Great Telescopes

The United States boasts of the largest refraeting telescope in the world, the object glass of which they have receutly purchased in England. It is a twentyfive inch glass, inagnifying three thou - sand times, and bringing our visión, to all intenta and purposes, to within a distanco of eighty miles f rom the moon. The construction of a lens of this size is a work of the most laborious and difficult nature, it being absolutely neuessary that the glass should be of uniform density, perfeetly pellucid, and free from the smallest iiaw. Tho telescope to this glïss belongs is made ot steel, with an inner zinc tubo, the whole mounted on a pillar twenty-nine feet in height, and woighing not lcjss than nina toas. The next largest telescopo is placed iu the Observatory ia Chicago, and has an object glesa eighteon moties and a halt in diameter. THe two next largest havo object glasses of fifteen and a hult' inches in diameter, and they are situated at Cambridge, Massachusetts, and at tho Obsoivatory of Pulkowa in Kussia. The most gigantic of teleseopes, howover, is still uniinished, though now rapidly approachiiig oomplstion. The object-glass tur this instrument will havo a diameter of twenty-sevén inches, and will alone coat 40,000. It is bemg manufacturad for the government of the United States, and will probably ba placed on an astronómica! station to be established by tho Coast tëurvey Bureau on the Sierra Nevada Mountains. It will_thus be situated at a height of frotn scven to ten thousand feet above the level of the soa, in an atmosphere of wonderful pui'ity and comparatively freo from clouds. That the establishment of this great instrument will add greatly Ho our knowledge of physical astronoiuy oannot be doubted, but it may reasonably be doubted whether any telescope will enablo us, as has been recently suggested, to settle by direct optical"proof whether the moon is inhabited or not.