Symud i'r prif gynnwys

Vol.X. AUGUST, 1908. No. 9 New Series. Some Optical Phenomena. By Miss E. Graham Hagerty. [Concluded.] Halos. Halos are luminous circles surrounding the Sun or Moon. They differ from Coronae in having definite radii, and when they show colours ihe red is inside. Traces of blue and green are sometimes seen, but the outside is generally nearly white. Venus has been seen with a halo in the tropics. The optical phenomena of halos, parhelia and pardselense are produced by the small ice-crystals which form the cirrus clouds, refracting* and inflect¬ ing the light of the Sun. These minute crystals telong to the hexagonal system, and their usual form is the regular hexagonal prism terminated by plane ends perpendicular to the axis. Sometimes the prisms are long and narrow, sometimes they are hexagonal pla'es. Two alternative faces of the hexagon give an ice-prism of 60 degrees; a face and an end give an ice-prism of 90 degrees, i'he