16 SCIENCE PROGRESS 



necessary for this catalogue with the 28-in. refractor of the 

 Royal Observatory, Greenwich. The catalogue includes all 

 the double stars with a separation of less than 5* discovered 

 from the date of the publication in 1906 of Burnham 's general 

 catalogue, and also those stars previously discovered, but not 

 included by Burnham. The region of the sky covered extends 

 from the North Pole to a north polar distance of 105 °. The 

 catalogue contains 3,950 stars, but the total number of known 

 double stars within this area amounts to 9,724, of which the 

 principal discoverers are Aitken (2,915), Jonckheere (1,282), 

 Hussey (1,138), W.Struve (1,110), and Burnham (855). A study 

 of the distribution of double stars leads to the conclusion that 

 the ratio of the number of close double stars to single stars 

 increases as the total density of the stars increases. 



Celestial Mechanics. — A resume of the lectures delivered at 

 the Sorbonne during the first term of the year 1916-17, by 

 Prof. H. Andoyer on the method of determining a planetary 

 or cometary orbit from three neighbouring observations is 

 printed in Bulletin astronomique , vol. xxxiv. 191 7, pp. 36-67. 

 The author follows generally the procedure of Charlier and 

 Moulton, and combines the advantages of the classic methods, 

 that of Gauss, and that of Laplace as recently modified by 

 Leuschner. For a concise and clear summary of this by no 

 means easy subject this paper can hardly be excelled. 



Several important publications have been issued recently 

 by certain observatories : amongst these may be mentioned : 



" Photometric Researches : The Eclipsing Variables RV 

 Ophiuchi, RZ Cassiopeise," by R. S. Dugan, Contributions 

 from the Princeton University, No. 4, pp. 38, 19 16. — RV 

 Ophiuchi is found to consist of two stars, one of which emits 

 nearly five times as much light as the other, and has a surface 

 brightness twelve times as great, the radius of the brighter 

 being two-thirds that of the fainter star. The distance between 

 their centres is five times the radius of the fainter star. The 

 sides of the stars facing one another are brighter than the 

 opposite sides, and both stellar discs are elliptical. There is 

 evidence of a difference in brightness of the advancing and 

 following sides of the brighter star. The star RZ Cassiopeiae 

 was interesting because it appeared as though it might be the 

 first eclipsing variable to be discovered which had no secondary 

 minimum. The observations show that this is not the case, 



