All conditions being favourable, however--that is to say, the eclipse being central, and the moon at such a position in her orbit as to present a diameter equal to, or slightly greater than, that of the sun--a picture of extraordinary beauty and wonder reveals itself the moment that totality has been established. The centre of the view is the black disc of the moon. From behind it on every side there streams out a wonderful halo of silvery light which in some of its furthest streamers may sometimes extend to a distance of several million miles. In the Indian Eclipse of 1898, for example, one streamer was photographed by Mrs. Maunder, which extended to nearly six diameters from the limb of the eclipsed sun (Plate VIII.). The structure of this silvery halo is of the most remarkable complexity, and appears to be subject to continual variations, which have already been ascertained to be to some extent periodical and in sympathy with the sun-spot period. At its inner margin this halo rests upon a ring of crimson fire which extends completely round the sun, and throws up here and there great jets or waves, which frequently assume the most fantastic forms and rise to heights varying from 20,000 to 100,000 miles, or in extreme instances to a still greater height. To these appearances astronomers have given the names of the Corona, the Chromosphere, and the Prominences. The halo of silvery light is the Corona, the ring of crimson fire the Chromosphere, and the jets or waves are the Prominences. [Illustration: PLATE VIII. Coronal Streamers: Eclipse of 1898. From Photographs by Mrs. Maunder.] The Corona is perhaps the most mysterious of all the sun's surroundings. As yet its nature remains undetermined, though the observations which have been made at every eclipse since attention was first directed to it have been gradually suggesting and strengthening the idea that there exists a very close analogy between the coronal streamers and the Aurora or the tails of comets. The extreme rarity of its substance is conclusively proved by the fact that such insubstantial things as comets pass through it apparently unresisted and undelayed. Its structure presents variations in different latitudes. Near the poles it exhibits the appearance of brushes of light, the rays shooting out from the sun towards each summit of his axis, while the equatorial rays curve over, presenting a sort of fish-tail appearance. These variations are modified, as already mentioned, by some cause which is at all events coincident with the sun-spot period. At minimum the corona presents itself with polar brushes of light and fish-tail equatorial rays, the latter being sometimes of the most extraordinary length, as in the case of the eclipse of July 29, 1878, when a pair of these wonderful streamers extended east and west of the eclipsed sun to a distance of at least 10,000,000 miles.