Burton — On Halos and Anthelia. 403 



LXII. — On Halos ak-d A^^ihelia. By Phuip Buetois-. 



[Eead, Xovember 30, 1880.] 



The formation of those luminous circles termed halos, and of the other 

 appearances -which sometimes accompany them, is now nniversally at- 

 tributed to the action of icy particles suspended in the air — an opinion 

 which seems to have been first entertained by Descartes, and was 

 afterwards established by Mariotte and Dr. Young. Of all these ap- 

 pearances, the halo of 22° distance from the sun is the only one which 

 occurs frec[uently in these countries ; it may be said, indeed, to be a 

 very common phenomenon usually preceding changes of weather. Its 

 formation is exemplified by bringing filaments of hoar-frost very near 

 the eye in sunlight, when colours corresponding to those of the halo 

 will appear at the same elongation. A similar circle is seen less 

 frequently at the distance of 46° from the sun, and, according to 

 Cavendish, is produced by the ends of the crystals when they are re- 

 gularly formecL Parhelia, or "mock suns," occasionally occur about 

 the places where these halos are intersected by horizontal and vertical 

 circles passing through the sxin ; these are, also, explicable on prin- 

 ciples suggested by Mariotte. But other halos have been described 

 which are not so easily accounted for. Thus, on the 1st of December, 

 1819, Captain Parry observed one which surrounded the moon at a 

 distance of about 38°, and was accompanied by paraselense, and it does 

 not appear evident why such phenomena should occur at this parti- 

 cular elongation. 



On the 6th of March, 1869, I observed a circular halo smaller than 

 any of the preceding, its semi-diameter, as I considered, not being 

 greater than 10° or 12°. Its colours were in the usual order, the red 

 being next the sun, and it was visible for more than two hours. It 

 could not have been a corona, as such a phenomenon can seldom be 

 seen unless when reflected from water, being generally too faint to be 

 otherwise perceived, whereas in the present instance the halo was very 

 bright. The part of the sky where it occuri'ed was free from clouds, 

 though presenting a sort of hazy appearance ; and the estimate which 

 I made of the diameter of the circle showed it to be about half that of 

 the common halo. As calculation shows that prisms of ice whose bases 

 are regular pentagons would produce a halo similar to the ordinary 

 one, and at the distance of 11° 46' from the sun (which very well 

 agrees with the estimated position), it appears very probable that this 

 circle must have been formed by such crystals. And that water may 

 sometimes be frozen in particles of that shape, I have no doubt, having 

 on one occasion seen a shower of hail which consisted entii'ely of pen- 

 tagonal prisms. These were perfectly formed, of a whitish colour, and 

 nearly opac^ue, and were remarkable for their shortness in comparison 



E. I. A. PKOC, SEK. II. VOL. in. — SCIEXCD. 2 'M 



