and the Theory of Halos. 77 
tance and position must be given, or the color transmitted to a 
particular point is indeterminate. In the accompanying figure, 
the eye is supposed to be at E; the sun in the direction of ES, 
AS, &c. Arepresents acrystal at the inner edge. D and D’ show 
the different positions at the outer edge. The other parts of the 
figure will be understood from the preceding description. 
Ss Ss is s 
The halo of 91 degrees in diameter is much more rare than 
the kind just described. It is believed to be formed in the 
same manner by crystals of 90°; and this angle occurs in the 
bib eae ose a of water with much less frequency than the angle 
of 60° 
Halos occur in summer as well as in winter; for vapor often 
floats higher than the limit of perpetual Sdagelaiion, 
The halo of 44° in diameter is a phenomenon of very fre- 
quent occurrence. Ihave reason to believe that it might be seen, 
