444 ' LECTURE xxxvir. 



fi-eezing, at an angle of 60 degrees, is sufficiently established, to allow us 

 to assume this as the constant angle of the elementary crystals of snow, 

 which are probably either triangular or hexagonal prisms:, the deviation 

 produced by such a prism differs very little from the observed angle at which 

 the first circle is usually seen; and all the principal phenomena, which attend 

 this circle, may be explained, by supposing the axis of the crystals to assume 

 a vertical or a horizontal position, in consequence of the operation of gravity: 

 thus the parhelia, which are sometimes a little more distant from the sun 

 than the halo, are attributed by Mariotte to the refraction of the prisms 

 which are situated vertically, and produce a greater deviation, on account of 

 the obliquity of the rays of light with respect to their axes. The horizontal 

 circle may be deduced from the reflection, or even the repeated refractions 

 of the vertical facets ; the anthelia from two refractions with an intermediate 

 reflection, and the inverted arch from the increase of the deviation, in the 

 light passing obliquely, through prisms lying in a horizontal position. The 

 external circle may be attributed either to two successive refractions through 

 different prisms, or with greater probability, as Mr. Cavendish has suggested 

 to me, to the effect of the rectangular terminations of the single crystals. 

 The appearance of colours, in halos, is nearly the same as in rainbows, but 

 less distinct; the red being nearest to the luminary, and the whole halo being 

 externally very ill defined. (Plate XXIX. Fig. 431, 432.) 



From the observed magnitude of these halos, I had concluded that the 

 refractive power of ice must be materially less than that of water, although 

 some authors had asserted that it was greater: and Dr. Wollaston afterwards 

 fully confirmed this conclusion by means of the very accurate instrument 

 which has already been described: his measurement agreeing precisely with 

 the mean of the best observations on these halos; so that ice must be con^ 

 sidered as the least refractiv'e of any known substances not aeriform. 



Sometimes the figures of halos and parhelia are so extremely complicated, 

 as to defy all attempts to account for the formation of their different parts: 

 but if we examine the representations which have been given,by various authors, 

 of the multiplicity of capricious forms frequently assumed by the flakes of 

 snow, we shall see no reason to think them inadequate to the production of 

 all these appearances. (Plate XXIX. Fig. 433, 434.) 



