150 ASTRONOMY TOR 1889, 1890. 



The last (ieteruiinatioii of the thickness of Satiiin's ring', as Piijf«-ssor 

 Hall has pointed out, was made in 1848 by W. C'. Bond, who found 

 that it was less than 0."01; Dus^jour estimated its thickness at 0."2, 

 aud Schroeter at 0."13. At the disappearance of the ring in Septem- 

 ber and October, 1891, the conditions of observation are not very 

 favorable, a better o[)portunity occurring in 1892. 



In connection with the approaching disai)p('arance of the ring, an 

 account of observations made by M. E. L. Trouvelot upon the passage 

 of the sun and earth through the plane of the rings in 1877-'78 is of 

 especial interest. 



SaturTi's satellites. — Dr. Hermann Struve has published the second 

 installment of his work on the theory of Saturn's satellites. In this he 

 discusses the orbits of Mimas and Enceladus, and their connection with 

 the other satellites, and he has been able to account satisfactorily for 

 the large corrections to the computed position of Mimas required 

 d'jring the past few years. In his j)revious paper Dr. Struve was led 

 to assume a sensible mass for the ring-system of Saturn, but he now 

 concludes that this hypothesis must be rejected, the mass of the ring 

 being so small that the terms to which it would independently^ give 

 rise in the disturbing function are as yet undetected by observation. 



A determination of the orbit of Titan and the. mass of Saturn, the 

 result of several years' work with the Yale observatory heliometer, is 

 published by Mr. Asaph Eall, jr., in the Transactions of the Yale 

 Observatory, 1889. His value for Saturn's mass is 1:3500.5 ± 1.44, 

 agreeing well with BessePs value 1 : 3502, and that obtained by Struve 

 1:3408. 



Uranus. — Dr. Huggins has found evidence of solar lines in the plioto- 

 grai)hic spectrum of Uranus, with an exposure of two hours (June 3, 

 1889). All the i)rincipal solar lines were seen, but no others either 

 bright or dark. Mr. Tayior, on the other hand, has reported bright 

 flutings seen with a direct vision spectroscope attached to the five foot 

 reflecter of Common's observatory, Ealing, and if this observation is 

 confirmed it will of course prove that the planet is at least in part self- 

 luminous. 



THE MINOR PLANETS. 



The discovery of additional members of the zone of asteroids goes 

 on without the least signs of abatement, and the number has iiow 

 reached 301, no fewer tlian (> having been found in 1889, and 11 in 189i'. 

 Twice during 1890 (April 25 aud September 9) two were discovered on 

 the same evening by the same observer ; and the two discovered by 

 Palisa on April 25 were independently discovered by Charlois on the 

 following evening, April 26. 



