PRESIDENT S ADDRESS. 1 3 



and whose motion is the most purel}' periodic that is known. 

 Since 1908, every visible ecHpse of this satellite has been 

 observed at the Union Observatory, so that in the course of 

 time we may expect that our observations may assist in the 

 solution of an obscure problem. 



In dealino^ with the structure of the sidereal universe, or 

 in a smaller way with the dynamics of a star-cluster, it is often 

 tacitly assumed that gravitation is the only force at work. 

 That gravitation is not universally applicable we see in the 

 solar system in the phenomena of comets' tails, and even more 

 so in the disintegration and disappearance of periodic comets 

 such as those of Biela and Holmes. Many double stars are un- 

 doubtedly subject to the law of gravitation in all its 

 purity, l)ut in far many more gravitation appears to be 

 at most only only a secondary force ( thus in the case 

 of double stars of which both components are of the helium 

 tj'pe, there do not appear to be any signs of gravitative 

 action between the two stars.* It is true that stars with 

 variable radial velocities have been found spectroscopicallv. and 

 their orbits deduced by purely gravitational principles, but in 

 many of these cases it is not indubitablv certain that the shift 

 in the lines of the spectrum is due to recession or approach. 

 The difficulty is that in the so-called earlier type of stars, it is 

 found that the H and K lines (»f calcium do not share in the 

 variable motion on which the binary orbit is based. The inter- 

 pretation of spectra — the contradictory behaviour of different 

 lines, their thickness and intensities— still })rovides problems to be 

 solved. In this connection one must refer to the illuminating 

 papers by Dr. Nicholson on the relation between atomic struc- 

 ture and the lines in the spectrum. Nicholson's work makes 

 much use of the spectra of nebulae, in which we see matter 

 under simpler conditions than is possible on Earth. At this 

 meeting Professor Malherbe is reading a paper ui^on " Atoms. 

 Old and New," which will go further into this subject than is 

 possible here. 



Organisation of Astronomy. 



In the earlier part of this address I dwelt upon the power 

 of organisation under scientific direction. I am tempted to 

 develop the subject, limiting my example of organisation to the 

 science of Astronomy, which is truly international in its aims. 

 Astronomers are scattered all over the world, and pursue their 

 work independently of the people amongst whom they live, and 

 who pro\ide the money necessary for their existence. The 

 people are not ungenerous, but they cannot be critical. The 

 Astronomer is on his honour as it were, and this is nearly good 

 enough, but not quite. If the Astronomer is a man of sufficient 

 initiative and energy with a regulated imagination, he will not 

 require much supervision, but he may feel that without the co- 

 operation of his colleagues spread over the world his work may 



* This question is discussed more closely in my paper on the Masses 

 of Visual Double Stars read at this meeting. 



