THE GREAT STAR MAP 3 



larger number of stars, called " clock stars," is required. These 

 have had the first claim upon the attention of astronomers at 

 our great observatories during a couple of centuries. A second 

 reason for confining attention to these brighter stars arises 

 from the limitations of instruments. The observations were 

 generally made by watching the star cross the field of view, in 

 which were certain spider lines for reference. Now these lines 

 cannot be seen unless the field of view is illuminated, and a 

 faint star is then lost in the illumination. In these days of 

 electric light it is comparatively easy to adopt a new instru- 

 mental method, whereby the wires themselves (and not the 

 background) are illuminated ; they then appear as bright lines 

 but are not sufficiently dazzling to obscure even a faint star, 

 which can thus be observed as well as a bright one. But in 

 former times this method had not been sufficiently developed 

 and in any case the brighter stars were easier to observe. For 

 these reasons therefore the fainter stars have not attracted at- 

 tention until comparatively recently. One motive for studying 

 them came with the discovery of the minor planets, which dates 

 from the first day of the nineteenth century. It had been 

 realised that there was a gap in the sequence of planets (as 

 arranged in order of distance from the sun) between Mars and 

 Jupiter. It was clear that there could not be any large planet 

 in this position, for it would have been noticed ; but there might 

 be a small one, so search was made for it. The method of 

 search was somewhat laborious. It was necessary to identify 

 all the stars within a certain region in order that any strange 

 body might be detected. It is now easy to accomplish this 

 by taking a photograph of the region ; but at the end of the 

 eighteenth century no such compendious process was available ; 

 then the positions of individual stars were either patiently and 

 laboriously measured one by one, or learned by the astronomer" 

 so that he could carry a picture of the region in his memory. 

 In default of an actual material photograph he practically photo- 

 graphed the image on his own retina. It is astonishing to think 

 how much was accomplished by this toilsome process. Not 

 one only but hundreds of minor planets were discovered in this 

 way, though not without difficulty and delay. Four were found 

 at first in rapid succession and then came a long blank during 

 nearly half a century, so that it seemed as though the number 

 were complete : but though this view proved quite erroneous, 



