The most brilliant are situated at the intersections of the inner halo and the parhelic circle; these are known as parhelia (denoted by the letter p in the figures) (from the Gr.
Less brilliant are the parhelia of the outer halo.
The parhelia are most brilliant when the sun is near the horizon.
From the parhelia of the inner halo two oblique curves (L) proceed.
The parhelia (p) were explained by Mariotte as due to refraction through a pair of alternate faces of a vertical prism.
When the sun is near the horizon the rays fall upon the principal section of the prisms; the minimum deviation for such rays is 22 °, and consequently the parhelia are not only on the inner halo, but also on the parhelic circle.
As the sun rises, the rays enter the prisms more and more obliquely, and the angle of minimum deviation increases; but since the emergent ray makes the same angle with the refracting edge as the incident ray, it follows that the parhelia will remain on the parhelic circle, while receding from the inner halo.
The axes will take up any position, and consequently give rise to a continuous series of parhelia which touch externally the inner halo, both above and below, and under certain conditions (such as the requisite altitude of the sun) form two closed elliptical curves; generally, however, only the upper and lower portions are seen.
The fourth essay is a systematic treatment of the nature of colour, with a description of many curious experiments and a discussion of the rainbow, halos, parhelia, diffraction, and the more purely physiological phenomena of colour.