1891.] on An Astro7iomer's Work in a Modern Observatory. 413 



tive force is necessary to compel the body to keep in that orbit, and 

 thus we arc able to weigh these bodies. The components of /? AurigaB 

 are two suns, which revolve about each other in four days ; they are 

 only between 7 and 8 millions of miles (or one-twelfth of our distance 

 from the sun) apart, and if they are of equal weight they each weigh 

 rather over double the weight of our sun. 



I have little doubt that these facts do not represent a permanent 

 condition, but simply a stage of evolution in the life-history of the 

 system, an earlier stage of which may have been a nebular one. 



Other similar double stars have been discovered both at Potsdam 

 and at Cambridge, U.S., stars that we shall never see separately with 

 the eye aided by the most powerful telescope ; but time does not 

 permit me to enter into any account of them. 



I pass now to another recent result that is of great cosmical 

 interest. 



The Cape photographic star charting of the southern hemisphere 

 has been already referred to. In comparing the existing eye estimates 

 of magnitude by Dr. Gould with the photographic determinations of 

 these magnitudes, both Professor Kapteyn and myself have been 

 greatly struck with a very considerable systematic discordance 

 between the two. In the rich parts of the sky, that is in the Milky 

 Way, the stars are systematically photographically brighter by com- 

 parison w^ith the eye observations than they are in the poorer part of 

 the sky, and that not by any doubtful amount but by half or three- 

 fourths of a magnitude. One of two things was certain, either that 

 the eye observations were wrong, or that the stars of the Milky Way 

 are bluer or whiter than other stars. But Professor Pickering, of 

 Cambridge, America, has lately been making a complete photographic 

 review of the heavens, and by placing a prism in front of the telescoj)e 

 he has made pictures of the whole sky like this. [Here two 

 examples of the plates of Pickering's spectroscopic Durchmusterung 

 were exhibited on the screen.] He has discussed the various types 

 of the spectra of the brighter stars, as thus revealed, according to 

 their distribution in the sky. He finds thus that the stars of the 

 Sirius type occur chiefly in the Milky Way, whilst stars of other types 

 are fairly divided over the sky. 



Now stars of the Sirius type are very white stars, very rich rela- 

 tive to other stars in the rays which act most strongly on a photo- 

 graphic plate. Here then is the explanation of the results of our 

 photographic star-charting, and of the discordance between the photo- 

 graphic and visual magnitudes in the Milky Way. 



The results of the Cape charting further show that it is not alone 

 to the brighter stars that this discordance extends, but it extends also-, 

 though in a rather less degree, to the fainter stars of the Milky Way. 

 Therefore we may come to the very remarkable conclusion that the 

 Milky Way is a thing apart, and that it has been developed perhaps in 

 a different manner, or more probably at a different and probably later 

 epoch from the rest of the sidereal universe. 



