SOLAR HALO. 65 



an equal distribution of crystals be found if the cloud be 

 heavy at one point and light in another, as shown in Figure 5. 

 Take, then, an ununiform thickness of cloud strata and an 

 unequal distribution of crystals, then observers at A, B and C 

 would each see vastly different details, although difference in 

 cloud thickness and in crystal distribution might not be great 

 enough to seriously affect the principal features of the phe- 

 nomenon being seen with little or no change from each of the 

 points of observation. 



It is considered of interest in connection with the descrip- 

 tion of the sun halo of May 20th, as well as all others, to 

 remember that the tiny ice crystals that are formed by 

 extreme cold, from the moisture suspended in the clouds, usu- 

 ally assume the shape of hexagonal and right hexagonal 

 prisms. These prisms have been discovered to have many 

 varied shapes, that may be thin like a pane of glass or a 

 snow flake, long and slender like a pencil, cone shaped, etc. 

 Of course, their position in the air depends upon their shape, 

 and they will fall or attain their suspended position in such a 

 way as to offer the least resistance to the air, but will in every 

 case maintain the same general positions on their axes. 



In right hexagonal prisms there are three possible angles 

 from which light may be refracted, namely, 60°, 90° and 160°. 

 Light refracted between two alternate faces would form the 

 angle of 60° ; between any side and the base of a perfect 

 right prism the angle would be 90°, and between adjacent 

 faces 120°. The different angles of refraction account for the 

 double halos. Edme Marriotte explained the inner halo as 

 being due to refraction through a pair of alternate faces, also 

 that the minimum deviation of a ray of light through an ice 

 prism having a 60° refracting angle is approximately 22°. 

 The outer halo, 46° from the luminary, is in a similar manner 

 accounted for by Henry Cavendish as due to refraction 

 between the base and a side of a right prism, at 90°. 



Man-iotte also explained the fact that red invariably 



