On the Nature of the Hydrogen Flocculi and Their Structure at Different Levels in the Solar Atmosphere

If we compare two monochromatic photographs of the sun, taken respectively with the H_2 line of calcium and the H_α line of hydrogen, we find that dark areas on the hydrogen plate correspond roughly in form with bright areas on the calcium plate. The exquisite details of structure, which on sharply defined H_α negatives appear like the delicate tracery of hoar-frost, have no counterpart on H_2 negatives, where the bright clouds of calcium vapour, or foccutli, more nearly resemble cumulus clouds in the earth's atmosphere. Thus there is no precise agreement in form, through the larger regions occupied by dark hydrogen and bright calcium flocculi usually do not differ greatly in area or general outline. In certain places dark hydrogen seems to be replaced by bright hydrogen, which frequently assumes the form of a narrow ring partially or completely encircliing a sun-spot.* Brilliantly luminous areas, rapidly changing in form, are often photographed with Ha, usually in the neighbourhood of active spots (see Plate 1, fig. 1).