426 



OPTICAL INSTRUMENTS. 



661. Astronomical Telescope; Refractor. 



Astronomical telescopes are of two kinds refractors and 

 reflectors. "Fig. 330 represents the arrangement of the 

 lenses and the direction of the rays in the refracting 

 telescope. The object-glass is of large diameter that it 



FIG. 330. 



may collect many rays for the better illumination of the 

 image. The inverted, real image formed by the objective, 

 0, is magnified by the eye-piece, as in the case of the 

 compound microscope. The visible image, cd, is a virtual 

 image of ab, the real image of AB. 



662. Reflecting Telescopes. A reflecting tele- 

 scope consists of a tube closed at one end by a concave 



FIG. 331. 



mirror, so placed that the image thus formed may be mag- 

 nified by a convex lens used as an eye-piece. Sometimes 

 the eye-piece consists of a series of convex lenses placed 

 in a horizontal tube, as shown in Fig. 331. The rays 

 from the mirror are reflected by the cathetal prism mn 

 ( 621 [<?]), and a real image formed at ab. This image is 



