348 LECTURE XXXVII. 



without any previous knowledge of what Mariotte had done. The natural 

 tendency of water to crystallize, in freezing, at an angle of GO 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 tb? i 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 after- 

 wards 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 considered as the least refractive of any known substances not 

 aeriform. 



Sometimes the figures of halos and parhelia are so extremely compli- 

 cated, 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.) 



The most singular of all the phenomena of refraction is perhaps the 

 property of some natural substances, which have a double effect on the 

 light transmitted through them, as if two mediums of different densities 

 freely pervaded each other, the one only acting on some of the rays of 

 light, the other on the remaining portion. These substances are usually 

 crystallized stones, and their refractions have sometimes no further pecu- 

 liarity ; but the rhomboidal crystals of calcarious spar, commonly called 

 Iceland crystals, possess the remarkable property of separating such pencils 

 of light, as fall perpendicularly on them, into two parts, one of them only 

 being transmitted in the usual manner, the other being deflected towards 



