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THE ADVANCE OF PHOTOGRAPHY
times as injurious as a similar error in the figuring of a lens. Any flexure or distortion of the mirror damages the image infinitely more than the same distortion of a lens. So sensitive is a mirror in this respect that even slight temperature differences, which would have no effect upon a lens, would render photographs obtained by the mirror useless.
For a given size a refracting telescope furnishes more light, and it also has the larger undistorted field. In the case of a refracting telescope which has been designed for the purpose of viewing the heavenly bodies, the rays most effective in vision, namely, the yellow, green, and light blue, are brought as nearly as possible
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Fig. 116.
to the same focus. The extreme violet rays, which have the greatest actinic value, and also the extreme red rays, deviate a considerable amount from this visual focus. The result of this is, that when such an instrument is used for photographic purposes without proper correction, only a more or less blurred image is obtained.
The Arrangements of Telescope for Photography. — The mode of preparing astronomical pictures differs little from that of ordinary photographs. An ordinary photographic apparatus could be used for this purpose, were it not that it gives too minute images of very remote objects, such as the stars. The size of the picture is in direct proportion to the focal length of the lens ; therefore, in taking astronomical photographs, by converting an astronomical telescope into a photographic camera, lenses are used the focal length of which is very long.