Lesson xxvn.] HALOS AND PARHELIA. if? 



might possibly have, besides a spherical one, he 

 could find no other so simple as that of a cylinder: 

 and, indeed, he had often observed, that snow 

 consisted of several slender oblong particles, mixed 

 with those of other shapes : and seeing that small 

 globules were sufficient for the production of 

 Halos, he imagined that a great number of small 

 cylinders, floating in the air, might produce simi- 

 lar appearances. He also remembered that Da- 

 cartes had taken notice of certain small columns, 

 which he had seen lying on the ground, the ex- 

 tremities of which were bounded with flat star-like 

 figures, consisting of six rays. 



The large white horizontal circle, observed in 

 some of these phaenomena, M.Huygens supposed 

 to be produced by the reflection of the sun's rays 

 from the outsides of the upright cylinders ; since, 

 when the sun shines upon a number of such cylin- 

 ders suspended in the air, a white circle must 

 necessarily appear to pass through the sun parallel 

 to the horizon. This he shews very distinctly by a 

 large figure of a cylinder, and by pointing out the 

 progress of the sun's rays reflected from it. For 

 every point of the sun's verticle diameter, as well 

 as his centre, will illuminate a circle of cylinders, 

 of the same apparent height as the illuminating 

 point. It is observable that no thick clouds are 

 seen in the air when these circles appear; but only 

 such as are very thin, and scarcely visible. For in 

 most of these observations the sky is said to have 

 been very clear and serene ; which agrees quite 

 well with this hypothesis ; since these minute 

 1 4 cylinders 



