TELESCOPE

an instrument employed for the purpose of viewing distant objects. There are two cla es of the instrument, the refracting and the reflecting. The common astronomical refracting telescope consists e entially of an objective and an eye-piece or ocular fitted into a tube, with geo me mechanical contrivance for adjusting the distance between the two. The objective is a double convex lens. The rays of light, proceeding from the distant object to which the telescope is directed, are brought to a focus somewhere witbin the tube, and an inverted image of the object is formed at that spot. This inverted image is magnified by the convex lens of the ocular. To produce a bright and distinct image a large objective is nece ary, and in the telescopes employed in observatories this is often from 16 to 24 inches in diameter, and the tube from 20 to 30 feet in lencth. The inversion of the image is no drawback when a celestial, body is being observed, In terrestrial telescopes this is corrected by modifications in the ocular; either two additional convex lenses of equal curvature are introduced, or, as in the Galilean instrument, the ocular consists of a double concave lens. Opera-gla es and field gla es consist usually of a pair of Galilean telescopes. The reflecting telescope is a huch more powerful instrument. In this the objective is replaced by a concare mirror usually of speculum metal. The reflector forms an inverted image of the object under examination, and this is viewed through a magnifying ocular, placed in the side of the tube in Newton’s and Herschel’s telescopes, or in the end in the Greg or i an. In the latter instrument the imace is viewed through a central aperture in the main reflector, and a second smaller reflector is introduced facing the former, by means of which the inversion is corrected. the second image formed by it being pone the lens of the ocular. In the larczest ecting telescopes the reflector has a diameter of from 6 to 7 feet. [s. 297]