OPTICAL INSTRUMENTS. 



365 



image is formed ; but the second lens, L, reverses the image, which is viewed 

 therefore, in an erect position by the last eye-glass, M. 



Fig. 299. 



Fig. 300, 



What is the 725. The common opera-glass, also called 

 tC'^'opera- the Galilean telescope from Galileo, its in- 

 giassr ventor, consists of a single convex object-glass 



and a concave eye-glass. 



Fig. 30O represents 

 the construction of this 

 form of telescope. is 

 a single convex object- 

 glass, in the focus of 

 which an inverted image 



of t-he object would be naturally formed, were it not for the interposition of 

 the double-concave lens, E. Tliis receiving the converging rays of light, 

 causes them to diverge and enter the eye parallel, and form an erect image. 



wiiatis a Re- "^^G. A Reflecting Telescope consists essen- 

 BcopeT ^^^' tially of a concave mirror, the image in which 

 is magnified by means of a lens. The mirror 

 employed in reflecting telescopes is made of polished 

 metal, and is termed a speculum. 



The manner in which the rays of light falling upon the concave speculum 

 of a reflecting telescope are caused to converge to a focus is clearly shown 

 in Fig. 301. The image formed at this focus is viewed through a double- 

 convex lens. 



Fig, 301. 



Fig, 302 represents one of the earliest forms of the reflecting telescope, called 

 from its inventor, Mr. Gregory, the " Gregorian Telescope." It consists of 

 a concave metallic speculum, A B, with a hole in its center, and a convex 

 eye-glass, E, the whole being fitted into a tube. An inverted image, n' m\ 

 of a distant object is formed by the speculum, A B ; this image ia agaia 



