ON LI6HT. 343 



as in the case of the original incident ray, it will not be 

 further subdivided, but refracted singly: if an ordinary 

 ray, ordinarily; and if extraordinary, extraordinarily. Its 

 refraction will also be single, if the second crystal be 

 turned round on the ray as an axis exactly through a right 

 angle ; but in this case the second refraction, if an ordi- 

 nary ray have been used, will be extraordinay, and via 

 versa. In every intermediate situation of the second 

 crystal, it will be subdivided into two, the one ordinarily, 

 the other extraordinarily refracted, but the two fractions 

 will be found to differ in relative intensity : generally 

 speaking, the more according as the conversion of the 

 second crystal has been through a less angle from its 

 first position, and they are equal when the angle of con- 

 version is 45, 135, 225, or 315, i.e., exactly half-way 

 between the rectangular positions of the crystal. 



(121.) All these particulars are easily and elegantly 

 exhibited by means of two crystals of the mineral called 

 Iceland spar (crystallized carbonate of lime), a mineral 

 of perfect and colourless transparency, which, if frac- 

 tured, will always separate itself 

 along its three " planes of cleav- 

 age" (which in this mineral are 

 singularly distinct and palpable) 

 into forms whose type is the ob- 

 tuse rhomboid (whose six faces, 

 all equal and similar rhombs of Fig. K. 



101 32' and 78 28', are united three and three, by 

 their obtuse angles, at the opposite extremities of a 

 line called the axis of the rhomboid, the shortest that 



