| Tourmaline crystals are doubly refracting;
an incoming ray is split into an ordinary and an extra-ordinary ray. which
are polarized at right angles to each other. However, the E-ray is rapidly
absorbed, so if the crystal is a few millimeters thick, the transmitted
light is linearly polarized.
The tongs have a tourmaline crystal in each half, mounted with the transmission axes at right angles to each other. Specimens, such as thin plates of mica or plates stressed by a glass press are slipped between the two halves of the tongs, and observed by holding the system up to the eye; tourmaline does not transmit light very well. The device was invented by Jean-Baptiste Biot (1774-1862).
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