In green tourmaline, which is a monoaxial bi-refracting crystal which crystallizes in the trigonal system, dichroism is so accentuated that in a slice cut parallel to its optical axis a thickness of about one millimetre is sufficient to absorb the ordinary ray completely.
Such a slice represents a dichroic polarizer which is far simpler and more economical than the Nicol prism which transmits only the extraordinary ray which vibrates parallel to the optical axis.
Tourmaline tongs represent a polariscope composed of two slices of green tourmaline having a thickness of about one millimetre and a section of one or two square centimetres cut parallel to the optical axis and are held in the two brass disks open at the centre by a cork frame. They can rotate inside two disks at the end of a wire bent in the middle to form a round spring perpendicular to the rings holding the tourmaline disks.
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