George F. Chambers, The Story of the Solar System
“ Very often we next notice some groups of little black spots, as if the luminous stratum was becoming thinner in such a way as to disappear little by little and leave a large black nucleus uncovered. At the commencement of the business there is usually no clearly defined penumbra. This developes itself gradually and acquires a regular outline, just as the spot itself often takes a somewhat circular form. ”
