Stereoisomerism. 



i6 5 



beams of polarized light thus pass through the polari- 

 meter. On rotating the analyzer, a position will be found 

 in which one beam will be completely, and the other only 

 partially extinguished. On rotating the analyzer further, 

 a second position will be found in which the second beam 

 will be extinguished, and the part of the field which was 

 originally dark will be light, whereas the first beam will 

 be extinguished and the other half of the field will be 



o 



dark. In the intermediate position, both halves of the 

 field will be equally illuminated. This is the position to 



Fig. 20. 



which the analyzer must be set before a reading is taken, 

 and it is noted on a scale provided with a vernier with 

 which the instrument is furnished, the actual point being 

 read with a magnifying glass (see I in Fig. 20). The 

 eye-piece of the instrument is provided with a cross-wire 

 which divides the field of vision into two halves. When 

 the zero position has been attained, the substance under 

 investigation is introduced between the tubes containing 

 the analyzer and polarizer (this is covered by a piece of 



