THE GREAT STAR MAP 549 



individual stars ; it was soon seen that a peculiarity was 

 manifest in the individual errors depending upon the brightness 

 of the star, and from independent information it was known 

 what was the reason of the discrepancy. 



The Cambridge observations had been made by watching 

 the transit of a star across spider webs, recording the time of 

 transit according to the clock. It has long been known that 

 different observers have a persistent personal characteristic 

 which has been called their " personal equation," in virtue of 

 which they are systematically a little early or a little late in 

 their records. More recently it has been found that even the 

 same observer will vary in his habit according to the brightness 

 of the star, the general tendency being to be late for the faint 

 stars. The tendency is more marked in some individuals than 

 others. The Cambridge observer (the late Mr, A, Graham) 

 apparently had a strongly marked tendency of this kind. 



Various methods have been suggested for the valuation of 

 this habit, especially the method which depends on using gauze 

 screens to reduce the light of a star and thus to substitute for 

 it a virtually fainter star occupying exactly the same space as 

 the brighter one. If the observer were free from the magnitude 

 equation error, he would make a record of the transit precisely 

 the same in the two cases; but if he be subject to the malady, 

 his records will differ by an amount which affords a measure 

 of his predisposition. There are, however, some difficulties 

 of a practical kind in using this method, and it was pleasant 

 to realise that in the photographic plate we had found a simple 

 and effective means of determining the magnitude equation 

 without the necessity for any special observations on the part of 

 the observer. 



A few details may be given which bring out some interesting 

 points. The first attempt at detecting the magnitude equation 

 from the Oxford measures was made by Mr, Hinks in 1897, 

 when only seven plates had been measured (Moii. Not R.A.S. 

 Ivii. p. 473). The material available was only sufficient to 

 demonstrate the value of the method, Mr. Hinks recording 

 his opinion that " when the reductions for the Astrographic 

 Catalogue are completed, it will be possible to discuss very 

 accurately the personal equations depending on magnitude." 

 The reductions are now completed and the discussion is being 

 undertaken ; but we did not wait until now for confirmation of 



