\ 



small heavy Bodies in Air, &c. 253 



said to be observed on the tops of the highest mountains, to which 

 the finest particles of smoke, and of whatever sohd materials 

 may be adequately divided so as to be elevated and dispersed 

 in air, may rise, and in a given state of rest in the air above 

 unknown below, may quietly be deposited and undisturbedly 

 repose. 



These attractions of the air upon small bodies existing in it, 

 and again of bodies upon small portions of the air, or of other 

 bodies interposed between them, and of all these bodies upon 

 light passing through or between them, lead to an explanation 

 of optical phsenomena, which is impossible without due consi- 

 deration had of them. Thus I had been enabled to account for 

 the irides which exist around and immediately contiguous to 

 the sun, moon, and other luminous bodies, and are of various dia- 

 meters, and of various successive orders of colours, which I 

 shewed to be produced by the inflections of light in passing be- 

 tween bodies interposed between the eye and the luminary. 



The great halo, of forty-five degrees diameter, has long been 

 the opprobnum philosophorum. All who have attempted to 

 account for its exhibition and dimensions seem to have 

 failed. A diminished refractive power in the drops of water, 

 forming it after two refractions without any reflection, seems 

 to be necessary. Such a diminution of refractive power in 

 drops of sufficient tenuity to constitute the permanently-floating 

 vapour in which these irides appear, may be inferred from 

 other phsenomena, and being applied together with these to 

 account for all the appearances of that iris, confirms and is 

 confirmed by them. 



Newton, Des Cartes, Huygens, and others, having assumed 

 the number, or parallelism of certain rays, as principles of 

 formation to account for the phsenomena of the primary and 

 secondary bows, these principles, not correct in themselves, 

 nor extensible to other irides, failed in accounting for the 

 halo of forty-five degrees. The true principle of the formation 

 of irides is that of images, that is, by radiants ; and to these 

 radiants, not to parallel rays, nor to the number of rays, is the 

 existence of irides as well as images to be referred. For cal- 



VoL. VIII. S 



