Some idea of the aspect of these spots may be obtained from the accompanying reproduction of a photograph of the Sun (taken September 8, 1898, at the author's observatory at Juvisy), and from the detailed drawing of the large spot that broke out some days later (September 13), crossed by a bridge, and furrowed with flames. As a rule, the spots undergo rapid transformations. [Illustration: FIG. 30.--Telescopic aspect of a Sun-Spot.] These spots, which appear of insignificant dimensions to the observers on the Earth, are in reality absolutely gigantic. Some that have been measured are ten times as large as the Earth's diameter, _i.e._, 120,000 kilometers (74,500 miles). Sometimes the spots are so large that they can be seen with the unaided eye (protected with black or dark-blue glasses). They are not formed instantaneously, but are heralded by a vast commotion on the solar surface, exhibiting, as it were, luminous waves or _faculæ_. Out of this agitation arises a little spot, that is usually round, and enlarges progressively to reach a maximum, after which it diminishes, with frequent segmentation and shrinkage. Some are visible only for a few days; others last for months. Some appear, only to be instantly swallowed in the boiling turmoil of the flaming orb. Sometimes, again, white incandescent waves emerge, and seem to throw luminous bridges across the central umbra. As a rule the spots are not very profound. They are funnel-shaped depressions, inferior in depth to the diameter of the Earth, which, as we have seen, is 108 times smaller than that of the Sun. * * * * *