ACCOU-NT OF TWO HAIOS. 57 



It appears, that a much fimpler and more natural expla- 

 nation of thefe phenomena may be deduced from the regular 

 crj'ftallization of friow in the atmofphere. 



It is well known, that the crvftals of ice and fnow, tend '^^^ equilateral 



1 J. r 1 /• „„„ '' r r • prifmatic cryftals 



always to form angles ol 60° ; now a pnlm or water or ice, ^f \^ produce a 



of 60**, produces a deviation of 23° 37', for rays forming tl^yition of 230 



equal angles with its furfaces, and the angle of deviation ^^^^Jl^^^'j ^"'" 



varies at firft very flowly, as the inclination changes, the flowly. 



variation amounting to lefs than 3^, while the inclination 



changes 30**. 



Now if fuch prifms were placed at all poffible angles of in- The cafual ar- 

 clination, differing equally from each other, one half of them [hefe^^m' i*ve 

 would be fo fituated, as to be incapable oftranfmitting any light the halo, 

 regularly by two fucceffive refractions direfted the fame way ; 

 and ol the remaining two fourths, the one would refradl all the 

 light within thefe three degrees, and the other would difperfe 

 the light in a fpace of between 20*^ and 3C, beyond them. 



In the fame manner, we may imagine an immenfe number Refradion 



of prifmatic particles of fnow to be difpofed in all poffible ^^'""g^.^^'o ^ 

 .. . ^ . . . ^ "^ pnfms gives the 



dire6lions, and a coniiderable proportion of them to be fo greater halo of 



fituated, that the plane of their tranfverfe feclion may pais 47*« 

 within certain limits of the fun and the fpedator. Then half 

 of thefe only will appear illuminated, and the greater part of 

 the light will be tranfmitted by fuch as are fituated at an an- 

 gular diftance of 23° 37', or within 3** of it : the limit being 

 flrongly marked internally, but the light being externally more 

 gradually loft. And this is precifely the appearance of the 

 moft common halo. When there is a fufficient quantity of the 

 prifmatic particles, a coniiderable part of the light muft fall, 

 after one refradion, on a fecond particle ; fo that the effect 

 will be doubled : and, in this cafe, the angle of refra<5tion 

 will become fufficient to prefent a faint appearance of colour, 

 the red being internal, as the leaft refrangible light, and the 

 external part having a tinge of blue. 



Thefe concentric halos of 23|° and 47°, are therefore fuf- Very fhort 

 ficiently explicable, by particles of fnow, fituated promifcuoufly ^'^■',7^^?'^J'^'*' 

 in all poffible diredions. If the prifms be fo fhort as to form wifej and the 

 triangular plates, thefe plates, in falling through the air, vvill''?^^^^'°" °^. 

 tend to affume a vertical direction, and a much greater number give the hori- 

 of them will be in this fituation than in any other. The reflec-^""^^' circle, 

 tion from their flat furfaces will confequently produce a hori- tbn a uarh^r"' 

 zontal circle of equal height with the fun j and their refraction withwinjs, 



will 



