HOW TO OBSERVE THE HEAVENS. 



347 



The quantity of light which will enter the eye will depend conjointly on the 

 magnitude of the pupil and the density of the light. If sufficient light to pro- 

 duce vision do not enter the pupil, there are two and only two ways to make 

 it sufficient. We must either enlarge the pupil, or augment the density of the 

 light so as to send in through the unenlarged aperture an increased quantity. 



Since the density of the light which diverges from a visible object dimin- 

 ishes in a very high proportion as the distance from the object is increased, 

 we can increase the density, and thereby render an invisible object visible by 

 diminishing our distance from it ; that is, by approaching nearer to it. This 

 expedient every one is familiar with, but it is an expedient not practicable by 

 a creature whose movements are limited to the surface of the earth. 



Since we can not then approach the object, we must see whether we may 

 not enlarge the window by which light is admitted to the pupil. This the 

 telescope has happily accomplished. In fig. 9, a star is represented with a 

 diverging cone of light proceeding from it toward the eye. The number of 



rig. 9. 



Fig. 10. 



Fig. 11. 



