ILLUSIONS 



117 



excessive bending of light-rays in traversing 

 adjacent layers of air of widely different densi- 

 ties, whereby distorted, displaced, or inverted 

 images are produced." * 



This is no doubt the true explanation of that 

 form of mirage in which people on Sahara see 

 caravans in the sky trailing along, upside down, 

 like flies upon the ceiling ; or on the ocean see 

 ships hanging in the air, masts and sails down- 

 ward. But the explanation is very general and 

 is itself in some need of explanation. Perhaps 

 then I may be pardoned for trying to illustrate 

 the theory of mirage in my own way. 



The rays of light that come from the sun to 

 the earth appear to travel in a straight line, 

 but they never do. As soon as they meet with 

 and pass into the atmospheric envelope they are 

 bent or deflected from their original direction 

 and reach the earth by obtuse angles or in long 

 descending curves like a spent rifle ball. This 

 bending of the rays is called refraction, which 

 must not be confounded with reflection — a some- 

 thing quite diiferent. Now refraction is, of 

 course, the greatest where the atmosphere is the 

 densest. The thicker the air the more acute the 

 bending of the light-ray. Hence the thick lay- 

 * Century Dictionary. 



Need of ex- 

 plamation. 



of light- 

 rays. 



