684 PROGRESS OF ASTRONOMY FOR 1891 AND 1892. 



but 110 instance lias hitherto been known of a star bright enough to be 

 visible to the naked eye having a small definite nebula within even sev- 

 eral times the distance of this from Merope. 



ASTRONOMICAL CONSTANTS. 



The Constant of Aberration. — Prof. Oomstock, of the Washburn Ob- 

 servatory, has been making careful trial of a modification of the method 

 of determining the constant of aberration first suggested by M. Loewy. 

 The essential feature of M. Loewy's method is the introduction of 

 reflecting surfaces in front of the objective of a telescope, by means of 

 which images of different portions of the heavens are simultaneously 

 produced in the focal plane of the objective. By means of the microm- 

 eter the apparent distance between the images of two stars thus pro- 

 duced may be measured, and the angular distance between the stars 

 determined from a simple relation involving the measured quantity and 

 the angle included between the reflecting surfaces. It is obvious that 

 great difficulties would attend the determination of this angle, and M. 

 Loewy avoids these difficulties by measuring the distances of two pairs 

 of stars and taking the dift'erence of these distances, thus eliminating 

 the angle between the mirrors. Prof. Comstock has found it advan- 

 tageous to place before the objective three reflecting surfaces instead 

 of two, making approximately equal angles among themselves, and to 

 employ successively each pair of surfaces in measuring the distance 

 between two given stars. If the normals to these surfaces all lie in 

 the plane passing through the two stars and the earth, the mean of 

 the three dihedral angles formed by the surfaces will be exactly 120°; 

 and by taking the mean of the results furnished by the three pairs of 

 surfaces the distance between a pair of stars may be determined in- 

 dependently of the angles between the mirrors. Prof. Comstock's pro- 

 visional result for the constant of aberration is 20".494 iO".Ol7. 



MM. Loewy and Puiseux's work on the Constant of Aberration is 

 summarized as follows in a communication to the Comptes Rendus for 

 March 16, 1891. 



1. Struve's value 20".415 is very near the truth. It would, in our 

 opinion, be premature to alter it. 



2. M. Fizeaii's result, that reflection does not affect the behavior of 

 rays with regard to aberration, is confirmed. 



3. The new method for determining aberration can be regarded as 

 satisfactory and definitive. 



STAR CATALOGUES AND CHARTS. 



The Star Catalogue of the Astronomische Gesellschaft. — The zone under- 

 taken by the Harvard College Observatory +50° to +550 declination 

 has been published as the fifth part of the great catalogue. The obser- 

 vations were made with the new meridian circle in the years 1870-'78 

 and 18S3-'84, chiefly by Prof. W. A. Kogers*, under whose direction 



