128 Very — Stellar Revolutions within the Galaxy. 



per sec, and that of the inner ring one-fourth as great, or 

 76,000 km. per sec. 



On September 20, 1901, Mr. G. W. Kitchey obtained a 

 photograph"^ which shows nebulous material on the south 

 southeast side at a distance of 20', after an interval of 211 days 

 from the appearance of the nova. Subsequent photographs 

 demonstrate that this material had almost ceased to move by 

 the end of September. We may assume, therefore, that the 

 tangential component of motion has carried the material of this 

 projection along a magnetic line of force nearly to the limiting 

 radial distance ; and as the path along such a line is 50 per 

 cent, greater than the direct one, the radial distance on Sep- 

 tember 20th was : 



1X300,000X211X86,400 = 3-65X10'' km., 

 and 1"= (3-65X10'')h- 1,200 = 3-04X10' km. 



This gives a parallax of 0''-04:9, agreeing well with the deter- 

 mination from the outer arc of March 29th. 

 ' The parallax 0''-05 may therefore be definitely adopted as 

 that of the nova,f and inferentially of that of the Milky Way 

 in its vicinity. 



We must next form some idea of the structure of the galac- 

 tic system, and of the nature of the motions within it. 



The investigations of Kapteyn:]: show that when the total 

 proper motion (which forms the best criterion of stellar 

 distance available at the present time) falls between /x = 0^^-04 

 and /x = 0'''*05, the stars having spectra of type II exhibit 

 no tendency to aggregate in the neighborhood of the 

 Milky Way, but those of type I already have a marked 

 preponderance in the lower galactic latitudes. For ^l — O'^'OO 

 to /* = 0''*03, the solar stars also begin to be condensed in 

 low galactic latitudes, while the stars of the first type 

 have nearly seven times as great a density in the Milky Way 

 through a zone 20° wide, as in the galactic zones ±60° to 

 ±90°. Consequently, at a mean distance corresponding to 

 /z = 0'^-04:, the confines of the galactic stream have been entered. 

 If this distance also corresponds to the parallax tt = 0''-05, the 

 annual motion of these galactic stars is 1*2x10^ km., and the 

 mean linear velocity is 120/150x4*75 = 3-80 km. per sec. 

 The mean galactic distance must be five times as great as this, 

 or TT = 0^^-01, if the mean linear velocities are equal to that of 

 the sun. 



* Astrophysical Journal, xv, 129, pi. vl, 1902, 



f The most reliable parallax of Nova Persei by the direct method is that of 

 Dr. O. Bergstrand of Upsala, in which the effect of the dispersion of the air 

 on the photographic measures is taken into account, giving absolute paral- 

 lax = 0"-033 (Astronomische Nachrichten, No. 3834). 

 :f On the Distribution of the Stars in Space," Knowledge, xvi, 114, 1893. 



