PROGRESS IN ASTROPHYSICS — ABBOT. 187 



these he has deduced the most probable positions in right ascension 

 and declination of the stars, and the proper motions of the stars as 

 indicated by the observations of them at long separated epochs. The 

 result obtained with regard to proper motion have led him to a most 

 interesting series of papers, some conclusions of which will be re- 

 ferred to in what follows: 



The third great piece of work to which I have referred is not yet 

 published in extenso, but has been published in part. It exists in 

 maunscript, and the principal conclusions to be immediately derived 

 from it have already been published in a series of interesting papers 

 by Prof. W. W. Campbell, director of the Lick Observatory. 



It is now 16 years since Director Campbell described the Mills 

 spectrograph of the Lick Observatory. This fine instrument, re- 

 modeled in 1902, has been unremittingly used by him until the pres- 

 ent time. A companion Mills spectrograph was installed under Di- 

 rector Campbell's direction in Chile in the year 1903, and this also 

 has been diligently employed by successive observers sent down from 

 the Lick Observatory. 



It was Director Campbell's intention in this long campaign to ob- 

 serve the spectra of all stars brighter than the fifth magnitude, in 

 both the northern and southern hemispheres, in a manner adapted 

 to determine accurately the motion of each of these stars in the line 

 of sight; that is to say, in a direction toward or from the earth. This 

 motion is also termed radial velocity. The time of exposure neces- 

 sary for photographing a single spectrum ranges from a few minutes 

 up to several hours, according to the brightness of the star and the 

 quality of the atmospheric conditions. It is necessary in such a 

 campaign as that which the Lick Observatory has been making to ob- 

 serve each of the stars several times in order to confirm the velocity 

 found or to detect the presence of variability of velocity, such as 

 often leads to the most interesting results. The Lick Observatories 

 in .California aiid Chile have observed between 1,000 and 2,000 stars 

 for radial velocity, and these, Avith a considerable number of others 

 observed by other observatories, made up a list exceeding 1,700 in 

 number, which was discussed by Director Campbell in a series of 

 papers in the year 1911. 



About one-fourth of the stars observed were found to be spectro- 

 scopic binaries. That is to say, although they appeared to be single 

 points of light to the telescope, yet certain peculiarities in the dis- 

 placements of their spectrum lines from time to time indicated that 

 each of the apparent points of light embraced a system of celestial 

 objects comparable in some respects with the solar system. This 

 similarity, however, doas not extend to details, for the objects in- 

 cluded in a spectroscopic binary, or multiple star, are each usually 

 hot enough to give light by itself, and in general are objects of more 



