ADVANCED PHYSIOLOGICAL CHEMISTRY 421 



direction ; but, of course, plane polarised. To detect the polarisation a similarly 

 constructed prism, or analyser, must be used. 



Certain other bodies, e.g. a quartz plate, a solution of siTgar or albumin, have the 

 power of rotating the plane of polarised light. Thus, supposing that the plane 

 polarised light vibrates along a vertical plane, one of the bodies may twist it inta 

 an oblique plane. If the analyser be so placed that none of the plane polarised 

 light can pass through it (i.e. the field is black), and if a piece of quartz be inserted 

 between the polariser and analyser, it will be found that now a certain amount of 

 light passes through the analyser (i.e. the field becomes opaque), and, in order to 

 obtain darkness again, it is necessary to rotate the analyser in the direction of the 

 hands of a watch, as seen by the observer. Consequently, rotation has taken place 

 to the right, i.e. dextro rotation is said to have occurred. If a solution of albumin or 

 laevulose be employed the rotation of the analyser must be to the left, i.e. against 

 the hands of the watch. When the plane of white light passes through the quartz 

 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 intermediate 

 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 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 

 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 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 interposed in the path of the polarised beam 

 can be varied by means of a screw. In other instruments the quartz 



