466 



REFRACTION THROUGH LENSES. 



prisms, with their refracting edges downwards, viz., the 

 prisms /, </, A, will, if their refracting angles have the re- 

 quired magnitude, refract the rays from a also towards c. 



FIG. 265. 



Finally, the central ray between the prisms passes from a 

 to c without refraction. It follows that, with a suitable 

 arrangement of prisms, a number of rays which diverge 

 from a may be brought to converge again at a point c. 



It will be easily seen that the series of prisms need 

 not be arranged one vertically above the other. Fig. 

 265 may be supposed to represent a section through a 

 series of prisms arranged in a horizontal line ; and in 

 fact, whatever the arrangement of the series, whether 

 horizontal or vertical, or inclined to either of these 

 directions, the effect will be the same : the rays from a 

 which impinge upon the prisms will be refracted towards 

 a point c. 



It follows further, since the deviation of a ray does 

 not depend on the distance of the refracting surfaces 

 from one another, but solely upon the angle between 

 tbem, that a mass of glass of the shape represented in 

 fig. 266 B must have precisely the same effect as the com- 

 bination of separate prisms represented at A in the same 

 figure. The upper and lower portions of 5, viz., a and 

 g, are exactly equal to the prisms a and g in A ; b and f in 



