1 66 Biological Chemistry. 



metal on hinges, seen only in the illustration of the complete 

 instrument, to keep away light ; the instrument is used in a 

 dark room). If the substance is optically active, one side of 

 the field will be rendered darker than the other, and the 

 analyzer is turned until both halves of the field show 

 equal illumination. The point at which equal illumination 

 is attained is then read off on the scale ; according to the 

 direction in which the analyzer is turned, the substance 

 under examination is either dextro- or laevo-rotatory, and 

 the magnitude of the rotation is indicated by the differ- 

 ences in the readings between the zero point and the 

 point of equal illumination of the whole field when the 

 polarized light passes through the tube containing the 

 substance under investigation. 



It will be noticed that when a half-shadow instrument 

 is employed, two fields are observed which come simul- 

 taneously within the field of vision. No attempt has 

 been made here to give the complete theory of the instru- 

 ment, for which reference must be made to text-books on 

 physical optics. 



The specific rotation of a substance is indicated by the 

 symbol [a]. If taken with the sodium line (the so-called 

 D line), this fact is indicated by the symbol [a] D . The 

 temperature is also indicated by writing it above the 



symbol [a] for example, [a] signifies that the specific 

 rotation is measured by sodium light at the tempera- 

 ture 20. 



The rotation is calculated from the following formulae: 



For a homogeneous liquid 



where [a] is the rotation actually read, I is the length of 

 the column in decimetres, and d is the density of the fluid. 



