254 Jordan on the Floatage of 



culation, rays are sometimes considered as parallel, but this 

 only with a reference to their remote radiants. 



Radiants, to constitute the before-mentioned first sort of 

 irides of various dimensions contiguous to the luminaries 

 around which they appear, are formed by inflections be- 

 tween the drops of water duly established by experiments : — 

 to constitute the primary and secondary rainbows, they are 

 formed by rays crossing one another, and thus forming radiants, 

 at the limits of the single and double reflections of the rays 

 within the drops, at points first ascertained by Des Cartes : 

 and to constitute the iris of forty-five degrees, by external rays 

 crossing each other at the limits of the points of refraction 

 without the drops corresponding with the limits of the first 

 reflections of the rays within, the refracted like the reflected 

 rays removing from, returning towards the axis of the drop 

 crossing each other at the points of return, and there forming- 

 refracted radiants, as the reflected rays form reflected radiants, 

 and issuing from the same points. These points, however, ac- 

 cording to the ordinary refractions of water, would give an iris 

 on Newton's principles of fifty or fifty-two degrees, according 

 to others of seventy-six degrees, forty-four minutes ; and a di- 

 minished refractive power in the drops of water constituting 

 floating vapour is necessary to exhibit an iris by two refractions 

 of forty-five degrees diameter. The application, therefore, 

 even of the true principle of formation by radiants, would not, 

 as in the case of the first sort of irides, have alone been suf- 

 ficient to account for the phsenomena. In that case, original 

 experiments and discoveries respecting the inflections of light, 

 furnished the clue to perfect the discovery. In this, a change 

 and diminution of the refractive powers of drops of water float- 

 ing in air is necessary to be established. The extension of 

 inflections to the phsenomena of thin plates, and of bodies of 

 considerable, of extreme tenuity, establishes what is required. 



The iris of forty-five degrees generally appears in what 

 may be called diffused vapour rather than clouds. At the dis- 

 tance of about twenty-two degrees and a half from the luminary 

 there appears a circle of three or more degrees broad, us circum- 



