PHYSIOLOGICAL CHEMISTRY 283 



posed in the path of the polarised beam can be varied by means of a screw. 

 In other instruments the quartz plates are dispensed with, the amount of 

 rotation being measured by rotating the next part of the instrument, namely 

 the 



(5) Analyser, so as to obtain uniformity of tint in the two halves of field. 



(6) A Lens. 



When the tube (3) is filled with water or an optically inactive fluid, and the 

 compensator or analyser rotated until a violet colour of uniform tint fills the 

 field, the indicator will be seen to stand at zero (if not so, the error must be 

 noted). If now, an optically active fluid be placed in the tube the two halves of 

 the field will become of different tints, i.e. rotation of the plane of polarised 

 light has occurred. In order to measure the amount of this rotation, we must 

 move the screw or pointer connected with the compensator or analyser until the 

 uniform tint is again obtained. 1 The amount of 'compensation' necessary is 

 read off on the scale and, if the holder be not 188-6 mm. long, the necessary 

 calculation is made in order to ascertain the strength of the solution (for 

 formula see below). 



Fio. 223. Diagram of scale and field of vision of polarimeter. Above is represented 

 the scale for measuring the compensation necessary. In the position represented in 

 the diagram the reading is 27 dextro rotation. The lower part of the diagram shows 

 the three appearances of the field of the polarimeter, the central one representing the 

 appearance at zero, i.e. when there is no rotation. 



To estimate the percentage of sugar in urine the chief precautions are, (1) to 

 see that it is perfectly dear, and (2) to see that it contains no protein. 



In order to obtain a specific or comparative number (i.e. a result always 

 obtained under the same conditions) it is necessary to adopt a standard. This 

 consists of the rotation, in degrees of a circle, produced by 1 gr. of the 

 substance dissolved in 1 c.c. of fluid and contained in a tube 1 dcm. long. 

 This is called the specific rotatory power and is represented by (a) D. 2 It is 

 determined by the following formula : 



where a = the observed rotation, 



Z = the length, in decimeters, of the tube in which the solution is placed, 

 p = the weight, in grammes, of the substance contained in 1 c.c. solvent. 

 The rotation produced by a substance depends upon its concentration in a 

 solution ; if, therefore, the index (a) D of any substance be known, and its 



1 In the best modern polarimeters the field is divided into three ; when at zero 

 these are of the same tint otherwise the central band takes a different colour. 



2 The ' D ' indicates that sodium light is used. 



