Here it is only necessary to distinguish halos from coronae.
Thus, while halos have certain definite radii, viz.
Prior to Young, halos and coronae had not been clearly differentiated; they were both regarded as caused by the refraction of light by atmospheric moisture and ice, although observation had shown that important distinctions existed between these phenomena.
The chromatic halos which frequently encircle these images are due to diffraction.
The two halos are the only phenomena which admit of explanation without assigning any particular distribution to the ice-crystals.
Luminous arcs (T), tangential to the upper and lower parts of each halo, also occur, and in the case of the inner halo, the arcs may be prolonged to form a quasi-elliptic halo.1 The physical explanation of halos originated with Rene Descartes, who ascribed their formation to the presence of icecrystals in the atmosphere.
According to a local legend, Athamas was king of Halos in Phthiotis from the first (Schol.
These appear ancos differ from halos and coronae inasmuch as their centres are at the anti-solar point; they thus resemble the rainbow.
The impurity of the colours (due partly to the sun's diameter, but still more to oblique refraction) is more marked in halos than in rainbows; in fact, only the red is at all pure, and as a rule, only a mere trace of green or blue is seen, the external portion of each halo being nearly white.