. The book of photography; practical, theoretical and applied. surface. Be-sides, it is a difficult matter to cast sucha large piece of glass so that it shall havea uniform structure throughout and alsobe free from local strain. The troublesalso, due to expansion, with such largemasses of glass are serious, and with alonger focus a longer tube is needed forthe telescope. This tube, moreover, mustbe rigid and mounted so as to point in anyrequii-ed direction. Reflecting Telescopes. As metal mirrors are more easily pre-pared in large sizes than glass lenses, 596 THE BOOK OF PHOTOGRAPHY. they have
. The book of photography; practical, theoretical and applied. surface. Be-sides, it is a difficult matter to cast sucha large piece of glass so that it shall havea uniform structure throughout and alsobe free from local strain. The troublesalso, due to expansion, with such largemasses of glass are serious, and with alonger focus a longer tube is needed forthe telescope. This tube, moreover, mustbe rigid and mounted so as to point in anyrequii-ed direction. Reflecting Telescopes. As metal mirrors are more easily pre-pared in large sizes than glass lenses, 596 THE BOOK OF PHOTOGRAPHY. they have been used in the constructionof the largest telescopes. Sir JohnHerschels telescope had a concave mirrorwhich was four feet in diameter. Metalis, however, more subject to expansionerrors than glass, and it is very difficultto mount metal mirrors so that the sur-faces are without strain and truly differences of curvature impercep-tible to the eye produce a marked effecton the definition of the image. Even apiece of plane glass, for example, may. EYEPIECE Fig. 79J.—Her^schels Reflecting Telescope. seem to have a uniformly even surface,but if a ray of light be made to fall on itand then enter the eye by reflection itrarely remains a distinct spot from everypoint of view. A great drawback in thesetelescopes is the presence of the appara-tus for directing the reflected ravs into
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