Curvature and Refraction 



The phenomenon which produces the mirage in very hot or 

 extremely cold climates also affects more or less, under all climatic 

 conditions, barometric pressure and rectilinear vision in solar obser- 

 vations, in hypsomctry and precise differential leveling. 



When a luminous ray passes obliquely from one air stratum to 

 another, the incident ray is bent or re- 

 fracted from its original direction. In a 

 general way it may be said, if it passes 

 into a denser medium, it is bent nearer to 

 the perpendicular or normal line NM, 

 and conversely, if into a rarer medium it 

 is bent away from the normal line. 



In Fig. 18, let S be the source of light 

 and AB the line of separation between 

 two strata of different density. The 

 angle of incidence is SON and the 

 Fisr- 8 angle of refraction is ROM. Assuming 



that the region ARMB is the denser, the beam SO will be refracted 

 in the direction OR; but had the region ARMB been superheated and 

 therefore less dense, the angle ROM would have been greater than 

 the angle SON. 



The effect of normal atmospheric refraction is to make distant 

 objects appear higher than they really are because a ray of light, 

 starting from the target as shown by the arrow at B, Fig. 19, is bent 

 downward and enters the telescope at L as though it had come in a 

 straight line from A. 



Curvature tends to increase rod readings, as between C and A. 

 This error can be determined with reasonable accuracy. It varies 

 directly as the square of the distance and may be computed by the 

 formula : 



.667 X D - miles 3 



35 



