Chap. XXX.] 



TOURMALIN PLATES. 



43 



With the plates of the crystal TOURMALIN, cut 

 parallel to the optic axis, polarisation may be shown. 

 When the plates are laid on one another, so that the 

 axes are parallel, the light is transmitted. When one 

 is rotated the light becomes more and more dim, till 

 when they are crossed it is extinguished. If the 

 rotation be continued till they are again parallel, the 



Fig. 182. Tourmalin Pjates. 



light is again transmitted (Fig. 182). The tourmalin, 

 plates, if sufficiently thick, completely extinguish the 

 ordinary ray. 



Polarisation by reflection of light was dis- 

 covered by Malus in 1810. An apparatus for pro- 

 ducing it is shown in Fig. 183. When a ray of light 

 falls on an unsilvered polished surface of glass, placed 

 at a particular angle to the incident ray, the reflected 

 ray is polarised. This may be shown by permitting 

 the ray to fall on a prism of Iceland spar, when the 

 phenomena already described will be produced. It is 

 also shown by receiving the reflected ray on a second 

 reflecting surface placed at the same angle as the 

 former. If the surfaces are parallel the light from 

 the second surface will be perceived by an eye 

 placed in the direction of the reflected ray. If the 

 second surface be now turned the intensity of the 

 light diminishes, till when the two surfaces are at 

 right angles it is extinguished, but is again reflected 

 on turning till the surfaces are again parallel. The 

 Fig. 183 shows the two reflecting surfaces A and B, 



