16 



hope, he apprehends, must be relinquished of ever effecting the de- 

 composition of the muriatic acid in the way of simple elective attrac- 

 tion ; its basis being probably some unknown body, which nothing 

 but the application of complicated affinities will perhaps ever enable 

 us to discriminate. 



On double Images caused by atmospherical Refraction. By William 

 Hyde Wollaston, M.D. F.R.S. Read March 6, 1800. [Phil. 

 Trans. 1800,_p. 239.] 



The remarkable instances of double and triple images of the same 

 object produced by aerial refraction near the horizon, lately commu- 

 nicated to the Society by Mr. Huddart, Prof. Vince, and Mr. Dalby, 

 have given rise to the present paper, in which the author attempts to 

 explain these phenomena on theoretical principles, and to illustrate 

 his conclusions by artificial experiments. 



Admitting the inference given by Professor Vince, that these ap- 

 pearances arise from certain unusual variations of increasing density 

 in the lower strata of the atmosphere, our author undertakes, 1 st, to 

 investigate the successive variations of increasing or decreasing den- 

 sity to which fluids in general are liable, and the laws of the refrac- 

 tions occasioned by them ; 2dly, to illustrate and confirm the truth 

 of this theory by experiments with fluids of known densities ; 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 circumstances observed in the above phenomena 

 seem to depend. 



Under the first head we find the demonstrations of three propo- 

 sitions, deduced from the general laws of refraction. The first im- 

 ports, that if the density of any medium varies by parallel, indefinitely 

 thin strata, a ray of light moving through it in the direction of the 

 strata, will be made to deviate during its passage ; and the deviation 

 will ever be proportionate to the increment of density where it passes. 

 From the second it appears, that when two fluids of unequal'densities 

 are brought into contact, and unite by mutual penetration, if the den- 

 sities at different heights be expressed by ordinates to a perpendicular 

 line drawn across the fluids, the curve drawn through the terminations 

 of these ordinates will have a point of contrary flexure. And in the 

 third proposition it is shown, that if parallel rays pass through a me- 

 dium, varying according to the preceding proposition, those rays above 

 the point of contrary flexure, where the line will be concave, will be 

 made to diverge, while those below the same point, where the curve 

 will be convex, will converge after their passage through it. The 

 converging rays, it follows hence, will at a certain distance, propor- 

 tionate to the quantity of convergency, meet in a focus, beyond which 

 they will diverge again, and thus produce effects perfectly similar to 

 those caused by a medium of uniform density, having a surface similar 

 to the above-mentioned curve of densities, whether convex or concave, 

 according to the nature of that curvature. Hence may be inferred 



