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Page:Popular Science Monthly Volume 56.djvu/304

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will be signalized in the history of astronomy by the detection among these little objects of one which has entirely cast into the shade all other discoveries of the same nature. On the night of the 13th of August, 1898, a German astronomer, Herr Witt, exposed a photographic plate to the heavens in his telescope in the Observatory of Urania, at Berlin. On that plate a picture of the heavens was obtained, and in that picture a new planet was revealed. At first the discovery of one more asteroid does not imply very much. Hundreds of such planets might be found, and indeed have been found, and yet no particular comment has been called forth. But this planet found by Witt is a unique object; it is more interesting than the whole of the four hundred and thirty-two other minor planets which have preceded it—not, indeed, on account of its size, for Witt's planet is a wholly insignificant object from this point of view. The special interest which this new planet has for us dwellers on the earth lies in the fact that it seems to be the nearest to the earth of all the other worlds in space—the moon, of course, excepted. This is the reason why the attention of all who are interested in the science of astronomy has been concentrated on Witt's discovery. It is certainly the most interesting telescopic revelation which has been made for many years.

It may illustrate a characteristic feature in the progress of modern astronomy if I describe how Witt succeeded in obtaining this picture. He had selected one of the most rapid plates that the skilled manufacturer can supply to the photographer. He put this plate into his telescope, and he directed it to the heavens. If that plate had been used in broad daylight for the more ordinary purpose of obtaining a photographic portrait, an exposure of half a second would have been quite long enough. But the very faint stars can not work their charm on the plate with equal rapidity; a second is not long enough, nor is ten seconds, nor even ten minutes. If we desire to secure an imprint of the faintest stars we must expose the plate for an hour, and sometimes for even much longer than an hour. Of course, an exposure of such duration would utterly ruin the picture if a gleam of any other light obtained access. But in the darkness of night the plate is secure from this danger. Each star is thus given time enough to impress its little image at leisure.

The photographer has often occasion to deplore the poorness of his light. It is, of course, in the endeavor to counteract the poorness of the light that so long an exposure is frequently given. But it will not be any longer supposed that, from the astronomer's point of view, a tedious exposure must necessarily be a disadvantage. Let it be henceforth recollected that it was the very require-