Chap, xxv.] PRISMS. 311 



The refractive index of the diamond is very high, 

 2755; that of flint glass is 1-576, of water 1'336, of 

 the aqueous humour of the eye, 1'337, of vitreous 

 humour, 1'339, of crystalline lens, 1*337 to 14. 



CHAPTER XXV. 



THE ACTION OF PRISMS AND LENSES. 



Refraction by a plate with parallel faces. 



If a ray of light pass through a transparent body 

 and out at the other side, it is evident that it will 

 be twice refracted, first when it enters the body, 

 secondly when it leaves, and that the two refractions 

 will be in different directions. This is shown in Fig. 

 137, where the ray AB falls on a plate whose faces 

 are parallel. On entering the plate 

 it is bent towards the normal, and 

 becomes EC ; on emerging it is bent 

 awav from the normal, and becomes , 



V ..- s v - r , v ^7 



CD. Since the plate is parallel and fc~-.- ..- ^ . '...-..-' 41 

 the divergence in both cases of 

 similar amount, the ray will issue, 

 pursuing the same direction as 



wlip-n it pnfprprl TVmt i<s to <?nv Fi Z- 137. Refraction 

 ea< Sa 7> lay a Plate with Paral- 



the entering and the emergent rays lei Faces, 

 will be parallel, but, owing to the 

 refraction on entering, the beam will be displaced. The 

 result is, that, supposing D to be a luminous object, 

 and an eye to be at A, the object would appear dis- 

 placed to D' in the direction indicated by the dotted 

 line. 



Refraction by a prism. Rays of light which 

 have passed through a transparent body, whose 



