be required than what I have been 

 able to devote to spectrum analysis to-day. Besides, I wish 

 before I sit down, to say a few words on a scheme, lately pro- 

 posed, for making a complete photographic survey of the 

 heavens, including thousands of stars invisible to the eye, even 

 in a telescope. Since the invention of the photographic dry- 

 plate process, such a survey has become possible, and it has been 

 found that, by making an exposure of from three to four hours, 

 the images of stars down to the fifteenth magnitude can be re- 

 corded on the plate. To all present appearances, many years 

 will not elapse before this great and unique representation of the 

 heavens will be successfully accomplished. 



The scheme was first proposed to English astronomers in 

 1885 by Admiral Mouchez, Director of the Paris Observatory, 

 who, in a letter addressed to me as President of the Eoyal 

 Astronomical Society, advocated the formation of an international 

 committee to take into consideration the possibility of practically 

 carrying out this great astronomical enterprise. At a subse- 

 quent conference of astronomers of all nations, held at Paris, in 

 April, 1887, it was resolved that the progress made in astronomi- 

 cal photography demands that the astronomers of the present 



