72 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



construction and methods of use are necessary in such an 

 instrument if accurate results are to be obtained. It is only, 

 it must be remembered, with rays of a given wave length 

 that absolute darkness will, as a rule, be obtained when the 

 Nicols are crossed, because the angle of refraction is different 

 for rays of differing wave length, so that a prism that was cut 

 in such a way that the ordinary violet ray was just totally 

 reflected might not completely cut off the ordinary ray of red 

 light, and so a small proportion would come through even 

 when the Nicols were crossed. It is better, therefore, always to 

 use light of a definite wave length, and for this purpose the 

 yellow light obtained when a compound of sodium, such as 

 a bit of melted carbonate of soda, is 

 y held in the flame of a Bunsen burner, is 

 employed ; even then the point of com- 

 plete darkness is not altogether easy 

 to distinguish. It must be remem- 

 bered that unless the Nicols are exactly 

 at right angles a certain component of 

 the vibration will pass through, in- 

 creasing in amount in proportion as the 

 Nicols become more nearly parallel, 

 B/ This may be rendered clearer by the; 



F IG . 14. following diagram; keeping to the an- 



alogy of the barred screen, if we assume- 

 the barred screen (Fig. 14) placed at an angle to the 

 polarised ray vibrating along AB this vibration will be- 

 resolved into two, one, ba, parallel to the bars which will pass- 

 through and the other, 6c, at right angles which will be* 

 extinguished. Obviously the component passing through will 

 depend on the angle of the barred screen to the polarised ray y 

 and in the diagram db represents the portion of light passing 

 through. 



In order sharply to define the point of darkness in the 

 polarimeter, half the field of view of the instrument is taken 





