70 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



to each other, it has the property of diverting the ordinary 

 ray and only allowing the extraordinary ray to pass, whose 

 vibrations are parallel to the long axis of the crystal. Such 

 a ray is polarised, and a ray which is polarised at right angles 

 to this direction will not pass through the crystal. Con- 

 sequently when two plates of tourmaline identical in structure 

 are held at right angles no light passes, and it follows also 

 that a plate of tourmaline is capable, as already indicated, of 

 enabling us to recognise whether a ray of light is polarised or 

 not. By means of such a plate of tourmaline it can be shown 

 that the rays issuing from the rhomb of 

 calcspar are polarised at right angles one 

 to the other. The use of tourmaline for 

 studying polarised light is unsatisfactory, 

 owing to the green colour of the crystal 

 and the consequent loss of intensity in the 

 light passing. Obviously a better source 

 of polarised light would be one of the 

 emergent rays from calcspar. If means 

 could be found to cut off one of these rays, 

 the other would remain as a ray of undi- 

 minished intensity whose direction of vibra- 

 tion was known. Such a means is found 

 in the Nicol prism. This prism is made by 

 taking an elongated rhomb of calcspar and 

 dividing it so that the plane of division aaf 

 forms an angle of 68 with the vertical sides 

 of the rhomb as in Fig. 12, and the two portions are then 

 reunited by a film of Canada balsam. If now a ray of light 

 cd impinges upon the shorter face of the prism, double refrac- 

 tion will take place, but the ordinary ray suffers total reflec- 

 tion at the surface of the Canada balsam and so passes out of 

 the crystal in the direction hi. The extraordinary ray def, on 

 the other hand, suffers no refraction on the surface of the 

 Canada balsam, and so passes on with its direction unaltered, 



9 



FIG. 12. 



NICOL PRISM. 



