192 ASTRONOMY. 



logue of binary and other interesting stars by Mr. GtLEDHILL, compris- 

 ing altogether some fifteen hundred objects, contains chapters on the 

 equatorial and the observatory by Mr, Crossley ; on methods of observ- 

 ing and the instruments which have been used in this work by Mr. Gled- 

 HiLL; on micrometers by Mr. Ceossley; on the methods of computing 

 double-star orbits, analytically and graphically, by Mr. Wilson and Dr. 

 DoBERCK, and the bibliography of the subject by Mr. Gledhill. The 

 catalogue is conveniently arranged in order of E. A., but the measures 

 of each star are placed opposite the names of the respective observers, 

 an arrangement far less convenient than a simple chronological one. 

 On the whole the work is an excellent one, and contains many valuable 

 suggestions to amateurs, not ordinarily found in astronomical books. 



Professor Holden has tabulated the magnitudes and colors of 1G2 

 stars which are certainly binary, according to a list furnished by Mr. 

 BuRNHAivr, and finds that of these the components of 122 binary stars of 

 the same color differ in magnitude on the average only 0™- 5, whilst those 

 of 40 of different colors differ 2"^- 4. The colors and magnitudes are for 

 the most part taken from M. Strtjve's estimations. Professor Holden 

 quotes a conclusion by Dr. Huggins and Professor Miller, that the 

 characteristic colors of stars are due to the absorptive action of their 

 atmospheres, and compares it with the two following facts : (1) The color 

 of a solid body cooling would as it cooled pass through the shades tvhite, 

 yellow, orange, red, but not through green, hlue, or purple. (2) We do not 

 find isolated stars of decided green, blue, or purple colors. A few such 

 have been recorded, but in most cases erroneously. In general such 

 stars are small, and apparently invariably associated with larger stars. 

 That is, the isolated stars appear always of the colors which would arise 

 in the cooling of solid or liquid masses, and never as if necessarily sur- 

 rounded with absorptive atmospheres — i. e., never decided violet or 

 purple. 



Professor Pickering bas published a pa7)er on the Actual Dimen- 

 sions of the Fixed Stars, in which the hypothesis that stars are of equal 

 intrinsic brilliancy is made the basis of computation. While this hyi)oth- 

 esis is perhaps as little objectionable as any general one that can be made, 

 it still leads to conclusions which cannot be at once accei^ted, as has 

 been shown by Professor Holden in a paper extending Professor Pick- 

 ering's work. 



Variable stars. — For some years it has been generally received that 

 tlie star Alpha Urs(e Majoris x)eriodically changed its color from yeUow 

 through red to orange, etc., every thirty-two days. This supposed dis- 

 covery was made in 1867, by Klein, and has been confirmed, or at least 

 not disproved, by several subsequent observers. 



This question is now tolerably well settled by Safarik, who has 

 chosen three other stars to observe simultaneously with this. He has 



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