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.
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.