252 Dr. Wollaston on double Images 



b and d, would be made to diverge to a and c ; consequently, 

 visual rays could not, under these circumstances, intersect each 

 other, and no objects could appear inverted. 



But, whenever the lowest strata of the air become saturated 

 with moisture, the variations between the saturated stratum and 

 the incumbent atmosphere of the common density, will follow 

 a law similar to what is found at the confines of other fluids of 

 unequal density ; hence, inversion will become visible, as there 

 will be a point below which the increment of density will de- 

 crease, and where the refractions will consequently be less, 

 although through a denser medium. 



Exper 10. To produce these appearances, I procured a trough 

 of thin deal, 5 feet long, 1 inch wide, with sides 2-§- inches high, 

 and closed the extremities of it with glass. A section of it is 

 given in Fig. 10. 



When the bottom was wetted with ether, the greatest refrac- 

 tion was, at intervals, more than -| of an inch from the bottom of 

 the trough ; and, beneath this height, I saw a second image in- 

 verted, when my eye was removed to 14 or 15 feet distance, 

 and the object at about 70 feet. The focus seemed at the same 

 time to be about 9 feet distant. 



There was not depth enough of uniformly saturated atmo- 

 sphere for the object itself to be seen through it, but its true 

 place, compared with that of the images, is represented at a. 



Exper. 11. When I made use of rectified spirit in the same 

 apparatus, I had also sufficient proof that the laws of evapora- 

 tion would admit of such appearances being produced; for the 

 same object now appeared curved downwards, as in Fig. 11, 

 so that rays nearer to the bottom were manifestly less refracted 

 than such as passed at some distance above. A degree of con- 

 vergency must therefore have been produced, although the dis- 



