Knowledge. 



With which is incorporated Hardwicke's Science Gossip and the Illustrated Scientific News. 



A Monthly Record of Science. 



Conducted by Wilfred Mark \\'ebb, F.L.S., and E. S. Grew, M.A. 



OCTOBER, 1910. 



ON THE STUDY OF DOUBLE STARS BY AMATEUR 



OBSERVERS. III. 



Bv G. F. CH.\.MBERS, F.R.A.S. 



THE LABOURS OF THE STRUVES. 



It may be desirable to give some particulars of the 

 labours of at any rate the two first of the three 

 generations of Struve associated with Double Stars. 



The system adopted by the first and greatest of the 

 Struves (F.G.W'.S.) was to divide all the Double 

 Stars measured b}- him into eight classes (and each 

 class into two sub-classes), according to the angular 

 distance of the components. The eight principal 

 classes were as follows : — 



Clais. Llistance of 



Component:). 



I. ... ... ... Less than 1" 



II. 



III. 



IV. 



\'. 



\"I. 



VII. 



VIII. 



The arrangement of the suli-classes had regard to 

 the magnitude of the Component Stars. The first 

 sub-class of every principal class consisted of con- 

 spicuous doubles, termed by Struve diipliccs liicicfijc ; 

 the second of less important doubles termed dupliccs 

 reliijiiijc. The former section comprised stars, each 

 component of which exceeded in brightness the 8^ 

 magnitude ; the latter, stars between the magnitudes 

 8^ and 12 — which last was assumed to be the 

 smallest visilile in the telescope emplo\ed hv him, 

 the refractor at the Dorpat Observator\- in Russia of 

 fifteen English inches aperture. Struve's system 

 was arbitrary and inconvenient for man\- reasons. 

 Amongst others, because Double Stars which are 

 Binaries (and there are man\' such) frequenth' pass 

 from one Strove class to another in the course of a 

 few }ears. .\nd a scale of magnitude such as 1 — 12 



took no account of the possibility of bigger telescopes 

 coming into use, nor of the fact that such a scale 

 was altogether out of touch with the common scale 

 made familiar b\- the labours of the Herschels and 

 man\' other astronomers long before Struve's time. 

 .\s a matter of fact the Struve Classification has 

 never been followed by anybody : and tlie Struve 

 Scale of Magnitudes only to a limited extent. 

 References to Struve's Catalogue are always, and to 

 himself personally often, indicated b}- the Greek Z ; 

 u hilst stars catalogued hv his son. Otto Struve. are 

 frequenth' indicated by (JS, and sometimes in' rr. 

 His grandson Hermann goes as HS. 



Struve's Main Dorpat Catalogue had three 

 appendices. Appendix I. was made up of sixt\' stars 

 between 32" and 7' in their distances. Appendix II. 

 comprised seventeen stars selected from Appendix I. 

 which were, therefore, more than 32" apart, but some 

 of which were possessed of decided proper motion. 

 Appendix III. was supplementar\', containing later 

 measures of many of the more remarkable stars in 

 the Catalogue. 



On the next page is a table, modernised from 

 one published by Sir J. Herschel. which ser\'es the 

 two-fold purpose of indicating how Stru\e's SNStem 

 appears in print, and of furnishing a series of tests 

 for trying the efficiencN' of telescopes. This latter 

 was the motive which inspired Herschel to prepare 

 the table, but I recollect once having a conversa- 

 tion w ith Dawes respecting this table, when he told 

 me that he attached no great importance to lists of 

 the sort so far as regards the testing of telescopes. 

 I think, however, that such a list is really of some use, 

 at any rate to an amateur, and therefore I have 

 decided to give it, after having brought it up to date. 



381 



