SECT, xviii.] TEKKESTRIAL REFRACTION. 173 



irregular variations in the density of the air near the surface 

 of the earth, which are sometimes the cause of very singular 

 phenomena. The humidity of the air produces no sensible 

 effect on its refractive power. 



Bodies, whether luminous or not, are only visible by the 

 rays which proceed from them. As the rays must pass 

 through strata of different densities in coming to us, it 

 follows that, with the exception of stars in the zenith, no 

 object either in or beyond our atmosphere is seen in its 

 true place. But the deviation is so small in ordinary cases 

 that it causes no inconvenience, though in astronomical and 

 trigonometrical observations due allowance must be made 

 for the effects of refraction. Dr. Bradley's tables of refrac- 

 tion were formed by observing the zenith distances of the 

 sun at his greatest declinations, and the zenith distances of 

 the pole-star above and below the pole. The sum of these 

 four quantities is equal to 180, diminished by the sum of 

 the four refractions, whence the sum of the four refractions 

 was obtained ; and, from the law of the variation of refraction 

 determined by theory, he assigned the quantity due to each 

 altitude (N. 186). The mean horizontal refraction is about 

 35' 6", and at the height of forty-five degrees it is 58"'36. 

 The effect of refraction upon the same star above and below 

 the pole was noticed by Alhazen, a Saracen astronomer of 

 Spain, in the ninth century ; but its existence was known 

 to Ptolemy in the second, though he was ignorant of its 

 quantity. 



The refraction of a terrestrial object is estimated dif- 

 ferently from that of a celestial body. It is measured by 

 the angle contained between the tangent to the curvilineal 

 path of the ray where it meets the eye, and the straight 

 line joining the eye and the object (N. 187). Fear the 

 earth's surface the path of the ray may be supposed to be 

 circular; and the angle at the centre of the earth cor- 

 responding to this path is called the horizontal angle. The 

 quantity of terrestrial refraction is obtained by measuring 



