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critical angle it 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 dilTers 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 colom-, 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 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 



h\ Be [^d. D . "H 



1 



Fig. 31. — Diagram of Laurent Polarimeter. Monochromatic light from tlie source L 

 passes through the lens A which renders the rays of light parallel, and then through the 

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

 the analysing Xicol prism. The field of view is observed through the telescope EF. At C 

 the circular opening of the tube carrying the polarising prism is half covered by a thin quartz 

 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. 



light from the first, is called the analyser (Fig. 31, D). 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 +) 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 

 on a similar laevorotatory piece would be optically inactive. 

 Physical examination of quartz crystals shows that <^/-crystals 

 differ from /-crystals in one respect only, viz. : the position of 

 their secondary facets. The ordinary form of a quartz crystal 

 is a six-sided prism topped by a six-sided pyramid. The alternate 



