SECT, xvni.] ASTRONOMICAL REFRACTION- 171 



SECTION XVIII. 



Refraction Astronomical Refraction and its Laws Formation of Tables of 

 Refraction Terrestrial Refraction Its Quantity Instances of Extraor- 

 dinary Refraction Reflection Instances of Extraordinary Reflection 

 Loss of Light by the Absorbing Power of the Atmosphere Apparent 

 Magnitude of Sun and Moon in the Horizon. 



NOT only everything we hear but all we see is through the 

 medium of the atmosphere. Without some knowledge of 

 its action upon light, it would be impossible to ascertain 

 the position of the heavenly bodies, or even to determine 

 the exact place of very distant objects upon the surface of 

 the earth ; for, in consequence of the refractive 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 towards 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 refracted from its course 

 towards 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 perpendicular is in a constant ratio to the sine of the 

 angle contained by the refracted ray and the same perpen- 

 dicular ; 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 atmosphere 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 



