142 THE CARBOHYDRATES 



left the plane of a beam of circularly polarized light passing 

 through it. 



The polarimeter is much used in ascertaining the strength 

 of sugar solutions, but before describing the mode of using 

 it, it is desirable to consider briefly the principles which are 

 involved. 



When a ray of light enters a crystal of any system other 

 than the cubical, it is broken up into two rays, the ordinary 

 and the extra-ordinary, provided the beam of light is not 

 coincident with the optical axis of the crystal. This pheno- 

 menon is known as double refraction. 



These two rays, the ordinary and the extra-ordinary, do 

 not behave similarly ; the former conforms to the ordinary 

 laws of refraction, but the latter does not ; further, the two 

 rays are polarized in directions at right angles to one another. 



In order to make use of these facts, it is necessary to be 

 able to examine the extra-ordinary ray alone ; that is, the two 

 rays must be separated one from the other. This is effected 

 by a Nicol's prism, which consists of two plates of Iceland 

 spar fixed together by means of Canada balsam. A ray of 

 light enters one side of the prism, and is broken up into the 

 ordinary and the extra-ordinary ray ; on reaching the layer of 

 balsam, the former is totally reflected, whilst the latter passes 

 on through the other plate and emerges at the side opposite 

 to its entry. If a second Nicol be placed in the path of this 

 ray, the latter will pass through in different amounts according 

 to the angle which the second prism makes with the first. If 

 the interposed Nicol be parallel to the first Nicol, the ray will 

 pass through entirely ; if the second Nicol be rotated, the 

 light passing through will be less and less in amount until, 

 when the two prisms are at right angles to each other, no 

 light passes at all. If the rotation be continued, the light 

 will again pass through in gradually increasing quantities 

 until the prism has been rotated through an angle of i8o° 

 from its original position, when the whole light will again pass 

 through freely. 



Many liquids and solutions of solids possess what is known 

 as optical activity, which means that they can rotate the plane 



