110 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



beyond the sensitive range of the apparatus, the light, on 

 comparison, appears to a certain extent of the same intensity 

 on either side of the vertical line, and this point may be 

 mistaken for the zero position. Under these circumstances, 

 even if the circle is turned through ten, fifteen, or even a 

 greater number of degrees, hardly any change will be observed. 



It is a matter, therefore, of the greatest importance, parti- 

 cularly after the sample to be examined has been placed in 

 the apparatus, to see that when the circle has been turned a 

 few degrees on either side of the zero line, the transition from 

 light to shade, and vice versa, is instantaneous. 



On placing the sample to be tested within the apparatus, 

 the first thing to do is accurately to adjust the telescope so 

 that the field is quite clear and equally divided by the vertical 

 line ; then the circle is turned until the shades are exactly of 

 the same intensity on either side of the line. 



The angle rotated by a column of 10 per cent, solution of 

 a sugar ten decimetres long is known as the specific rotatory 

 power of the sugar. 



In the case of a 10 per cent, solution of pure cane sugar 

 ten decimetres long the angle is 66*5 degrees when sodium 

 light is used ; this is generally known as [a] D . In practice 

 it is convenient to use tubes one or two decimetres long. 



If the quantity of sugar per 100 c.c. is known, the specific 

 rotatory power is given in the following equation : When E, 

 = the reading of the polarimeter, L the length of tube, and C 

 the number of grams per 100 c.c., then 



R 



On the other hand, if the specific rotatory power of the 

 solution is known, the quantity present can be calculated 

 from a determination of the specific rotatory power, thus : 

 C_ B 



100 



