REFRACTION OF LIGHT. 319 



As ordinarilj constructed, a prism is an Fia. 257. 



oblong, triangular, or wedge-sbaped piece of 

 glass, with sides inclined at any angle, as 

 is represented in Fig. 257. 



Explain the ac .^"^ ^°°^^"S *^^™"Sh a 



tion of the prism, all objects are seen 

 pnsm. removed from their true 



placa Thus, let C A B, Fig. 258, bo a prism, and D E a ray of light inci- 

 FiG. 258. dent upon it ; it will be refracted ia 



the direction E F, and on emerging, 

 c o._ /\ will again be refracted in the direc- 



tion F H; and as objects always 

 appear in the direction in which the 

 last ray enters the eye, the object 

 D will appear at G, in the direction 

 of the dotted line, elevated above its 

 real position. If the refracting angle, A C B, had been placed downward, 

 the object would have appeared as much depressed. 



The prism, although of simple construction, is one of the most important 

 of optical instruments, and to its agency wo are indebted for most of the in- 

 formation we possess respecting the nature and constitution of light. Tho 

 beautiful and complicated results of its practical application belong to that 

 department of optics which treats of the phenomena of color. 



678. A Lens is a piece of glass or other 



What is a Lens ? \ ■, ■,■,,. ■, 



transparent substance, bounded on both sides 

 by polished spherical surfaces, or on the one side by a 

 spherical, and on the other by a plane surface. Eays of 

 light passing through it are made to change their direc- 

 tion, and to magnify or diminish the aj^pearance of objects 

 at a certain distance. 



^o^r many Thcrc arc slx different kiuds of simplc leuscs, 

 I'e^'ii^sare'there? ^^^ of which may be considered as portions of 



the external or internal surface of a sphere. 

 Four of these lenses are bounded by two spherical sur- 

 faces, and two by a plain and spherical surface. 



Fig. 259 represents sectional views of the six varieties of simple lenses. 



Expi.-xinfhedif- -^ doublo couvcx Icus is bouudcd by two 

 lenses.'''"''^ °^ COUVCX sphcncal surfaces, as at A, Fig. 259. 



To this figure the appellation of lens was first applied from 

 its resemblance to a lentil seed (in Latin, kns). 



A plano-convex, or single convex lens has one side 



