ASTRONOMY 361 



from tbo sun. It is not possible as yet to draw any conclusion with re- 

 gard to the distribution of the whole mass between the comjjonents, 

 or of the position of the center of gravity of the system. Peters has 

 computed from his elements the following ephemeris : 



Epoch 1885-0 188G-0 1887-0 1888-0 



Position 1190 44' ll>0o V 120o 31' 120° 55' 



Distance 20"-(i0 20"-71 20"-81 20"-92 



Double star measures. — ITos. 2662-G3 and 2677-78 of the Astronomisehe 

 Naclirichten contain a series of measures of double stars made by II err 

 R. Engelmann during the years 1882-'84, preceded by a comparison of 

 the differences between the observer's positions and distances of a num- 

 ber of stars, with those measured by Dembowski and Asaph Hall, and 

 by other particulars bearing upon his own results. For several of the 

 more interesting binaries the following epochs are given : 



o '/ 



Castor - 1882-88 234-3 5-56 



C Cancri 1884-28 67-0 0-94 



fij Leouis 18«4-23 91-4 -ea 



I Ursffi Majoris 1884-41 249-6 1-92 



X Viro-inis 1883-07 155-6 5-22 



42 Comai Berenices : 1882-93 192-1 0-56 



I Bootis 1884-45 266-6 3-65 



Another important series of double-star measures will be found in 

 Astro nomisclie. Nachrichten, 2684-85. They are communicated by M. 

 Perrotin, of the Nice Observatory, and are in continuation of the obser 

 vations published in Nos. 2529 and 2559-60. 



The series of micrometrical measures of double stars made at the 

 Temple Observatory, Eugby, is continued in a "Fourth catalogue," 

 published in volume 48 of the Memoirs of the Royal Astronomical So- 

 ciety. 



A cause of systematic errors in measures of double stars. — M. Bigourdan 

 {Bull. Astro7i., I, 439) has recently pointed out a possible cause of system- 

 atic error in observing i^osition angles. In the case of the Secretan 

 equatorial of 0™ -305 aperture of the Paris Observatory, he finds that 

 the diffraction-rings surrounding the images of the brighter stars are 

 discontinuous; so that when the objective is turned through 180° round 

 its optical axis, as in observing east and west of the meridian, there is 

 sufficient difference in the relative appearance of the components of a 

 double star to cause systematic errors in the observed position-angles. 

 And, in fact, from a series of experiments which M. Bigourdan has car- 

 ried out with this instrument, he finds that there are considerable dif- 

 ferences in the position-angles measured under the same conditions by 

 the same observer, and within a few minutes of each other ; so that the 

 line joining the stars makes much the same angle with the line joining 

 the eyes (always horizontal) in the two cases, one being east of the pier 

 and the other west. M. Bigourdan points out that the errors cannot be 

 eliminated by determination of personal errors from observations of arti- 



