128 ENZYMES 



stationary while the other will rotate round it as a centre. This 

 phenomenon 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 ])e 

 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. 



L— :^ 



Fig. 29. — Diagram of the paths of the ordinary and extraordinary rays of liulit tlirough a 

 rhombohcdron of Iceland spar. The liglit, 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. 



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



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



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 



sin a 

 of refraction is constant, —. — 7, = u.) 



sm p ' 



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. so. — 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 EC (Fig. 30) 

 perpendicular to the principal plane i'or the face AC. The cut 

 surfaces were carefully polished and then cemented in their 

 original position by a thin film of Canada balsam. If now the 

 ordinary ray falls on the surface BC at an angle greater than the 



