102 IN STARRY REALMS. 



the incalculable clouds overcast the sky, and the whole 

 object of the enterprise is frustrated. 



But even when not charged with clouds the atmosphere 

 is baneful to the practical astronomer, for air when clear- 

 est and purest still obstructs a great deal of light. It 

 makes the stars appear fainter than they would otherwise 

 be, while rays from very small stars are extinguished by 

 it, though those rays would have been quite sufficient to 

 render their source perceptible in our telescopes if we 

 could observe without the intervention of the atmosphere. 



An airless globe like the moon would, for merely tele- 

 scopic purposes, present the most favourable condition 

 conceivable, though how it would fare with the astro- 

 nomers involves questions of a different character. We 

 have not only the imperfect transparency of even the 

 purest skies to contend with, but there are other diffi- 

 culties. Even under the stillest and clearest sky it is 

 of the essence of the atmosphere to distort the places 

 of the celestial bodies. To see a star we have to 

 point the telescope, not at the real position of the star, 

 but in a somewhat different direction. Astronomers 

 speak of this derangement as refraction ; they are obliged 

 continually to bear it in mind, and their observations have 

 to be corrected for it so as to place the star in its true 

 instead of its apparent position. The amount of the dis- 

 placement of a celestial object depends, among other 

 things, upon the temperature of the air. If the tempera- 

 ture changes the amount of the displacement will change. 

 While, therefore, there is any fluctuation of temperature 

 the place of the star will appear to be continually dis- 

 turbed The astronomer will say tha.t "the stars are 



