682 PKOGRESS OF ASTRONOMY FOR 1891 AND 1892. 



literature, to be found in English, while the excellent reviews in Nature 

 and the more popular notes of the Athenwum need no special comment 

 here. The Astronomische Nachrichten and the Astronomical Journal 

 contain occasional notices of important works. 



The '^iSTotes on some points connected with the progress of astron- 

 omy during the past year" in the Monthly Notices of the Eoyal Astro- 

 nomical Society have been increased in scope and fullness, and as the 

 reviews in difterent branches of astronomy are furnished by specialists, 

 these notes form a most valuable commentary on the year's work. The 

 Vierteljahrsschrift der astronomischen GeseUschaft is, of course, the crit- 

 ical astronomical review, and is the recognized authority for summa- 

 ries of cometary and planetary discoveries. 



STELLAR SYSTEMS. 



The Milky Way. — The independent researches of Prof Pickering at the 

 Harvard observatory and of Dr. Gill at the Cape of Good Hope have 

 led to the conclusion that the stars of the Milky Way form a veritable 

 sidereal system, separate and individual. This conclusion is entirely 

 opposed to the views Sir William Herschel reached from his earliest 

 observations (1785) which are still generally received by those who 

 have not given much attention to this special question. Miss Gierke 

 points out in the Observatory for September, 1891 (p. 302), that "the 

 study of nebular distribution might alone, and long ago, have driven 

 out of the iield every form of ' projection theory' of the Milky Way. 

 For it showed the great majority of gaseous nebuhiB to be embraced 

 within its circuit, and this alone amounted to a demonstration that a 

 physical reality, and not simply a geometrical appearance, was in 

 question," 



A brief statement of the arguments of Prof. Pickering and of Dr. 

 Gill is contained in a lecture by the latter delivered at the Koyal 

 Institution of Great Britain, May 29, 1891. Dr. Gill said: 



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 Prof. 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 comparison with 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 Prof Pickering, of Cambridge, America, has lately 

 made a complete photograiihi(^ review of the heavens and by placing 

 a prism in front of the telescope he has made pictures of the whole 

 sky. . . . He has discussed the various types of the spectra of the 

 brighter stars, as thus revealed, according to their distribution in the 



