OHAP. IV.] 



THE TELRSCOPK. 



aperture of the pupil, the light is increased in the ratio of 36 to 

 72,900 or 1 to 2,025, the absorption of light by the lens being 

 neglected. This also explains the possibility of distinguishing in 

 the daylight with the telescope, stars which can only be seen with 

 the naked eye in the evening or during the night. 



Bodies not luminous of 'themselves, such as the moon and planets, 

 appear in the telescope much less brilliant than with the naked eye 



FIG. 200. The same portion of the heavens seen with a telescope of ~H centimetres aperture. 



and it therefore follows that the magnifying power is limited for a 

 given aperture. 



Astronomical instruments require such perfection in their manu- 

 facture as makes them costly acquisitions. The object-glass requires, 

 besides purity of material, a long and difficult process of grinding 

 and polishing, without which the sharpness and achromatism of the 



