282 



PEACTICAL PHYSIOLOGY 



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

 fore 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 polaiimeter 

 or sacchaiimeter. 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. 



Fio. 222. Polarimeter of Mischerlich with Laurent s polariser. P, polariser and device 

 for obtaining half shadow ; jK, fluid container ; T, scale with vernier c attached to 

 pointer ; A, compensator and analyser ; F, lens. 



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 semi- 

 circular 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. 



(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 inter- 



