254 SCIENCE PROGRESS 



there had been, owing to an accident, a longer interval than 

 usual. After two exposures had been successfully given, clouds 

 had come up and it was impossible to give the third on that 

 night. But in these da3''s of "dry plates" such an occurrence 

 does not mean the loss of the work already done ; it was only 

 necessary to close up the plate securely until the next fine night 

 when the telescope was again pointed in precisely the right 

 position and the third exposure given. But meantime the star 

 had changed in brightness a little, and so M. Baillaud's careful 

 scrutiny enabled him to discover that it was variable — a success 

 which he afterwards repeated in another instance. Indeed the 

 accident was suggestive ; just as an accidental effect at rehearsal 

 is often deliberately adopted subsequently in the play itself, so 

 it was made clear by the unavoidable separation of the exposures 

 in question that there would be a distinct advantage in separa- 

 ting them deliberately. M. Baillaud made this suggestion at the 

 recent meeting of the Permanent Committee and the wisdom of 

 it was at once recognised. 



The detection of variable stars, however, is not a regular part 

 of the work on the Map. These objects are few and exceptional : 

 we are concerned chiefly with the many and the average. The 

 many are counted by thousands and millions, thousands of stars 

 on a single plate and millions on the whole collection of plates. 

 Perhaps a few definite figures may be given here : not too many 

 of them, for they are apt to be tiresome ; but one or two repre- 

 sentative figures will give a crisper idea of the magnitude of the 

 work. 



There are eighteen observatories concerned : the share of each 

 is about 1,200 plates, taken twice over with short and long expo- 

 sures. Fixing our attention on the short exposures, there are 

 on the average 400 to 500 stars on each, the places of which are 

 to be measured and recorded. But this average is struck from 

 numbers which diverge widely ; on some plates there may be 

 5,000, on others less than 100, the rich plates being of regions 

 in the Milky Way and the poor ones of regions far from it. 

 Each observatory has thus to measure about half a million star 

 images and as the number of figures required to record each 

 measure may be several dozen, it is easily seen that many millions 

 of figures are used by each observatory. These measures took 

 a staff of four or five people at Oxford some ten years or so to com- 

 plete : and the printing of them another four years. The checking 



