148 ASTRONOMICAL REFRACTION. SECT. XVIII. 



Tractive power of the air, no distant object is seen in its 

 true position. 



All the celestial bodies appear to be more elevated 

 than they really are ; because the rays of light, instead 

 of moving through the atmosphere in straight lines, are 

 continually inflected toward the earth. Light passing 

 obliquely out of a rare into a denser medium, as from 

 vacuum into air, or from air into water, is bent or re- 

 fracted from its course toward a perpendicular to that 

 point of the denser surface where the light enters it 

 (N. 184). In the same medium, the sine of the angle 

 contained between the incident ray and the perpendic- 

 ular is in a constant ratio to the sine of the angle con- 

 tained by the refracted ray and the same perpendicu- 

 lar ; but this ratio varies with the refracting medium. 

 The denser the medium the more the ray is bent. 

 The barometer shows that the density of the atmos- 

 phere decreases as the height above the earth increases. 

 Direct experiments prove that the refractive power of 

 the air increases with its density. It follows therefore 

 that if the temperature be uniform, the refractive power 

 of the air is greatest at the earth's surface and dimin- 

 ishes upward. 



A ray of light from a celestial object falling obliquely 

 on this variable atmosphere, instead of being refracted 

 at once from its course, is gradually more and more bent 

 during its passage through it so as to move in a vertical 

 curved line, in the same manner as if the atmosphere 

 consisted of an infinite number of strata of different den- 

 sities. The object is seen in the direction of a tangent 

 to that part of the curve which meets the eye, conse- 

 quently the apparent altitude (N. 185) of the heavenly 

 bodies is always greater than their true altitude. Owing 

 to this circumstance, the stars are seen above the hori- 

 zon after they are set, and the day is lengthened from 

 a part of the sun being visible, though he really is behind 

 the rotundity of the earth. It would be easy to de- 

 termine the direction of a ray of light through the at- 

 mosphere if the law of the density were known ; but as 

 this law is perpetually varying with the temperature, 

 the case is very complicated. When rays pass perpen- 

 dicularly from one medium into another, they are not 



