476 CHAPTER XXIV 



attaching a pointer and circular scale to A this angle can be measured, 

 giving means to determine an unknown concentration of active material 

 when the rotation for one known concentration has been obtained. This 

 combination of two nicols serves to fix the position of the plane of the polarized 

 light emergent from P and also to determine the rotation produced by an 

 active material. It thus forms an elementary type of polarimeter, in which 

 the critical position is that of total darkness. 



White and Monochromatic Light.— In the discussion immediately above 

 light qua light was mentioned. The statement therein made refers only 

 to a beam of monochromatic or homogeneous light. White light is not homo- 

 geneous, but is composed of the spectral colours red, orange, yellow, green, 

 blue, indigo, violet. These components are rotated differently, and hence 

 when an active material is interposed between two nicols, no position of 

 total darkness obtains, since a position crossed with reference to one spectral 

 colour will permit light from all the others to pass. Consequently, on the 

 rotation of one of two nicols, between which is interposed an active material, 

 through which passes a ray of white polarized light, the eye receives in suc- 

 cession the sensation of all the colours of the spectrum. In such a case, 

 however, the position of total darkness can be restored by the interposition of 

 an active material of opposite sign and of the same rotation dispersion as 

 that of the material undergoing examination. 



Many textbooks make very confused statements on this matter, and 

 frequently imply that it is the means adopted for obtaining the critical 

 position which determines the kind of light to be used. Actually the de- 

 termining factor is the means adopted for compensation {v. infra.), mono- 

 chromatic light being capable of compensation by a number of means, 

 whereas white light cannot be compensated by analyser or polarizer 

 rotation, but requires special devices. This error is of special occurrence in 

 descriptions of the Laurent apparatus, which has been regularly built for 

 use with white light since 1882.1* Similarly, Wild in 1883^5 adapted his 

 instrument for the use of white light. 



Critical, Positions. — The various critical positions that are or have been 

 in use are described below. Mathematical analysis of the devices is not 

 introduced, for which reference to specialized works or to original papers 

 must be made. 



Extinction of Extraordinary Ray. — In one particular position of a natural 

 prism of Iceland spar with reference to an incident beam of light the double 

 image does not appear. This position was used by Biot^ in the first polari- 

 meter on record as a critical position. 



Elimination of other than Red Rays. — If between two nicols an active 

 material such as cane sugar be interposed, the position of total darkness 

 cannot be obtained when white light is employed as the iUuminant. On 

 rotation of one nicol the spectral colours in turn appear. Ventzke,^^ in a 

 fundamental paper on sugar analysis, took as a critical position the appear- 

 ance of the red field as matched against a standard cell of iron anilate. 



Total Darkness. — This position has already been described. It is applic- 

 able (with rotation compensation) only to homogeneous light, and was first 

 used by Mitscherlich.^' 



