210 SENSE OF SIGHT. 



tinctly visible, the ray from it proceeding in the direction A C F, but 

 appearing to come straight to the eye in the direction C F. 



All transparent bodies, at the same time that they refract light, re- 

 flect a portion of it. This is the cause of the reflections we notice in 

 the glass of windows, and of the image perceptible in the eye. The same 

 substance has always the same refractive power, whatever may be its 

 shape: in all cases, the sine of the angle of refraction holding the 

 same ratio to the sine of the angle of incidence, whatever may be the 

 incidence. The angle of incidence is the angle formed by the incident 

 ray with a perpendicular raised from the point of immersion ; the 

 angle of refraction that formed by the refracted portion of the ray 

 with the same perpendicular. In Fig. 77, A C K is the angle of in- 

 cidence of the ray A C ; and L C F the angle of refraction. The 

 sines of these angles respectively are the lines P Q and L F. But 

 although media may refract the rays of light equally, the form of the 

 refracting body materially modifies their arrangement. The perpen- 

 diculars to the surface may approach or recede from each other ; and 

 if this be the case the refracted rays will approach or recede from each 

 other likewise. 



Where the body has plane and parallel surfaces, as the glass of our 

 windows, the refraction, experienced by the ray on entering the glass, 

 is corrected by that which occurs on its emergence ; the light does not, 

 therefore, proceed in one straight line, but in parallel lines separated 

 by a space dependent upon the thickness of the refracting body, and 

 the obliquity of the incident ray. If the medium be thin as in a pane 

 of glass, the rays do not appear deflected from their original direction. 

 In Fig. 77, the interval between the direct ray and the ray A C F after 



its emergence is that 



Fig. 78. between G and H. If 



the surfaces of the dia- 

 phanous body be plane, 

 but inclined towards 

 each other, as in the 

 common prism ; the re- 

 fraction experienced by 

 the ray, on emerging, 

 instead of correcting 

 Prism. that experienced during 



its passage through the 



body, is added to it ; and the rays are deflected from their course to an 

 extent equal to the sum of the two refractions. The ray A B, Fig. 78, 

 after impinging upon the side D L of the prism at B, instead of con- 

 tinuing its course in the direction B J, is refracted toivards the perpen- 

 dicular C B F, the medium being denser than air. and on emerging 

 into the rarer medium, instead of continuing its course in the direction 

 G I, it is refracted in the line G H, or from the perpendicular K J. 

 Again, if the surfaces of the medium be convex, the rays are so situate, 

 after refraction, as to converge behind the refracting body into a point 

 called the focus, which is nearer to the medium the less the divergence 

 of the rays, or in other words, the more distant the luminous object. 



