8 president's address. 



eleven times the earth's periodical displacement, and the gain is 

 continuous. Hence the mere lapse of time will tell us the dis- 

 tance of the stars, but the problem is complicated because the 

 proper motions of the stars are not mere reflexes of the sun's 

 proper motion ; the stars themselves are also in motion, so that 

 a process of unravelling is necessary. Without any unravelling, 

 but by sim])le averaging, the elder Herschel found out that the 

 sun was travelling in the direction of the constellation Hercules. 

 At Capetown, in 1905, Professor Kapteyn announced his dis- 

 covery that the proper motions of the stars divided themselves 

 into two distinct drifts. The elder Boss found that the proper 

 motions of a widely-spread group of stars converged to a point. 

 The same astronomer also found, from a study of the proper 

 motions, that there was a marked relation between the amount 

 of proper motion and a star's spectrum. 



Investigations based upon proper motions — the thwart or 

 across the line of sight motions — were powerfully aided by spec- 

 troscopic results, and especially by the application of the Doppler 

 piinciple. which tells us almost directly the radial velocity of the 

 star, or its motion in the line of sight. The interpretation of 

 stellar spectra is far from complete, and its problems will not be 

 discussed to-night. The broad facts are that stellar spectra, with 

 a few exceptions, fall into four great classes, which may be called 

 the helium stars, the hydrogen stars, the metallic stars, and the 

 carbon stars, in which the gradation from one class to another 

 is so well marked that it is very plausibly assumed that a star 

 of one class can in the course of time change into its contiguous 

 class, and from that into its next class. At present it is assumed 

 that the helium class is degrading or cooling into the hydrogen 

 class, and that the hydrogen class is similarly approaching the 

 metallic class (in which our sun is), and that later the metallic 

 class will degrade into the carbon class, and that, finally, the 

 carbon class will cool down and become dark stars. This con- 

 tinuous degradation is a convenient inciiwria tcchnica, but it is 

 not based upon any facts. Sir Norman Lockyer, by a closer study 

 of spectra, asserts that there is both a descending and an ascend- 

 ing scale. The assumption that there are the dark stars above 

 referred to is unsupported by any fact. But to-night we are 

 only concerned with spectrum analysis as an aid to interpreting 

 the proper motions of the stars. Radial velocities fully confirm 

 the motion of the sun through space as disclosed by the ])roper 

 motions. The recent spectroscopic determinations of the 

 direction and amount of the solar motion made by Dr. Campbell 

 in America, and by Messrs. Hough and Halm at the Cape, agree 

 Avithin a reasonable margin with the determinations of Newcomb 

 and Boss, which are based in proper motions. Further, as with 

 the proper motions, it is found that as the stars degrade from 

 helium to hydrogen to metallic to carbon spectra, their velocities 

 increase. Professor E. C. Pickering and others have shown how 

 certain species of stars aggregate in certain parts of the sky. 

 Thus the helium stars are only found near the Milky Way, that 



