LIGHT. 67 



where nothing intervened between the prisms. Substances which 

 have such an effect on polarized light are said to turn the plane of 

 polarization, and are called optically active. The amount of this 

 turning or rotation of the plane of polarization can be measured by 

 noting to what extent the second prism must be turned to the right 

 or left to produce the original condition, namely, darkness. 



Substances which act in such a manner that the analyzer must be 

 turned to the right to produce darkness are called dextrorotatory, or 

 right-handed; while those substances acting in the opposite manner 

 are called levorotatory , or left-handed. Substances acting in neither 

 way are called optically inactive. 



The degree of rotation varies with the quantity of substance in 

 solution for a definite length of a column of the latter, and hence is 

 used to determine the percentage strength, for example, that of sugar 

 in urine and other liquids. Polariscopes are therefore valuable quan- 

 titative analytical instruments. 



All polariscopes consist essentially of a polarizer and an analyzer, 

 with a tube between them for containing the substance to be exam- 

 ined, and a scale for reading the amount of rotation produced, all 

 carried on a suitable metallic support. In addition to these essential 

 parts, the polariscopes of different makers are provided with numer- 

 ous contrivances rendering the instruments more perfect and the 

 analytical results obtained more accurate. 



Plates of crystalline substances, as quartz, have a noteworthy 

 effect when placed in the path of polarized light between a polarizer 

 and an analyzer. Not only do they prevent the light from being 

 entirely shut off when the Nicol prisms are crossed, but they produce 

 sharply defined appearances, as color tints, alternate lines of light 

 and darkness, equal or unequal illumination of the two halves of 

 the field of view, etc. The appearances depend on the particular 

 arrangement and direction of cutting of the crystalline plates. A 

 full explanation of the cause of these effects is beyond the scope of 

 this work. 



Fig. 28 shows Lippich's polariscope, used chiefly for sugar solu- 

 tions. The optical arrangement is shown above the drawing of the 

 instrument, and consists of a telescope, a-a, the analyzer 6, a station- 

 ary polarizer, c, a movable polarizer, d, and the condensing lens e ; 

 /represents a number of diaphragms. The liquid to be examined is 

 placed in the tube, and the rotatory power of the substance is deter- 

 mined by turning the polarizer to the right or left, as the case may 

 require. The amount of turning necessary to establish the same con- 



