ELEMENTARY CHEMICAL PHYSIOLOGY 



213 



plate, however, the various colours of the spectrum are rotated to a different 

 degree, so that, instead of having a mere opacity (as is the case with inter- 

 mediate positions of two "tourmaline " plates), different colours are obtained 

 according to the amount of rotation. There are also samples of quartz which 

 rotate the plane of light to the left. 



Dextrose and a quartz plate produce the same amount of rotation, and 

 therefore it is possible to determine the rotatory power of a solution of the 

 former by compensating its rotation by means of a quartz plate of known 

 rotatory power. 



We are now in a position to understand the construction of a polarimeter 

 or saccharimeter. It consists of the following parts : 



(1) A Nicol's prism, called the polariser. This polarises light in a vertical 

 plane. 



(2) A biquartz, or other device for rotating, in opposite directions, the two 

 halves of a polarised beam. A biquartz consists of a disc of quartz made of 

 two semicircular halves of equal thickness, but of opposite rotatory powers. 

 Each half is of such a thickness that it rotates the plane polarised light to 90 

 in opposite directions so that, on 



emerging from the disc, the plane of 

 light is now horizontal. Instead of a 

 biquartz many instruments contain a 

 semicircular plate of quartz. 



(3) A tubular liquid holder to hold 

 10 c.c. of the liquid to be examined. 

 If the length of this tube be 188-6 

 mm. the amount of rotation in angular 

 degrees will correspond to percentage 

 of dextrose in the fluid (e.g. urine) 

 examined. 



FIG. 175. Diagram of scale and 

 field of vision of polarimeter. 



Above is represented the scale for measuring 

 the compensation necessary. In the posi- 

 tion represented in the diagram the read- 

 ing is 2'7 dextro rotation. The lower 

 part of the diagram shows the three 

 appearances of the field of the polari- 

 meter, the central one representing the 

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

 rotation. 



(4) A Compensator. This shows how 

 much rotation has been produced by 

 the solution. It is connected with a 

 scale representing angular degrees, 

 and the pointer carries a vernier, so 

 that tenths of a degree can be read off. 

 In some instances the compensator 

 consists of two wedge-shaped pieces of 

 quartz, so arranged on one another 

 that the total thickness of quartz in- 

 terposed 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 rotat- 

 ing 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). 



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. 



