ASTRONOMY. 351 



refraction is a miuimum, the distanee between the images is again 

 measured. The comparison of these two observations gives the means 

 of detern)ining the amount of the refraction with great accuracy. It is 

 of course necessary for the success of the method that the measures 

 should be absolutely independent of every possible displacement of the 

 apparatus in the interval between the observations. This result is ob- 

 tained by placing the double mirror in such a position that the planes 

 of reflection for the two stars coincide, as M. Lcewy finds that under 

 these circumstances, whatever small displacements the mirror may un- 

 dergo, the distance between the images in the field of the telescope, 

 measured in the plane of reflection, or the projection of this distance on 

 the trace of the plane of reflection in the field, remains invariable. M. 

 Lcewy claims for this method that, on account of its simplicity and the 

 ease with which it can be put in practice, it completely solves the prob- 

 lem to which he applies it. A few experiments ought to show whether 

 it is capable of affording results suflSciently accurate to make the method 

 of practical value. (Observatory.) 



STAR CATALOGUES, ETC. 



Glasgow catalogue. — ''The Glasgow Catalogue,* recently published 

 by Mr. Eobert Grant, is the subject of a very detailed review by Dr. 

 Auwers in the Yierteljahrsschrift for 1884. The catalogue depends 

 ui)on observatious made since 1860 by the Ertel meridian circle, and 

 comprehends, besides some hundreds of bright stars selected from the 

 B. A. C, more than 6,000 telescopic stars from Weisse's first cata- 

 logue. The stability of the instrument has been satisfactory, although 

 not comparable with that of the Greenwich circle. The process of re- 

 duction employed by Mr. Grant is not entirely beyond criticism. Dr. 

 Auwers has undertiiken a minute comparison of this catalogue with 

 that of the Astronomische Gesellschaft in order to determine the relative 

 precision of Mr. Grant's positions. He finds the mean error of these 

 positions to be i 0^-06 and ± 0"-9 for the unit of weight (which depends 

 upon the number of observations). This mean error being O^-Ol and 

 0"-6for the best catalogues. Dr. Auwers thinks the relative weight, one- 

 half, may be attributed to the Glasgow positions." (Bull. Astron., 

 March, 1885.) 



Cordoba Zone catalogue. — " In a notice in the Observatory for January, 

 1885, devoted to Dr. Gould's zones, Mr. Downing shows the high value 

 of this great work, which it has required no less than thirteen years co 

 bring to completion. Wo already owe to Dr. Gould the Uranometria 

 Argentina, which the Royal Astronomical Society hoiiored in 1883 with 

 its gold medal. The publication of a general catalogue is looked for 

 soon, and this, with the catalogues of the Cape and Melbourne, will 

 measure the extent of our knowledge of the southern heavens. 



* Catalogue of 6,415 stars for the epoch 1870, «&c., by Robert Grant. Glasgow, 

 1883. 4to. 



