“ Photographs of the solar disk, taken with the H or K line, show extensive luminous clouds (flocculi) of calcium vapour, vastly greater in area than the sun-spots. By setting the camera slit so as to admit to the photographic plate the light of the denser calcium vapour, which lies at low levels, or that of the rarer vapour at high levels, the phenomena of various superposed regions of the atmosphere can be recorded. The lower and denser vapour appears as bright clouds, but the cooler vapour, at higher levels, absorbs the light from below and thus gives rise to dark clouds. ”

“ The spectroscope is then moved parallel to itself, admitting to the collimator slit light from all parts of the sun's disk. Thus a monochromatic image of the sun, formed of a great number of successive images of the spectral line employed, will be built up on the plate. As the only light permitted to reach the plate is that of the calcium line, the resulting image will represent the distribution of calcium vapour in the sun's atmosphere. The calcium clouds or flocculi thus recorded are invisible to the eye, and are not shown on direct solar photographs taken in the ordinary way. ”

“ The first photographs of the sun in hydrogen light were made with the spectroheliograph in 1903. These reveal dark hydrogen flocculi, which appear to lie at a level above that of the bright calcium flocculi. They also show less extensive bright flocculi, usually in the immediate neighbourhood of sunspots, and frequently eruptive in character. These rise
↑ Two screws, of different pitch, are provided, to give different speeds. ”