Worlds of David Darling
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reflecting goniometer
A scientist measuring the angles of a crystal using an older type reflecting goniometer. The angle through which the crystal has to be turned in order that successive faces can reflect a beam of light into the telescope is determined by this instrument.

A goniometer is an instrument for measuring angles, especially those between crystal faces. The simplest form is the contact goniometer, a protractor whose base is laid against one face, a movable arm being turned until it contacts the adjacent face.

conatct goniometer
For rough purposes, a contact goniometer can be used to measure the angles between the faces of a crystal.

reflection goniometer
Modern reflecting goniometer
The more accurate reflecting goniometer (or reflection goniometer) mounts the crystal axially on a graduated circle, or more usually two graduated circles, horizontal and vertical, rotatable independently. The crystal is rotated until each face in turn reflects a collimated light beam into a fixed telescope, and so the direction of the normal to each face is determined. (The term may also be used for the direction finder.)

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