DOUBLE REFRACTION. 65 



boliedron, and is equally inclined to the three faces which 

 meet there. The angles at which the faces themselves are 

 mutually inclined are 105 5 X and 74 55'. 



(84) If a transparent piece of this substance be laid upon 

 a sheet of white paper, on which a small black spot is marked 

 with ink, we see two images of the spot instead of one, on 

 looking through the crystal ; and if the eye be held perpen- 

 dicularly above the surface, and the crystal turned round in 

 its plane, one of these images will appear to describe a circle 

 round the other, which is immoveable, the line connecting 

 them being in the direction of the shorter diagonal of the 

 rhombic face. We may vary this experiment, by substituting 

 for the dark spot on the paper a luminous point on a dark 

 ground, as, for example, the light of the sky seen through 

 a small aperture ; but the most direct mode of performing 

 the experiment is to transmit a ray of the Sun's light 

 through the crystal, and receive the emergent pencils on a 

 screen. 



If now we examine the course of the two rays within the 

 crystal, we shall find that, at a perpendicular incidence, the 

 deviation of one of them is nothing ; that, at any other inci- 

 dence, the ray is bent towards the perpendicular, the sines 

 of the angles of incidence and refraction being in the constant 

 ratio of 1 '654 to 1 ; and that these angles are always in the 

 same plane. This ray, therefore, is refracted according to 

 the known law, and is called the ordinary ray. On examin- 

 ing the other ray, however, we find that, at a perpendicular 

 incidence, the deviation, instead of vanishing, is 6 12'; that, 

 at other incidences, the refracted ray does not follow the law 

 of the sines ; and that, moreover, the angles of incidence and 

 refraction are in different planes. This ray, therefore, is re- 

 fracted according to a new and different law, and is called the 

 extraordinary ray. 



