KEFLECTING TELESCOPES. 347 



object viewed, it results that the image ultimately perceived by the 

 eye is upside down. 



For astronomical observations this inversion is of no particular 

 consequence ; for land use some extra lenses are introduced, the 

 effect of which is to produce a second inversion of the image, and 

 consequently to render it visible erect, much as the reflecting 

 plane mirror in a camera obscura erects the inverted image formed 

 by the lens (Expt. 370). Telescopes thus adjusted are termed 

 terrestrial telescopes. 



Expt. 375. Eeflecting Telescopes. By employing a concave 

 mirror (preferably of parabolic curvature, Expt. 361, footnote) 

 instead of a convex lens to receive light from a far off object, the 

 rays of light are converged to a focus, and form a real image in 

 front of the mirror. In what is termed the front view reflecting 

 telescope the real image thus formed is magnified by lenses acting 

 as eyepiece exactly as the analogous image formed by the convex 

 lens of an astronomical telescope, the mirror being so large that the 

 observer's head in front only blocks out a fraction of the total light 

 (mirrors 6 feet and upwards in diameter being used). As with 

 the astronomical telescope, the magnifying power is nearly the 

 quotient of the focal length of the mirror divided by that of the 

 eyepiece. In Newton's reflecting telescope fitted with a concave 

 mirror not of such large dimensions, a small plane mirror is sup- 

 ported by a wire, &c., a little nearer to the concave mirror than its 

 principal focus and inclined at 45 to its axis ; the rays of light are 

 then reflected to one side at right angles to the axis of the tele- 

 scope, and are thus enabled to form an image in a position where it 

 can be magnified by an eyepiece without requiring the observer's 

 head to be in front of the mirror ; very little light is obstructed 

 by the plane mirror on account of its size, which is small in com- 

 parison with that of the concave mirror. Instead of a mirror a right- 

 angled prism can be used acting like the prism-lens in Expt. 370. 



In Gregory's reflecting telescope the concave mirror has a circular 

 hole in its centre ; the converging rays of light reflected from the 

 outer annular portion are reflected back a second time from a 

 smaller concave mirror, supported some distance in front of the 

 larger one and directly facing it ; the rays thus reflected back a 

 second time, pass through the cavity in the larger mirror and form a 

 real image, which is then magnified by an eyepiece as before. In 

 Cassegrairis telescope a small convex mirror is used instead of a 

 concave one to reflect back the rays, being placed in a different 

 position in reference to the larger concave mirror. 



In all kinds of telescopes the use of tubes to enclose the paths 

 of refracted or reflected rays is chiefly to support the lenses, &c., 



