LIGHT. 



211 



Double Convex Lens. 



Fig. 79 exhibits a pencil of 

 rays, proceeding from a ra- 

 diant point at A, and meet- 

 ing at a focus at B; the 

 dotted lines being the per- 

 pendiculars drawn to the 

 surface at the points of 

 immersion and emergence. 

 Lastly; if the surfaces of 

 the medium be concave, as 

 in Fig. 80, the luminous 



rays, proceeding from a radiant point as at A, are rendered so divergent, 

 that if we look for a focus here it must be anterior to the medium or 

 at Gr. 



A knowledge of these facts has given occasion to the construction 

 of numerous invaluable optical instruments, adapted to modify the 

 luminous rays so as to change the situation in which bodies are seen ; 

 to augment their dimensions; and to render them more luminous and 

 visible, when remote and minute. It is to this branch of science that 

 we are indebted for some of the most important information and ad- 

 vantages, that we 



possess in the do- Fi s- 80 - 



mains of science and 

 art. The simplest 

 of these instruments 

 are bodies shaped 

 like a lentil, and 

 hence called lenses. 

 They are composed 

 of two segments of 

 a sphere. The me- 

 dium in Fig. 7 9 is a 

 double convex; that 

 in Fig. 80, a double 

 concave lens. The 

 manner in which 

 they modify the 

 course of the lumi- 



Double Concave Lens. 



nous rays passing through them has been sufficiently described. 



The study of the refraction of light leads to the knowledge of an 

 extremely important fact ; which, when it was first made known by 

 Newton, excited universal astonishment ; that a ray of light is itself 

 composed of several coloured rays differing from each other in their 

 refrangibility. If a beam of the sun's light be admitted through the 

 hole of a window-shutter, E F, Fig. 81, into a dark chamber, it will 

 proceed in a direct line to P, and form a white spot upon the wall, or 

 on a whitened screen placed there for the purpose. But if a glass 

 prism, B A C, be placed, so that the light may fall upon its surface, 

 C A, and emerge at the same angle from its second surface, B A, in 



