CHAPTER XIX. 



THE EXTENT OF THE SIDEREAL HEAVENS. 



IN discussing the extent of the visible universe, it must 

 always be borne in mind that the farther a source of light 

 is from us the fainter is the illumination which we receive 

 from it. Suppose that a star which just lies on the limits 

 of naked-eye visibility were somehow to be transported to 

 a distance which is twice as great, then the lustre of that 

 star would be diminished to one-fourth of its original 

 amount. It would, therefore, be of course invisible to the 

 unaided eye, but could still be easily perceived by a tele- 

 scope. Indeed, the very word telescope means an instru- 

 ment for looking at objects a long way off, and the effect 

 of the telescope is to reduce the apparent distance of the 

 object. Thus the binocular glass that the mariner uses at 

 sea has the seeming effect of reducing distances to about 

 one-third of their amount, so that if a star were carried off 

 to three times its actual distance from us it would still be 

 shown as clearly in a binocular as it would be seen by the 

 unaided eye when placed at its original distance. A star 

 just on the verge of naked- eye visibility would still remain 

 within the grasp of such an instrument even were it three 



