Page:Popular Science Monthly Volume 22.djvu/502

486 constitution is, that the surface of the sun presents the aspect of a network, the web of which is formed of strings of more or less regular grains, with here and there elongated bodies drawn out in all directions. An attentive study of these curious phenomena leads us to a very simple explanation of them.

The stratum of luminous matter to which the sun owes its power of radiation is, as we know, very thin. If this stratum was in a state of perfect equilibrium, the fluid matter of which it is constituted would form a continuous envelope around the nucleus of the sun; and the granular elements being confounded together, the solar surface would have everywhere a uniform brightness. But the ascending currents, of which the eruptions of metallic vapors and the hydrogen-protuberances are evidences, rupture the fluid stratum which is tending to form at a great number of points. It is then broken up and divided into more or less considerable fragments. Wherever the perturbing forces leave the elements of the photosphere in a state of relative repose, they take a more or less pronounced globular form. At those points, on the other hand, which are the seats of ascending currents, these elements give evidence in their aspect of the violence of the actions to which they are subjected. Hence the variable forms of the elements of the photosphere, concerning which there has been so much discussion. Hence, also, the explanation of that net-work-like structure of the solar surface which has been revealed by photography.

These images also show the enormous difference between the luminous power of the elements of the photosphere and that of the medium in which they float, which seems quite dark by the side of them. A result of this constitution is, that the radiating power of the sun will be affected according to the number and brightness of these elements. The spots, then, can no longer be regarded as the principal element in the variations of the solar radiation; a new factor, the action of which may be preponderant, must hereafter be added to them.

These photographs permit another study, which promises results of extreme importance—the study of the motions which the granular elements take on under the action of the forces that rumple the photosphere. For the study of these motions, successive images of the same point on the surface of the sun are taken at very brief intervals with the photographic revolver. A comparison of the images demonstrates that the matter of the photosphere is animated by movements of the violence of which our terrestrial phenomena can convey only a very feeble idea.

Following the example of spectrum analysis, photography is making a circuit of the heavens. The year 1881 witnessed the first taking of the photograph of a comet, with a considerable portion of its tail. This picture has revealed some curious particulars of structure and has permitted a number of photometric measurements, the most notable of which is one showing that the tail, notwithstanding the