32 SUGAR 



would have to be turned until it arrived at the angle 

 to which the ray had been deflected. When the light 

 had fully reappeared, the angle through which the 

 crystal had been turned would indicate the amount 

 of deflection. An instrument could be imagined in 

 which the scale should be so adjusted, and the parts so 

 constructed that the zero should be the point where, 

 with pure water in the tube, the two crystals coincide 

 and the light passes through ; and the one hundred 

 point should be that to which, with a fixed weight of 

 pure sugar dissolved in a fixed volume of water in the 

 tube, the crystal would exactly arrive in order to trans- 

 mit the full light of the deflected ray. In practice, 

 it is necessary to adopt a more complicated arrangement 

 in order to secure accuracy. Between the tube and the 

 second crystal is inserted a " compensator " con- 

 structed of two wedge-shaped crystals, which can be 

 shifted across each other by means of a screw until 

 they exactly compensate or neutralize the deflection 

 caused by the sugar, and allow the ray once more to 

 pass through the second crystal. The screw at the same 

 time moves a vernier scale on which the operator can 

 read off the percentage of sugar contained in the tube. 

 But this does not give absolute accuracy because the 

 exact point of maximum light or maximum darkness 

 is difficult to detect. Behind the polarizing crystal, 

 between it and the tube, is therefore inserted a plate 

 of quartz, made of two plates joined together side by 

 side, with their axes opposed, the one from right to 

 left and the other from left to right, the join being a 

 perpendicular line in the centre. The eye of the operator 

 now sees, when the ray is not deflected, a field of uniform 

 colour. But when the ray is deflected by the sugar 

 he sees the field divided by a sharp perpendicular 

 line into two different colours. As he moves the 



