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ASTRONOMICAL PHOTOGRAPHY 293
stances have led to a closer study of the spot formations, and photography has offered a valuable aid for this purpose. It gives at a particular moment a faithful view of the sun's surface, and photographs taken daily give us the most exact representation of the spots, their size and number. A comparison of the views during one month affords an instructive survey of the changes on the sun's surface, relating more faithfully than words the history of the central body of our planetary system. Lewis Rutherford, of New York, who made valuable contributions to astronomical photography, took a great number of these views at the photographic observatory built at his own expense.
These views, taken on successive days, exhibit mani
Fig. 121.
fold groups of spots, often of considerable size ; and the change in their form and position is thus accurately recorded (the latter consequent upon the revolution of the sun). These impressions are not prepared, as were the pictures of the eclipse, in the principal focus of the telescope, but in an additional apparatus (fig. 121) which answers the purpose of a magnifying apparatus. This contains a small lens L, which projects on the ground glass, G, an enlarged image of the small representation of the sun, S, produced by the great lens, 0.
In this manner Rutherford obtained directly a picture of the sun about two inches in diameter. This enlarging apparatus is not to be recommended in the case of eclipses ; for the brightness of the image produced by the great telescopic lens is materially weakened by the