256 IN STARRY REALMS. 



times as far away. The instrument the astronomer uses 

 omst, however, be much more powerful than that which 

 suffices for the mariner. A telescope small enough to be 

 held in the hands would be competent to retain a star 

 within its ken even though the star were removed to a dis- 

 tance ten times as great as when it was just visible with 

 the unaided eye. The larger instruments would be suffi- 

 ciently powerful to follow a star even though it were 

 removed to a distance one hundred times as great as that 

 at which it could be just glimpsed by the naked eye, while 

 the greatest instruments of which our observatories can 

 boast would still hold every lucid star in the heavens 

 within view even were those stars wafted off one thousand 

 times as far as they are at present. 



These facts give us some notion of the extraordinary 

 extent to which great telescopes increase the range of our 

 vision. We are permitted by their aid to sound to a depth 

 in space one thousand times as great as that to which the 

 unaided eye would enable us to penetrate. Above and on 

 each side, and around us in every direction the range of 

 our vision is increased a thousand- fold. At first it might 

 appear that the extent of space accessible to our telescopes 

 would also be increased to one thousand times the extent 

 of naked-eye vision. This would, however, give a very 

 inadequate conception of the truth. With our unaided 

 eyes we are able to see all the objects around us which lie 

 within a certain mighty globe of which our earth is the 

 centre. When we employ great telescopes we can explore 

 a globe of which the radius has been enlarged one 

 thousand-fold, therefore the quantity of space that we can 

 examine in the two cases must be represented by two 



