. The Americana : a universal reference library, comprising the arts and sciences, literature, history, biograhy, geography, commerce, etc., of the world . dapted to hispurpose, both as to magnification and light. The telescope giving the minimum power isthe opera glass, usually magnifying 2y^ or 3diameters, which is sufficient for indoor use,while for outdoor use the Galilean binoculsr hasa power of 4 or 5 diameters, and the Prismbinocular of 6, 8, 10, or even 12 diameters. The8-power is, however, considered as high as can of the two lenses results in clear and distinctdefinition, the image o
. The Americana : a universal reference library, comprising the arts and sciences, literature, history, biograhy, geography, commerce, etc., of the world . dapted to hispurpose, both as to magnification and light. The telescope giving the minimum power isthe opera glass, usually magnifying 2y^ or 3diameters, which is sufficient for indoor use,while for outdoor use the Galilean binoculsr hasa power of 4 or 5 diameters, and the Prismbinocular of 6, 8, 10, or even 12 diameters. The8-power is, however, considered as high as can of the two lenses results in clear and distinctdefinition, the image of the star being sharplyoutlined and colorless. Optical glass for suchlenses is of special manufacture. The worldssupply comes from three makers, one each inEngland, France, and Germany. The rnirror orspeculum of the reflecting telescope is madefrom a casting of ordinary glass of sufficienthickness to be handled without flexure. Thereflecting surface is ground and polished, withgreat precision, to a parabolic form of the focusrequired, and then, by a chemical process, coatedwith silver, which may be easily renewed whentarnished or otherwise E- — -J 5 Galilean telescope (opera glass). be held in the hands with sufficient steadinessto give the best results. The power used interrestrial telescopes steadily mounted on atripod usually ranges from 15 to 100 diameters,depending on the condition of the atmosphere. The most important element in a refractingtelescope is the objective, and, in a reflectingtelescope, the mirror or speculum. The objec-tive of the early refracting telescope was adouble convex lens, which could not give a dis-tinct image because it separated each ray oflight into its various prismatic colors, and eachcolor, having a refracting power different fromthe others, found a focus of its own. The redray, being most refracted, reached its focus first; Vol. 15 —6 The making of the optical parts of telescopesis a rare art, which, however, has been cul
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