222 SCIENCE FROM AN EASY CHAIR 



year is five and a half billion (that is, five and a half 

 million million) miles. The nearest sun to us after our 

 own sun is, therefore, about sixteen billion miles away 

 and if its light were suddenly extinguished, we should not 

 know of its extinction for three years. 



How many we may well ask how many of these 

 fixed stars suns like our own are there? Roughly 

 speaking, we can see with the naked eye, reckoning both 

 the northern hemisphere and the southern together (for 

 the stars seen from the former are other than those seen 

 from the latter), about 8000. Not many after all, one is 

 inclined to say. But stop a minute and hear what the 

 telescope reveals. With the best telescope about one 

 hundred million can be seen, less and less brilliant and 

 more difficult to see in proportion to their remoteness. 

 And now we go further even than that. For within the 

 last thirty years the great science of astronomy has been 

 rejuvenated by the application of photography to its task. 

 The invention of the " dry " plate, a sensitive photographic 

 plate which does not spoil by prolonged exposure as the 

 "wet" plate does, enables the astronomer to keep his 

 telescope fixed by slow-moving clockwork on to a given 

 region of the sky for four or five hours or more, and the 

 very faint stars, invisible by the aid of the most powerful 

 telescope stars the light from which is so feeble that it 

 could not affect the plate in a few seconds or minutes, 

 have time by the continued action of their faint light to 

 print themselves on the plate and sign, as it were, a 

 definite record of their existence for man to see and 

 measure, though they are themselves for ever invisible to 

 his eye. It is not possible to say how many may be 

 recorded in this way by photography; it depends on 

 length of exposure. But some thousands of millions of 

 stars can certainly be so recorded. These " unnumbered 

 hosts" are of various degrees of brightness, and by 



