106 CAUSE OF THE HALO, &c. CHAP. 2. 20. 



division of them into 1st, those which result 

 from the intervention of cloud between the 

 spectator and the luminous body. And, 2dly, 

 those which appear in a cloud opposite to the 

 rays of the sun or moon. Of the 1st sort are all 

 the different kinds of Halo, Corona, and Par- 

 lielion. Of the 2d sort are the different varieties 

 of the Iris. With regard to the first kind of 

 these, caused by intervention of clouds, many 

 attempts have been made by different philoso- 

 phers to explain them ; but they have been 

 generally founded more on vague conjecture than 

 fact. All that can be said about them is, that 

 they probably depend upon some peculiarity 

 in the refractive or reflective powers of the 

 intervening cloud, by which certain of the rays 

 are thrown off at a particular angle. I may, in 

 the first place, observe, that all the rays from 

 the sun or moon must fall nearly parallel on 

 the surface of the cloud. This will be evident, 

 if we consider the great distance of those bodies 

 when compared with the diameter of the largest 

 Halo. The rays which constitute the luminous 

 ring of the Halo, must be reflected at an angle 

 equal to the angle of the semidiameter of its 

 area ; or, in other words, to the angle subtended 

 by the distance from the sun or moon's centre 



