104 ENZYMES 



values for the ordinary ray (1-GG) and the extraordinary ray 

 (1-48). Therefore the ordinary ray falling on the surface AC at 

 an angle greater than the critical angle will be totally reflected, 

 while the extraordinary ray will pass through the prism. This 

 ray, as we have stated above, is plane polarised. To the unaided 

 eye it differs in no way from ordinary light, but, when viewed 

 through a second Nicol's prism, its condition is recognised by the 

 fact that on rotating the prism the beam of light from the first 

 prism alters in colour, passing through the various colours 

 of the spectrum and returning again to white when the 

 rotation has been carried through 180. If monochromatic 

 light has been used the field will be illuminated when the 

 principal planes of the two prisms are parallel. On rotating the 

 second prism through an angle of 90 the ray is extinguished and 



FiQ. 16. Diagram of Laurent Polarimeter. Monochromatic light from the source 

 passes through the lens^l which renders the rays of li.irht parallel, and then through 

 the polariser B. is the observation tube containing the fluid under examination, 

 while I) is the analysing Nicol prism. The field of view is observed through the 

 telescope EF. At C, the circular opening of the tube carrying the polarising prism is hal f 

 covered by a thin quart/ plate (shown at C'), the, thickness of which is such that the 

 light in passing through the plate is altered in phase by half a wave-length. 



the prisms are said to be crossed. If the rotation be carried on 

 to 180 the planes are again parallel and again the field is bright, 

 and so on. In two positions the planes are parallel and in two 

 at right angles. The first prism is called the polariser, the second, 

 by which alone we can recognise the polarisation of the light from 

 1, is called the analyser (Fig. 1C). If now a plate of quartz cut 

 with the faces perpendicular to the optic axis be placed between 

 crossed Nicols, it will be found that some light passes through the 

 analyser. That is, the quartz has rotated the plane of the light 

 polarised by the first prism. By rotating the analyser a position 

 can be found when all light is stopped. The amount of rotation 

 of the analysing Nicol is a measure of the rotation of the plane 

 of polarised light by quartz. A body which has this property of 

 rotating the plane of polarised light is said to be optically active. 



Some samples of quartz rotate the plane of polarisation in a 

 clockwise or right-handed (or -f) direction, other samples have a 

 reverse (or ) direction of rotation. (The direction is taken as 

 from the direction in which the light is travelling, not from the 

 analysing eye.) A dextrorotatory piece of quartz superimposed 



