Chap, xxx ] NICOL'S PRISM. 399 



one optic axis have a corresponding number of direc- 

 tions in which a ray may be transmitted singly. The 

 ordinary form of Iceland spar consists of six surfaces. 

 Three of the surfaces meet one another at an obtuse 

 angle, and at the lower opposite angle three surfaces 

 also meet at an obtuse angle. The other angles of the 

 crystal are acute. This is shown in diagram in Fig. 

 179. A line drawn diagonally through 

 the crystal to join the obtuse angles 

 is the axis of the crystal cd in the 

 figure. The plane of the axis is 

 called a principal plane, and any 

 plane parallel to it is also a prin- 

 cipal plane. In Fig. 179 cadb is 

 the plane of the axis, and 1 and 2 Fig i79._p r i nc i pa i 

 are other principal planes. It is seen planes and Optic 



,, 7, T f> n, ,1 Axis of Iceland 



that 111 the ordinary torm oi the spar, 

 crystal incident rays all form an angle 

 with the axis in whatever position the crystal lies. If 

 now the obtuse angles be cut off by a plane at right 

 angles to the optic axis, the new surface obtained will 

 be at right angles to the axis. Rays which fall per- 

 pendicularly 011 this surface will be parallel to the 

 axis, and they will be transmitted through the crystal 

 without double refraction, subject only, therefore, to 

 the laws of simple refraction. Therefore ivhen the 

 plane of incidence is at, rig lit angles to the optic axis 

 there is no double refraction. 



If the rays are made to fall obliquely, double 

 refraction appears, and is the more pronounced the 

 greater the obliquity of the rays. 



Ticol's prism consists of a rhombohedroii of 

 Iceland spar, which is divided into two by a section 

 through its obtuse angles. The cut surfaces are care- 

 fully polished and then cemented in their former 

 position with Canada balsam, which has an index of 

 refraction intermediate between that of the ordinary 



