256 Jordan on the Floatage of 



of first reflections. The black ring is formied by the edges of 

 the drops from which all the light is turned aside, the coloured 

 rings by light immediately adjacent to the dark ring, and there- 

 fore coming from the limits of refractions without and of reflec- 

 tions within the drop. There are no other points at which radiants 

 by adjacent rays can be formed, and these determine the re- 

 fractive power of the drops. 



The ratio of 4 : 3 gives an angle of 38° : 22', for 

 the radius of the iris by refracted rays, coming from the 

 limits of first reflections within the drop. By using Dr. 

 Halley's construction for finding the angles of incidence 

 and refraction of efficient rays after two refractions and one 

 reflection, and assuming various refractive powers, in various 

 trials, I found the refractive power of 65 : 61, to give 

 sufficiently nearly tire required angle of 22° : 30' between the 

 incident and emergent rays at the limits of first internal re- 

 flections, being double the difference of the angles of incidence 

 and refraction of the same rays at entering the drop. This is 

 the apparent radius of the iris, and therefore that is the re- 

 fractive power of the drops. 



The necessity of having small drops, all of equal adequate 

 refractive powers, and of equal sizes, to prevent the changes of 

 appearances which other drops would produce, might raise 

 against this doctrine a question, which is, however, answered 

 by the inference that permanent floating vapour can only con- 

 sist of such drops, and this inference is thus exalted into a dis- 

 covery. No other drops but of these refractive powers and 

 sizes have powers of aggregation sufficient to resist for a time 

 the solvent powers of air maintaining their floatage, whilst all 

 of larger sizes are precipitated, of smaller are dissolved. 



That a diminished refractive power of the drops is induced by 

 the attractions of the air surrounding them necessary to effect 

 their floatage as permanent vapour, is to be shown by reference 

 to the phsenomena of thin plates. 



The phsenomena of thin plates, contrary to the Newtonian 

 liypothesis of fits of easy transmission and reflection, have 

 been shown to be phaenomena of inflection. 



