74 Prof. Snell on a singular case of Parhelion, 
first place, though the colors were scattered promiscuously, yet 
there was evidently a prevalence of the red on the most distant, 
that is, the concave edge of the band. And in the next place, I 
ascertained, by a rude measurement, that the distance of the curve 
from the sun was about twenty twodegrees. These two particu- 
lars characterize that species of halo, most frequently seen encir- 
cling the sun and moon, respectively called the parhelion and 
the paraselene.* 
The only hypothesis offered for the explanation of the halo of 
AA or 45 degrees in diameter, which is at all satisfactory, is that 
of M. Mariotte and Dr. Young, who considered it the effect of 
the transmission of light through snow crystals, whose refracting 
angle is 60°. This is known to be one of the most frequent 
angles in such crystals. The index of refraction for ice is about 
bt. 
Now suppose a pencil of light to traverse a prism of ice, whose 
refracting angle is 60°, in such direction as to cut perpendicularly 
the bisecting line of the refracting angle ; this pencil will be found, 
by asimple calculation, to deviate 21° 50’ from its original direc- 
tion. If the prism be revolved either way from the position just 
named, the angle of deviation will increase; yet so slowly at first, 
that a change of 10° in the prism will not occasion a disturbance 
of more than one fourth of a degree in the emergent ray. But if 
the prism be revolved through large angles, the deviation increases 
more rapidly ; and the greatest deviation occurs, when the prism 
is revolved in one direction, till the angle of incidence equals 90°, 
or in the opposite, till the angle of emergence is 90°. In either 
case, the deviation I find to be 43° 27’; this is the maximum. 
Let there be a stratum of these crystals floating in the air, 
of such depth as to produce only haziness, through which, of 
course, the sun can be plainly seen. ‘The refracting edges must 
be supposed to lie in all possible positions. We are at present 
concerned only with those whose edges are perpendicular both to 
the line of vision and the solar ray, and whose refracting angles 
are turned away from the axis, or line joining the eye and the sun. 
And of all such, it is plain that no crystal, lying nearer the sun’s 
* These terms are often used to designate those bright images of the sun or moon, 
sometimes seen at the intersection or contact of two halos, also called mock-suns 
and mock-moons. ButI find good authority for applying the words to the halos 
themselves. 
S75 Se mes 
