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294 THE ADVANCE OF PHOTOGRAPHY
enlargement. When the image is twice the size, the weakening of the light is fourfold ; when it is three times as great, the weakening is ninefold, and so on. In views of the unclouded sun this is of no consequence, for its light is so intense that it bears a considerable enlargement, and yet remains bright enough to give a view on a momentary exposure. The protuberances, however, give out much less light, and if their image were magnified they would become so faint that a longer exposure would be required than the duration of the eclipse.
The Spectro-heliograph.1 — Our knowledge of the sun has in the last fewT years been greatly augmented by means of the spectro-heliograph. This instrument enables photographs of the sun to be taken in which use is made of monochromatic light only.
The spectro-heliograph was first systematically employed for purposes of solar research in 1892.
As will be seen from the following account, the function of the instrument is the photography of the invisible gases surrounding the sun, which are commonly spoken of as the solar atmosphere.
The principle of this apparatus can be readily grasped by a study of fig. 122, which represents a plan of the essential parts.
The whole of the parts here shown, with the exception of the plate G, are on a moving platform, which is so arranged that the primary slit A can move across an image of the sun formed in that position by means of a telescope. The slit A need not be of any very special construction, in fact, the ordinary spectroscopic slit will do very well for this purpose.
The whole may really be looked upon as a spectroscope having the ordinary collimator and telescope, but with a
1 For further information on this subject see (a) " Monthly Notices,'* B. A. S., vol. lxv. No. 5, pp. 473 et seq. ; (b) Nature, Nov. 4 and Nov. 11, 1909; (c) Astrophysics Journal, Sept. 1908.