THE GREAT STAR MAP 



IV. SOME INCIDENTS OF THE WORK 



By H. H. turner, D.Sc, D.C.L., F.R.S. 



Savilian Professor of Astronomy in the University of Oxford 



The general history of this enterprise as exempHfied more 

 particularly in the portion of the work undertaken at Oxford 

 has now been given and it remains to notice several incidental 

 investigations of different kinds which have branched from the 

 main project. In a piece of work already extending over about 

 twenty years, in a new department of science such as the 

 application of photography to astronomy, it is only natural 

 that the consequences of the departure should not have been 

 foreseen in their entirety at the outset. 



The first novelty which attracted our attention at Oxford 

 was what is called the " magnitude equation " of the Cambridge 

 meridian observations. In order to determine completely the 

 places of the stars on any one of our photographic plates, it 

 was necessary to know the places of a few of them in the sky, 

 so that we might virtually peg down the plate in its proper 

 place on the sky and refer all the new and previously un- 

 measured stars to their proper positions. For this purpose 

 a large number of meridian observations made at the Cam- 

 bridge Observatory some years before were ready to hand. 

 Having selected the stars required and used them without 

 difficulty for the purpose described, we found what are called the 

 "constants of the plates." For each plate two stars would have 

 sufficed, had everything been theoretically perfect ; but in order 

 to compensate for the small errors of various kinds unavoidable 

 in scientific work, it was desirable to make use of many more 

 stars than the theoretical minimum. From the general average 

 of all the stars considered we ascertained the relative errors of 



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