458 PHILOSOPHICAL TRANSACTIONS. [aNNO I67O. 



the sun, which seemed as in an eclipse. The weather was cold, considering 

 the season of the year, and it was affirmed for certain, that it had frozen the 

 night before. This halo appeared in the same beauty and splendor of colours 

 unchanged, from nine in the morning till about half an hour past ten ; after 

 which time it became fainter and fainter, till two o'clock in the afternoon, when 

 it ended, after it had resumed a little more force some time before it disap- 

 peared. 



The observation of this phenomenon engaged M. Huygens to propose to the 

 company, assembled at his Majesty's library, what he had meditated some years 

 ago, concerning the cause, not only of these halos, but also of the parhelia, 

 which have been hitherto considered by many as prodigies, and as prognostics 

 of some singular event. 



As for the halos, he said, that they were formed by small round grains, made 

 up of two parts, one transparent, the other opaque, the latter being inclosed in 

 the former, as a cherry-stone is in a cherry ; as may be seen in the fig. 2, where 

 A A represents one of these grains, and B the kernel or opaque part. He re- 

 lated the observations of those who have seen hail formed after this manner, and 

 among others, that of M. Descartes, in his Treatise of Meteors; and explained 

 how some of these little grains, which swim up and down in the air between 

 us and the sun, being less distant from the axis, which extends itself from the 

 sun to our eye, than of a certain angle, do necessarily hinder the rays, which 

 fall on them, from coming to our eyes, since the opaque kernel is the cause 

 that there is behind every such grain a space of a conical figure, as MNO in 

 fig. 2, in which the eye of the spectator being situated, cannot see the sun 

 through that grain, though it may see it when posited elsewhere, as somewhere 

 in P. 



And to make the company the more distinctly to understand the effect 

 which these grains suspended in the air must produce, he drew the 3d figure ; 

 in which B is the place of the eye; BA, the axis which passes from the eye to 

 the sun ; C, M, F, some of the icy grains with their kernel, making them semi- 

 opaque : Among which, the grain C being in the axis B A, and the lines CK, 

 LH, representing the rays of the sun nearest to the axis, the passage of which 

 is not hindered by the opacity of the kernel, it is certain, not only that the grain 

 C will not be able to transmit any ray of the sun towards B, but also that con- 

 ceiving the superficies of a cone, whose top is in the eye, and its sides BD, BE, 

 parallel to the rays CK, LH; all the grains MM, which this superficies may 

 comprise, will likewise not suffer any ray to pass to the eye, because it must 

 needs be in their cone of obscurity; but those that are without this superficies, 

 as the grains FF, will let them pass, because the eye is without their cone of 



