USEFUL MICROSCOPE ACCESSORIES 141 



average taken. It is obvious that in this series, the first measure- 

 ments must include rotation of the analyzer both to the right and 

 to the left. 

 The specific rotatory power of a substance for yellow light, 



() D , is found from the equation (O;)D = , where a is the 



angle of rotation found, c the number of grams of substance in 

 100 cubic centimeters of solution and / the length of the polariza- 

 tion tube employed expressed in decimeters. 



Since in most cases the specific rotatory power of a substance 

 is known, we may determine the per cent of the optically active 

 'substance by dissolving a known weight of the material contain- 

 ing it in water, making the volume up to 100 cubic centimeters 

 and determining the angle of rotation a in the Bates tube. This 

 tube is 100 millimeters long. In the above equation all the mem- 

 bers will thus be known but c, i.e., the number of grams of the 

 active substance present in the mixture. Solving for c will give 

 the result sought. 



For further details the student is referred to the standard 

 works on the polarimeter and saccharimeter. 



