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



107 



to the ring. To illustrate this, I subjoin the 

 following problem and figure. 



M 



B 



D 



E 



The distance of the sun or moon from the 

 cloud bears so great a proportion to the diameter 

 of the Halo, that the rays may be said to fall 

 physically parallel on all parts of it ; that is, if 

 of two rays coming from the sun's centre, one 

 should impinge on A, and the other on B, these 

 rays might be considered as parallel. Let A B 

 be the diameter of the Halo, M. the centre or 

 place where the moon appears : if a ray, x y, 

 proceeding from the moon in the direction x y> 



