78 POPULAR LECTURES AND ADDRESSES. 



46. (11) The refraction, of light is the change 

 of direction which a ray is found to experience 

 in passing from one transparent medium as 

 luminiferous ether, 1 or air or water, to another 

 transparent medium, as air or water or glass. 

 Light entering the earth's atmosphere from 

 the sun or moon or stars, in any other direc- 

 tion than the vertical experiences refraction, by 

 which its inclination to the vertical is diminished 

 as it passes through denser and denser strata of 

 the atmosphere down to the surface. Hence every 

 observed altitude must be corrected for refraction, 

 in order that the true altitude of the straight line 

 from the object to the observer may be determined. 

 The correction is clearly greater the farther the 

 object is from the zenith. The amount of the 

 correction is 33' when the line of vision is 

 horizontal. In this case the object is actually 

 below the horizon by this amount, so that a ray 



1 Luminiferous ether is a name given to the substance, ether or 

 aether, occupying space outside some indefinite limit, perhaps 20, 

 perhaps 50, perhaps 100 miles high, within which the earth's 

 sensible atmosphere is contained. 



