298 Halos. 



in the atmosphere is concerned in their production; in the case of 

 hail, he supposes that the spherical drops of water congeal into the 

 same form, and that the spicular, prismatic and stellar form of snow 

 flakes, depends on crystallization from the state of vapor. The 

 light which comes to the eye, in cases like that described above, 

 passes through a compound medium. 



A cloud of rain, snow or ice intervenes between the eye and the 

 luminous body. The cloud is near or distant. 



It will be large in direct proportion to the angle of vision which it 

 subtends, and the diameter will be in an exact ratio to the visual an^ 

 gle. The various sizes of these halos are explained by the case of 

 a rainbow, the continuity of whose arc depends on the continuity of 

 the cloud, and the edge of the cloud limits the extent of the refrac- 

 tion (for there the medium is broken off, and is at an end) ; so it 

 follows that the size of the cloud will control the limits of the reflec- 

 tion in the corresponding case of a halo, or simple corona. Sup- 

 pose the sun elevated, and a cloud of dry, glistening snow inter- 

 venes between it and the spectator. The whole mass becomes re- 

 flective like water, or like glass, or like a semi-opaque body. At the 

 extremity of the cloud, or at the densest part, the rays will be most 

 strongly attracted and reflected, since it is proved that light is subject 

 to this general law, and they will represent a circular image, for they 

 proceed in right lines from a spherical body. 



Although refracted by the earth's atmosphere, they move in equal 

 parallelism, as well after as before refraction. All the direct rays 

 from the sun passing through the cloud would represent that object, 

 while those striking the cloud, in other angles, would have the angles 

 of incidence and reflection equal, or in other words, would be reflect- 

 ed to different points of sight, and give to other spectators at the other 

 positions a similar image. If the circle bejivhite, the rays, as pos- 

 sessing equal refrangibility, are called homogeneous, if colored, het- 

 erogeneous, because unequally refrangible. The halo seen on the 

 7th of February was of pure white, the rays were homogeneous, 

 and the medium transmitting thern was consequently of the same 

 refracting and reflecting power throughout, a fact which goes very 

 far to support the theory already projected. 



We have heard of many facts which show that entire clouds be- 

 come the media of reflection. On the Alps the figure of a man 

 is seen by the wondering shepherds. In Sicily the Fata Morgana 

 are well authenticated, and within our own knowledge a fact exists 



