118 



THE DESERT 



Dense air- 

 strata. 



Illustration 

 of camera- 

 lent. 



ers of air lying along or a few feet above the 

 surface of the earth on a hot day are peculiarly 

 well-fitted to distort the light-ray, and conse- 

 quently well-fitted to produce the effect of mir- 

 age. These layers of air are of varying densi- 

 ties. Some are thicker than others ; and in 

 this respect the atmosphere bears a resemblance 

 to an ordinary photographic or telescopic lens. 

 Let us use the lens illustration for a moment 

 and perhaps it will aid comprehension of the 

 subject. 



You know that the lens, like the air, is of 

 varying thicknesses or densities, and you know 

 that in the ordinary camera the rays of light, 

 passing through the upper part of the lens, 

 are refracted or bent toward the perpendicular 

 so that they reach the ground-glass " finder " 

 at the bottom ; and that the rays passing 

 through the lower part of the lens go to the top 

 of the "finder." The result is that you have 

 on the " finder " or the negative something re- 

 versed things upside down. That, so far as 

 the reversed image goes, is precisely the case in 

 mirage. The air-layers act as a lens and bend 

 the light-rays so that when seen in our " finder " 

 the eye the bottom of a tree, for example, 

 goes to the top and the top goes to the bottom. 



