Page:Popular Science Monthly Volume 22.djvu/501

Rh the aid and the applications which physical astronomy can expect from it.

The first application which was made of photography to the study of the sky was in France, whatever may be said about it. The first image of a fixed star upon the daguerrean plate was that of the sun, and it was taken by the authors of the admirable processes for measuring upon the earth the velocity of light—MM. Fizeau and Foucault. Shortly afterward, images of the moon were obtained in the United States. These labors were followed by others, of which the sun and the moon were also the objects. Beautiful proofs of lunar photographs have been taken by Mr. Warren de La Rue and Mr. Rutherfurd. Photographs of the sun are taken regularly in many observatories, as aids in the study of the spots and faculae of that star. More recently, Mr. Rutherfurd and Mr. Gould have begun to make celestial maps, and photographs of the nebula in Orion have been obtained in New York (by Mr. Draper) and at Meudon.

These works are all very important; they bear upon the primary object of astronomical photography, that of obtaining durable and trustworthy images of the stars and the phenomena produced upon them, available for further studies and measurements. Hitherto, observers had only memory, a written description, or a drawing, to depend upon for the preservation of the recollection of a phenomenon. Photography substitutes for this the material image of the phenomenon itself. It is an admirable artifice, which in a certain manner prevents the extinction of the phenomenon and its passage to among the things that were, and keeps it always with us for examination or study. Important as these results may be, the latest labors of which photography has been the object, especially in what concerns the sun, have demonstrated that the method may be employed as a means of making discoveries in astronomy.

The large solar images which have been obtained in the latest years at Meudon have revealed phenomena of the surface of the sun which our largest observatory instruments could not have shown, and which open a new field of studies. By their aid we can at last distinguish the real form of those elements of the photosphere, respecting which so many different and contradictory assertions have been made. The elements in question are composed of a fluid substance readily obedient to the action of external forces. At points of relative calm, the matter of the photosphere assumes forms more or less approaching the spherical, and its aspect is that of a general granulation. Where-ever, on the other hand, currents and more violent movements of the matter prevail, the granular elements are more or less drawn out, and present aspects suggesting the forms of grains of rice, willow-leaves, or veritable threads. The regions, however, where the photosphere is more agitated, are limited spots, and the granular form is generally observed in the intervals between them. The result of this peculiar