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Ch. 2: Minerals: Physical Properties

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A POPULAR TREATISE ON GEMS.
When, on the other hand, the planes of reflection are parallel, the ray is wholly and at any intermediate angle partially reflected. A ray of light polarized by reflection is also incapable of transmission through a tourmaline slip in one position, which, however, is'at right angles to that in which a ray polarized by passing through another slip is not transmitted.
In order to observe the polarization of light, a very sim­ple instrument will be found useful (fig. •96). At one end of a horizontal board Β a black mirror a is fixed. In the middle is a pillar to which a tube c d is fastened, with its axis directed to the mirror at an-angle of 35-1/2°. On the lower end is a cover c, with a small hole in the centre, and at the upper end another cover with a small black mirror m attached to it by two arms, as in the figure, and also at an angle of 35-1/2°. With this instrument the mirror rn can be so placed in relation to a that the planes of reflection shall have any desirable inclination to exhibit the simple polarization of light.
. This instrument furnishes a simple test whether minerals that cleave readily into, thin lamella} are optically mono-axial or binaxial. Place the two mirrors with their, polari­zation-planes at right angles,· and fix a plate of the mineral with a little wax over the hole c, and then observe what takes place in the second mirror during the time that the cover c is turned round. If the mineral belongs to the bi
Ch. 2: Minerals: Physical Properties Page of 515 Ch. 2: Minerals: Physical Properties
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