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FIG. 15.—SPOT OF SEPT. 21, 1870. (REDUCED FROM AN ORIGINAL DRAWING BY S.P. LANGLEY.)

FIG. 16.—SPOT OF MARCH 5, 1873. (REDUCED FROM AN ORIGINAL DRAWING BY S.P. LANGLEY.)

The surface of the sun may be compared to an elaborate engraving, filled with the closest and most delicate lines and hatchings, but an engraving which during ninety-nine hundredths of the time can only be seen across such a quivering mass of heated air as makes everything confused and liable to be mistaken, causing what is definite to look like a vaguely seen mottling. It is literally true that the more delicate features we are about to show, are only distinctly visible even by the best telescope during less than one-hundredth of the time, coming out as they do in brief instants when our dancing air is momentarily still, so that one who has sat at a powerful telescope all day is exceptionally lucky if he has secured enough glimpses of the true structure to aggregate five minutes of clear seeing, while at all other times the attempt to magnify only produces a blurring of the image. This study, then, demands not only fine telescopes and special optical aids, but endless patience.