PRESJDLNT S ADDRESS. Q 



great girdle of stars which is the framework of the sidereal 

 system. The direct measurement of parallaxes, and the small- 

 ness of their proper motions, both indicate that the helium stars 

 are enormously distant ; and conversely, that stars near us are 

 generally of the metallic spectrum class. Besides the Taurus 

 group of converging stars found by Boss, several other groups, 

 with members spread all over the sky, have been found. The 

 stars in these groups appear to be moving with nearly equal and 

 parallel velocities through space. It is evident that once a star 

 is grouped correctly, and the parallax or distance and velocity 

 of any one star in its group is known, we can also determine its 

 distance. Unfortunately the Doppler principle, by which astron- 

 omers determine the radial velocities of the stars, is somewhat 

 limited in its application. In the helium and hydrogen classes 

 the lines of the spectrum are few, and are difficult to measure, 

 and in all classes it is only possible to measure the displacements 

 of the lines of the bright stars. Even if we anticipate improve- 

 ments in the art of spectrography, it would seem impossible to 

 obtain spectroscopic data in the form required for more than 

 twenty or thirty thousand of the brighter stars. Therefore, 

 although spectroscopy will be a useful ally, its help is limited. 



Let us now collect the data which are at the astronomers' 

 disposal for finding the distance of the more distant stars. The 

 most important datum is the star's proper motion. This is 

 compounded of the reflex of the sun's motion and of the star's 

 own proper motion, which latter may be eliminated b}- a process 

 of judgment ))y assuming that the star is an average member of 

 its group and spectral class, or that it belongs to one or other of 

 Kapteyn's two drifts. Although in individual cases these indi- 

 cations may be very erroneous, yet in the gross they are permitting 

 astronomers to classify the stars into manageable groups. 



What is wanted is a better knowledge of the proper motions 

 of the stars. Unfortunately at present these are not well known 

 except for perhaps 10,000 of the brighter stars. Hitherto, the 

 finding of the i^roj^er motions of the stars has been slow, arduous 

 and expensive work. .\t least ten meridian observations, spread 

 over half a century, were essential, and each meridian observation 

 cost about 20s., and meridian observations can only be made of 

 the brighter stars — of i)erhaps 100,000 out of i ,000,000,000 stars 

 now within the reach of the largest telescopes, or of one star in 

 every 10,000. This proportion is altogether too one-sided. 

 Hence astronomers hailed the advent of the photographic dry 

 plate. An organisation for a Carte dii del was formed, in which 

 our first President, the late Sir David Gill, was one of the chief 

 promoters, and this scheme has now been at work for twenty- 

 eight years ; but, so far, the first Carte is far from complete. 

 When completed in ten or twenty years time, we may expect it 

 to furnish us with precise positions of some 3,000,000 stars (or 

 of about I star in 300, still a very small proportion). We will 

 not know the j)roper motions of these stars. To achieve that, 

 another Carte dn Clel must be prepared, so that we must expect 



