304 IN STARRY REALMS. 



of this size would be required to cover completely the 

 whole vault above the horizon and below. If, then, there 

 be over sixteen thousand stars on one of these plates, it 

 follows that the total number over the sky capable of 

 being disclosed by photography cannot be less than one 

 hundred and sixty millions. It will be instructive to 

 compare these figures with the stellar statistics afforded 

 by other methods. 



If we take a position on the equator, from whence, oi 

 course, all the heavens can be completely seen in the lapse 

 of six months, the number of stars that can be reckoned 

 with the unaided eye will, according to Houzeau, amount 

 to about six thousand. If we augment our unaided vision by 

 a telescope of even small dimensions, such as three inches 

 in diameter, the number of stars in the northern hemi- 

 sphere alone is upwards of three hundred thousand. We 

 may assume that the southern hemisphere has an equally 

 numerous star- population, so that the entire multitude 

 visible with this optical aid is about six hundred thousand. 

 Thus we see that the use of a telescope small enough to 

 be carried in the hands suffices to multiply the lucid stars 

 one hundredfold. Great telescopes no doubt soon show 

 us that the hundreds of thousands are only the brighter 

 members of a host of millions, and now we receive the 

 assurance of photography that the telescopic stars are 

 only the more conspicuous members of that vast universe. 

 Mr. Roberts indeed declares that the multitudes of stars 

 on the photographic plate grow with each increase of 

 exposure to such a degree that it would almost seem as if 

 the plate would be a well-nigh continuous mass of stars if 

 the operations could be sufficiently protracted. 



