74 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



L represents a Bunsen flame in which is inserted a bead 

 of carbonate of soda to obtain monochromatic light. A is 

 a lens to render the rays of light parallel, B is the polariser, 

 C and C' the quartz plate. is the tube containing the 

 solution to be observed, D the analyser, and EF the 

 telescope. 



The Lippich model of polarimeter differs only from 

 the Laurent in having a small Nicol prism to produce the 

 half shadow instead of the quartz plate. The outward 

 appearance of the two instruments is identical, and is shown 

 in Fig. 13. In both cases a light filter consisting either of 

 a solution of potassium bichromate or a crystal of this salt 

 is placed in front of the lens A. 



n> 



FIG. 15. DIAGRAM OF POLABIMETEB. 



After passing through the polariser and the quartz plate, 

 the light emerges as two beams of polarised light, differing 

 in phase by half a wave length. If the polariser is rotated 

 so that the plane of polarisation forms an angle (0) with the 

 quartz plate, the planes of polarisation of the two beams 

 will also be inclined at an angle, equal to 20. This is the 

 half-shadow angle. On rotating the analyser, a position 

 will be found at which the one beam will be completely, 

 the other only partially, extinguished. The one half of the 

 field of view, therefore, will appear dark, while the other 

 half will still remain light. 



The details of practical use of the polarimeter will be better 

 considered in a later chapter ; it should here be stated that 

 the angle of rotation of the analyser can be accurately measured 



