CARBOHYDRATES 33 



SPECIFIC ROTATIONS OF MORE COMMON CARBOHYDRATES 1 



An instrument by means of which the extent of the rotation may be 

 determined is called a polariscope or polarimeter. Such an instru- 

 ment designed especially for the examination of sugar solutions is 

 termed a saccharimeter or polarizing saccharimeter. The form of polari- 

 scope in Fig. 5, page 32, consists essentially of along barrel provided 

 with a Nicol prism at either end (Fig. 6,) . The solution under exami- 

 nation is contained in a tube which is placed between these two prisms. 



FIG. 6. DIAGRAMMATIC REPRESENTATION OF THE COURSE or THE LIGHT THROUGH THE 

 LAURENT POLARISCOPE. (The direction is reversed from that of Fig. 5, p. 32.) 



a, Bichromate plate to purify the light; b, the polarizing Nicol prism; c, a thin quartz 

 plate covering one-half the field and essential in producing a second polarized plane; d, 

 tube to contain the liquid under examination; e, the analyzing Nicol prism; /and g, ocular 

 lenses. 



At the front end of the instrument is an adjusting eyepiece for focusing 

 and a large recording disc which registers in degrees and fractions of a 

 degree. The light is admitted into the far end of the instrument and is 

 polarized by passing through a Nicol prism. This polarized ray then 

 traverses the column of liquid within the tube mentioned above and 

 if the substance is optically active the plane of the polarized ray is 

 rotated to the right or left. Bodies rotating the ray to the right are 

 called dextro-rotatory and those rotating it to the left levo-rotatory. 



Within the apparatus is a disc which is so arranged as to be without 

 lines and uniformly light at zero. Upon placing the optically active 

 substance in position, however, the plane of polarized light is rotated 

 or turned and it is necessary to rotate the disc through a certain number 

 of degree in order to secure the normal conditions, i.e., " without lines 



1 The specific rotation varies with the temperature and concentration of the solution. 

 The figures here given are for concentrations of about 10 per cent and temperatures of about 

 2oC. Fresh solutions may give markedly different values due to mutarotation, the figures 

 here given representing the constant values obtained on standing. 



