414 



PHYSIOLOGICAL PHYSICS. [Chap. xxx. 



The amount of rotation is dependent upon the 

 length of the column of liquid, and the amount of the 

 substance in solution. In a tube, 20 centimetres 

 long, the rotation is 1'333 for every gramme of 



Fig. 185. The Saccharuneter. 



sugar in 100 cubic centimetres. Thus, by the amount 

 of the rotation can be determined the strength of the 

 solution. To make the determination more exact, 

 between c and b there is interposed the double 

 quartz plate of Soleil. It consists of a plate of quartz 

 rotating to the right, cemented to a plate rotating to 

 the left, each cut perpendicular to the axis, and of the 

 same thickness (3 '15 mm.). The effect of this plate 

 is, that with the Nicol's prisms in a particular position 

 (parallel), a circle of light will be seen, all of one 

 colour (a violet tint), teinte sensible. If now the 

 plane of polarisation be turned the field will change 

 in colour, one half changing to the red end, and 

 the other towards the violet end of the spectrum, 

 because the two halves of the quartz are opposite. 



