GAUGING THE UNIVERSE. 



This is what has been called the space-penetrating power of the 

 telescope. 



If the light of a star of the sixth magnitude be 100 times less 

 than that of a star of the first magnitude, a telescope which would 

 augment the light 100 times, would exhibit it with the same 

 apparent brightness as a star of the first magnitude. 



Thus, for example, the reflecting telescope used by Sir William 

 Herschel, in some of his principal stellar researches, had an 

 aperture of eighteen inches, and twenty feet focal length, with a 

 magnifying power of 180. The space-penetrating power of this 

 instrument was found to be seventy-five, the meaning of which 

 is, that when directed to a star of any given brightness, it would 

 augment its brightness so as to make it appear the same as it 

 would be if at seventy-five times less distance, or what is the 

 same, that a star which to the naked eye would appear of the 

 same brightness as that star does when seen in the telescope would 

 require to be removed to seventy-five times the actual distance, so 

 that when seen through the telescope it would have the brightness 

 it has when seen with the naked eye. Thus a star of the sixth 

 magnitude, if removed to seventy-five times the actual distance, 

 would appear in such an instrument still as a star of the sixth 

 magnitude would to the naked eye ; and if we assume with Sir 

 John Herschel, that a star of the sixth magnitude has a hundred 

 times less light than o Centauri, and is therefore at ten times a 

 greater distance, it will follow that o Centauri would require to 

 be removed to 750 times its actual distance, so that when viewed 

 through such telescope it would be seen as a star of the sixth 

 magnitude is to the naked eye. 



40. If, then, it be assumed, as it may fairly be, that among the 

 innumerable stars which are beyond the range of unaided vision, 

 and brought into view by the telescope, a large proportion must 

 have the same magnitude and intrinsic brightness as the average 

 stars of the first magnitude, it will follow that these must be at 

 distances 750 times greater than the distance of an average star 

 of the first magnitude, such as a Centauri. But it has been 

 already shown that the distance of a Centauri is such that light 

 would require 3-54217 years to come from it to the earth. It 

 would therefore follow, that the distance of the telescopic] stars 

 just referred to, must be such that light would take to come from 

 them to the earth 



3-54217 X 750 = 2656-6275 years. 



If it be desired to ascertain the distance of such stars, taking 

 the earth's distance from the sun as the unit, we shall have 

 225920 X 750 = 169,440000. 



It appears, therefore, that the distance of such a star would be 



o2 195 



