1 64 Biological Chemistry. 



the maximal intensity of the field of illumination. A 

 substance may rotate the plane of polarized light either in 

 one direction or another that is, it may be necessary to 

 rotate the analyzer either to the right or the left to 

 produce the maximal illumination of the field. According 

 to the direction in which the analyzer must be rotated, a 

 substance is either dextro- or laevo-rotatory. The instru- 

 ment for determining the rotatory power is known as the 

 " polarimeter." 



The apparatus actually employed for the measurement 

 of optical rotation is somewhat more complex than that 

 suggested by the foregoing description. If only an 

 analyzer and polarizer were employed, it would be difficult to 

 determine with accuracy the positions of maximal illumina- 

 tion or darkness. To obviate this difficulty the so-called 

 " half -shadow " instruments are employed (polarimeters of 

 Lippich and Laurent). In these instruments, the mono- 

 chromatic light from some source, such as a Bunsen burner 

 containing fused sodium chloride (for sodium light), or a 

 mercury lamp (for the mercury light), is focused on to 

 the polarizer (B, Fig. 19) through lens A. These are con- 



J)F d IDz ESJo. "0 > ISb xO. 



Fig. 19. 



tained in one tube of the polarimeter. The analyzer is 

 contained in another tube in which is a telescope EF. 

 The liquid under examination is placed in a tube O. The 

 end of the tube containing the polarizer is covered with 

 a half-disc of quartz (Laurent's polarimeter) or a small 

 Nicol prism (Lippich's polarimeter) which is of such thick- 

 ness that the light passing through the plate is altered in 

 phase a half-length, but still remains plane-polarized. Two 



