240 J3lr- Wollaston on double Images 



from " unusual variations" of increasing density in the lower 

 strata of the atmosphere. 



In the volume for 1795, Mr. Dalby mentions having seen 

 " the top of a hill appear detached, for the sky was seen under 

 " it." In this case, as well as in the preceding, it is probable 

 that inversion took place, and that the lower half of the portion 

 detached was an inverted image of the upper, as the sky could 

 not be seen beneath it, but by an inverted course of the rays. 



Since the causes of these peculiarities of terrestrial refraction 

 have not received so full an explanation as might be wished, I 

 have endeavoured, 



1st. To investigate theoretically the successive variations of 

 increasing or decreasing density to which fluids in general are 

 ' liable, and the laws of the refractions occasioned by them. 



2dly. To illustrate and confirm the truth of this theory, by 

 experiments with fluids of known density. 



And lastly, to ascertain, by trial upon the air itself, the 

 causes and extent of those variations of its refractive density, on 

 which the inversions of objects, and other phenomena observed, 

 appear to depend. 



The general laws may be comprised in three propositions. 



Prop. 1. If the density of any medium varies by parallel 

 indefinitely thin strata, any rays of light moving through it in 

 the direction of the strata, will be made to deviate during their 

 passage, and their deviations will be in proportion to the incre- 

 ments of density where they pass. 



For each ray will be bent towards the denser strata, by a 

 refracting force proportioned to the difference of the densities 

 above and below the line of its passage ; and, as their velocities 

 are the same, and therefore the times of action of the forces 



