sponding colours. These substances are known as dichroic. Biaxial crystals have three principal indices of refraction, and they possess three corresponding colours, and the principle is known as trichroism. Pleochroism includes both dichroism and trichroism. In order to separate the colours an instrument called ' dichroscope' is used. It
	consists of a rhombohedron of Iceland spar mounted in a cylindrical tube. At one end is a small square opening and at the other end an eyepiece.
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