If we compare two monochromatic photographs of the sun, taken respectively with the H
2
line of calcium and the H
a
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
negatived appear like the delicate tracery of hoar-frost, have no counterpart on H
2
negatives, where the bright clouds of calcium vapour, or
flocculi
, more nearly resemble cumulus clouds in the earth’s atmosphere. Thus there is no precise agreement in form, though 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 for of a narrow ring partially or completely encircling a sun-spot. Brilliantly luminous areas, rapidly changing in form, are often Photographed with H
α
, usually in the neighbourhood of active spots (see Plate 1, fig. 1). The bright hydrogen flocculi of the quiescent or slowly changing type, and the very bright hydrogen flocculi of the rapidly changing or eruptive type, are commonly found in regions where H
2
or K
2
photographs also show bright calcium flocculi. Moreover, dark calcium flocculi, usually long and slender in form, are of ten shown by H
2
or K
2
plates to correspond in position with exceptionally dark hydrogen flocculi. These dark calcium flocculi have recently been photographed by Deslandres with dispersion suffciently high to isolate the K
3
line. In this way they are found to be much more unmerous and extensive than when photographed with lower dispersion.
The bows and rings of dawn and dusk
