THE GREAT STAR MAP 559 



But Sir David Gill was not dealing with this general 

 rotation of all the stars together : he thought he had detected 

 a relative rotation of the bright stars and it seemed possible 

 that some evidence might be gathered from the photographic 

 measures in the following way. Of the stars whose places 

 had been determined at Cambridge about 1880, some had 

 been photographed at Oxford in 1892, others (say) in 1902, 

 Assuming that there was in reality a relative drift of the 

 bright stars as suggested, the plates of 1892 ought to show 

 ten years of it when compared with the Cambridge observa- 

 tions of 1882, whilst those of 1902 would show twenty years 

 of it. By simple subtraction we could get ten years of the 

 drift. The subtraction is rendered necessary by the existence 

 of the "magnitude equation" already noticed at the beginning 

 of this article, which would affect both determinations of drift 

 and prevent the drift from being identified from either source by 

 itself, though it could be found from their difference if it could 

 be rightly assumed that the effect of magnitude equation was 

 the same in both cases. The experiment was accordingly 

 tried, though scarcely under such favourable conditions as 

 sketched above : with the result that a drift of the bright stars 

 seemed to emerge of about the magnitude assigned by Sir 

 David Gill but in the contrary direction {Mon. Not. R.A.S. 

 Ixiii. p. 56). Attempts were made to find a reason for the 

 discrepancy but on extending the enquiry to movements in 

 declination (Mon. Not. Ixiv. p. 3), proper confirmation was not 

 forthcoming, and it was suspected that there was some un- 

 known source of error (toe. cit. p. 18). At that time it had 

 not been suspected that magnitude equation could occur in 

 photographic measures ; but when subsequently Mr. Hinks 

 came across a gross case of it in the work on the Eros photo- 

 graphs, as above mentioned, it was seen that the unknown 

 source of error had probably been detected and the significance 

 of the measures was thus destroyed (see Mon. Not. Ixv. p. 55). 



But if in this instance we failed to obtain what was searched 

 for with much labour, on another occasion we made a consider- 

 able find without looking for it at all. The history of our 

 Oxford portion of the map was made remarkable by the quite 

 unexpected discovery of a New Star. It would be possible to 

 institute a regular search for new objects by the use of star 

 maps, comparing one plate with another taken on a different 



