Charles Piazzi Smyth

“ That is the chief band of watery vapor; and to see it very dark, even black, do not look at a dark part of the sky or at black clouds therein, but look, rather, where the sky is brightest, fullest of light to the naked eye, and where you can see through the greatest length of such well-illumined air, at a low, rather than high, angle of altitude, and either in warm weather, or, above all, just before a heavy rain-fall, when there is and must be an extra supply of watery vapor in the atmosphere. ”

“ Tyndall and others have well shown, it moderates the excesses of hot solar radiation by day and cold radiation of the sky at night, and is more abundant in the hotter than in the colder parts of the earth. Wherefore, according largely to its temperature for the time being, the air—otherwise consisting almost entirely of nitrogen and oxygen—can sustain, and does assimilate, as it were, a specified amount of this watery vapor, invisibly to the naked eye, the microscope, or the telescope; but not so to the instrument of recent times, the spectroscope. ”

“ The classical "rain-band," which in the little instrument is merely a very narrow fringe to an almost infinitely thin black line, is so magnified laterally in the larger instrument as to fill the whole breadth of the field. The thin black line before spoken of is now not only split into two, but these are both strong, thick, sharply defined lines, separated from each other by six or seven times the breadth of either. These are the celebrated solar D-lines, D1 and D2, arising from the sodium metalloid burning or incandescent in the sun. ”