1. (Upper). Perspective view of the sky, showing the sun (_S_); ordinary halo of 22° (_a_); great halo of 46° (_b_); upper tangent arc of the halo of 22° (_c_); lower tangent arc of the halo of 22° (_d_); ordinary parhelia of 22° (_e_, _e’_); Lowitz arcs (_f, f′_); parhelia of 46° (_g, g′_); circumzenithal arc (_h_); infralateral tangent arcs of the halo of 46° (_i_); the parhelic circle (_m_); a paranthelion of 90° (_q_); light pillar, (_u, u′_); the observer (_O_). 2. (Lower). Perspective view of the sky, showing the observer (_O_); the parhelic circle (_m_): ordinary paranthelia of 120° (_p_); the paranthelion of 90° (_q’_); the oblique arcs of the anthelion (_r, r′_); and the anthelion (_n_).] The commonest halo is a circle of 22 degrees radius (the _22-degree halo_) about the sun or moon. When formed by the sun it generally shows a distinct reddish inner border and traces of other spectral colors. The lunar 22-degree halo usually appears colorless. This halo is visible, in whole or in part, to the attentive observer about once in three days, on an average. Less common, but by no means rare, are the _parhelia_ or “sun dogs” of 22 degrees (called _paraselenæ_ or “moon dogs” when formed by the moon), the beautiful _circumzenithal arc_, and a few other members of the halo family. Most forms of halo are so uncommon that their appearance is an event of some scientific importance. The accompanying diagrams, by Dr. Louis Besson of the Observatoire de Montsouris, show the positions, with respect to the sun (or moon), of the majority of known halo phenomena. The upper diagram shows the halos that occur on the same side of the sky as the sun (or moon), and the lower those that appear on the opposite side. Most of these halos, when bright, show the spectral colors. The circumzenithal arc, _h_ (commonly described, by the uninitiated, as a “rainbow”), and the parhelia of 22 degrees, _e, e′_, are especially brilliant in their coloration. The _parhelic circle, m_, which sometimes extends entirely around the sky, is white, and so are a few of the rarer forms of halo. The _upper_ and _lower tangent arcs of the halo of 22 degrees_, _c_ and _d_, undergo striking alterations, with changes in the altitude of the sun. When the luminary is more than about 40 degrees above the horizon, these two arcs become joined at their tips to form the _circumscribed halo_, and at still greater solar altitudes this halo contracts from an elliptical to a circular form, thus blending into the 22-degree halo as shown on the next page, where the solar altitudes corresponding to the different forms of the halo are indicated. The positions of the parhelia of 22 degrees, _e, e′_, also depend upon solar altitude. When the sun is on the horizon these “sun dogs” are 22 degrees from the luminary, and therefore lie in the 22-degree halo; at greater solar altitudes they lie outside this halo.