POLARIMETRY 103 



ary while the other will rotate round it as a centre. This pheno- 

 menon of double refraction may be demonstrated in another way. 

 If a strong beam of light be allowed to fall on one of the faces 

 of a crystal of Iceland spar and the transmitted light be received 

 on a screen, two spots of light will be seen, and if the crystal be 

 rotated as before one spot will circle round the other. That is, 

 the beam of light has been split into two rays of equal intensity. 

 One ray, the stationary one, has travelled through the crystal 

 just as it would pass through glass obeying the ordinary laws of 



FIG. 14.-Diagram of the paths of the ordinary and extraordinary rays of light through 

 a rhombohedron of Iceland spar. The light, falling on the face AC divides into 

 two rays, both of which are polarised. The extraordinary ray (E) is the lesser refracted 

 ray : the ordinary ray (0) is the more refracted ray. 



refraction (Snell's Law). It is called the ordinary ray. The other 

 ray is called the extraordinary ray, and it does not obey the 

 ordinary law of refraction. It is this ray which gives the movable 

 image when the crystal is rotated. (Snell's Law states that the 

 ratio of the sine of the angle of incidence to the sine of the angle 



of refraction is constant, - = . 



sin ft 



Both rays are plane polarised, but in planes at right angles to 

 one another. Nicol's problem was to get rid of one of these rays 



c 

 FIG. 15. Diagram of refraction in a Nicol's prism. 



so as to get light vibrating in one plane. The method he adopted 

 is very ingenious. The angular separation between the ordinary 

 and extraordinary rays is not very great, so that it is not possible 

 to screen off one of the rays unless a very thick crystal be employed. 

 A rhomb of Iceland spar was cut in two by a plane BC (Fig. 15) 

 perpendicular to the principal plane for the face AB. The cut 

 surfaces were carefully polished and then cemented in their 

 original position by a thin film of Canada balsam. The index 

 of refraction of balsam (1 -55) is intermediate between the minimum 



