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 most probable, then the several pictures will evidence this by their difference in outline. No doubt the different photographs may to some extent overlap, but though this will interfere with the pictorial effect, it will not prevent their interpretation in the sense that it is instructive to the astronomer.

One of the most remarkable features in the structure of the corona is the presence of streamers or luminous rays extending from the north and south poles of the sun. These rays are generally more or less curved, and it is doubtful whether the phenomena they exhibit are not in some way a consequence of the rotation of the sun. This consideration is connected with the question as to how far the corona itself shares in that rotation of the sun with which astronomers are familiar. I should perhaps rather have said—that rotation of the sun's photosphere, which, as the sun-spots prove, is accomplished once every twenty-five days. Even this shell of luminous matter does not revolve as a rigid mass would do. By some mysterious law the equatorial portions accomplish their revolution in a shorter period than is required by those zones of the photosphere which lie nearer the north and south poles of the luminary. As to how the parts of the sun which are interior to the photosphere may revolve, we are quite ignorant. Nor does there seem much likelihood of any discoveries being made which will clear up this matter. We have no means of knowing to what extent the corona shares in the rotation. It would seem certain that the lower parts which lie comparatively near the surface must be affected by the rapid rotation of the photosphere. But it is very far from certain that this rotation can be shared in to any great extent by those parts of the corona