SECT. XVIII. ASTRONOMICAL EEFRACTION. 



SECTION XYIIlf 



Refraction Astronomical Refraction and its Laws Formation of Tables 

 of Refraction Terrestrial Refraction Its Quantity Instances of 

 extraordinary Refraction Reflection Instances of extraordinary Re- 

 flection 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 posi- 

 tion 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. 189). In the 

 same medium, the sine of the angle contained between the in- 

 cident ray and the perpendicular is in a constant ratio to 

 the sine of the angle contained by the refracted ray and the 

 same perpendicular; 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 de- 

 creases as the height above the earth increases. Direct experi- 

 ments 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 diminishes upwards. 



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 



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