OPTICAL INSTRUMENTS. 



427 



magnified by the glasses of the eye-piece and a virtual 

 image formed at cd. The Earl of Rosse built a telescope 

 with a mirror six feet in diameter and having a focal dis- 

 tance of fifty-four feet. 



663. Terrestrial Telescope. The inversion of 

 the image in an astronomical telescope is inconvenient 



FIG. 332. 



when viewing terrestrial objects. This inconvenience is 

 obviated in the terrestrial telescope by the interposition of 

 two double convex lenses, m, n, between the objective and 

 the eye-piece. The rays, diverging from the inverted 

 imago at /, cross between m and n, and form an erect, 

 magnified, virtual image at db. 



FIG. 333- 



664. Magic Lantern. In the magic lantern, a 

 lamp is placed at the common focus of a convex lens in 

 front of it and of a concave mirror behind it. The light 

 is thus concentrated upon db, a transparent picture, called 

 the "slide." A system of lenses, m, is placed at a little 



