sceii at Porisea on the 29th of March 1848. 445 



crystal should be of ice, to imitate truly the effects observed in 

 nature ; but for merely showing the princijile of these effects, 

 any crystal of nearly similar shape, one of quartz for instance, 

 will answer the purpose, though from its higher refractive 

 power it will produce different curves from those produced 

 by an ice crystal of the same shape. 



Now if we admit into this arrangement a very small sun- 

 beam, barely large enough to illuminate the whole of the 

 crystal and nothing else, a portion of this beam, either passing 

 by the edges of the crystal or through two parallel faces of it, 

 will continue its course unaltered, and fall on the same spot 

 of the basin as if no crystal had intervened. We will call this 

 spot the direct image. Now the faces of the crystal will, by 

 reflexion and refraction (even omitting the consideration of 

 its double refraction), divide the remainder of the light into 

 several beams of various intensities, according as they have, or 

 have not, suffered one or more partial reflexions. These beams 

 proceeding to various parts of the basin, will there paint va- 

 rious spots, some white and others coloured, according as they 

 have been formed by reflexion or by refraction, or both. Now 

 to see all the positions of these spots, arising from all possible 

 positions of the crystal (with its axis vertical), we have only to 

 turn it round on its axis, and if we then mark with a pencil the 

 path of each moving spot, we shall obtain curves similar to the 

 various bands and halos seen in these phaenomena, and having 

 the same positions with regard to the direct image and the edge 

 of the basin, that the actual appearances have with regard to the 

 sun and the horizon. Some of the spots will remain stationary 

 or nearly so, and these will correspond with the places of the 

 mock-suns. The whole appearance however might be exhi- 

 bited to the eye far more correctly, by making the crystal 

 revolve round its vertical axis rapidly, so that the spots being 

 seen in all parts of their track at once, will appear as curves, 

 and the whole hemispherical surface will then form an inverted 

 representation of the sky during these phaenomena, the various 

 parhelia and bands being seen in their true colours, and with 

 their true gradations of intensity. If the crystal have three 

 or six vertical sides, it will on revolving always })roduce the 

 white horizontal circle passing through the sun, and the two 

 coloured mock-suns with tails. The rest of the appearances 

 will vary with the form of the upper and lower terminations 

 of the crystal. 



The common halo cannot be imitated in this way, for this 

 would re(]uire the crystal to revolve in every direction at once; 

 but if we substitute for the crystal a small globe of water, tiien 

 (provided the sun's altitude be below 4-2'^) the primary and 



