2 SCIENCE PROGRESS 



have a tendency to be greater on the larger scale ; and they may 

 indicate either a few bright stars or many faint ones. We are 

 familiar with the fact that there are only a few very bright 

 stars, more of a degree less bright, more still of fainter stars ; 

 and the increase continues as the luminosity diminishes, long 

 after they have ceased to be visible to our eyes, no limit being 

 reached even by the longest exposures given with our largest 

 telescopes. Completeness then can only be a relative term. 

 It is at present impossible to think of giving all the stars in 

 the sky ; we can only settle to give all those brighter than a 

 certain fixed standard. 



The earliest maps of the stars were probably made for 

 astrological purposes ; later they were required for the use of 

 sailors. But through all the centuries so little had been done 

 towards making accurate maps that in 1674, when there arose 

 a question of finding the longitude at sea by observations of 

 the moon and stars, it was pointed out by Flamsteed that no 

 sufficiently accurate maps or catalogues of the stars were avail- 

 able. King Charles II., to whom this information was brought, 

 was thoroughly alarmed at the state of affairs and immediately 

 said that he must have the omission rectified. Thus was 

 Greenwich Observatory established. When asked who was to 

 take charge of the Observatory, the King immediately replied 

 that Flamsteed, who had pointed out the need of such an 

 institution, was the man to put in charge. Modern observation 

 of the positions of the stars may be said to have begun at this 

 period. Greenwich took a great step forward half a century 

 later, when Bradley was made the third Astronomer Royal and 

 increased the accuracy of observation very considerably, so 

 that his results have formed the basis of our knowledge of the 

 positions of the stars to the present time. But Bradley and his 

 successors for the most part confined their attention to the 

 brighter stars, not concerning themselves with those much 

 fainter than can be seen with the naked eye. There are two 

 good reasons for this. In the first place, the number of stars 

 required for the use of sailors is not large ; indeed, sailors 

 themselves use remarkably few, for only the brightest are 

 suitable for observation by the small telescopes of their sextants. 

 Indirectly, however, sailors depend upon the keeping of accu- 

 rate time— Greenwich time is in use all the world over for 

 determining longitude': and for keeping accurate time a much 



