PROGKKSS OF ASTKONOMY I'OU 1891 AM) 1892. ('}[)?) 



the '' Wolf-Rayet," or us lie sug.uests the '• fiftli type," every one He.s 

 within 10^ of the Gnlaetie cciuator, two-thirds within 2° of it. 



n Virginis. — Dr. Vo<iers more recent observations oH a ^'ir,iiinis at 

 Potsdam aeeord with his earlier ol)servations of the same star, show- 

 in.i;' that it is a close biiiaiy. The method of observation is quite 

 iiiterestiii.ii' : The si)ec(rnni of the star and of terrestrial hydroijen are 

 ])hi)to<;ra])hed to<ietlier, and tiie <lisplaceniejit of the star lines on the 

 l)hotof>rai)li in the neighborhood of 11 ;- is afteiwards measured under 

 a microsc()])e. Stars with si>ectra of the second ;iiid tliir<l types j^ive 

 results of consideral'.le accuracy, as the lines in such stars ar<' numer- 

 ous aiid sharp. 



In the case of a \'ir<;inis tiie ditiiculties of observat ion were iireater, 

 the hydroii'en lines being- broad and diiiuse, without any delinite maxi- 

 mum of intensity, and there were no distinct lines in the vicinity of 

 H K to which the measurements coidd be referred. Dr. Vogei's me;is- 

 urements of twenty-four photograi)hs showed that the star lines were 

 disjdaced alteriiately toward the upi)er and the lower end of the sjjcc- 

 trum in a complete i)eri()d of about four days, the maximum disi)lace- 

 ment toward the violet iiulicating a motion of the star towaid the sun 

 of G.l.O English miles, and that toward the red a receding motion ot 

 47.5 miles per second. These observations are completely explained by 

 sui)posing that Spira is a binary star having a period of one comj)0- 

 nent about the other or the common center of gravity of about 4 days, 

 (the orbital velocity of the larger component being .">(). 7 miles per sec- 

 oml) and that the system is approaching the sun at the rate of 1).2 

 miles i)er second. On the assumption of a circular oibit, equal mass of 

 the components, and the data given by observation, the mass of the 

 system is 2.(1 times that of the snn, and the distance hetween the 

 comi)onents (>,200,000 miles. 



In <:ommenting' upon Dr. \'oge]'s work Pi'of. Keeler says, ''A won- 

 derful ])icture of stellar Jiiotion is presented to our mind, and one to 

 which the whole visible nin\erse as revealed to us l)y oui' greatest tele- 

 scopes otters no parallel. The spectacle of two great suns like our own, 

 revolving around ea<h other in only four da\s, at a distance no greater 

 than that which se])arates the sixth satellite of Saturn from its 

 primary, is one whi<di the inade(imicy of our o])tica] powers will ])r<)ba- 

 bly ever forbid us from actually beholding, but the indirect e\id(Mice 

 that such extraordinary circumstances of motion exist is so complete 

 that we must admit their reality." 



/j Aiir't(j(v. — The Potsdam obser\ations fuini.sh a complete <;on(irnui- 

 tion of Prof. Pickering's discovery of the duplicity of this star. The 

 lines in the sjn'ctrum of the star api)ear double on every s(>cond day, 

 and thecomponent, in the line of sight, of themotion of the system can 

 amount to nearly l.")0 miles a .second, while the v.hole system has a 

 motion relatively to the solar system of —4.0.'} miles; that is, a motion 

 of this amount per second towards the solar system. 



