Goniometer

Goniometer (from Greek gonia - angle and metron - measure), an instrument that allows measuring angles between individual faces of a crystal. The principle of the G. can be easily understood from the following. Let there be an angle in a crystal a (see Figure 1) and let this crystal be mounted on a stage such that its faces AB and AC are parallel to the axis O, around which the tube P, set at infinity, can rotate. If the tube P is positioned so that its axis is perpendicular first to one face (AC) and then to the other face (AB), then the angle β, by which the tube will rotate, will be equal to 180 minus the angle a of the crystal (β=180-α). Another way to determine this angle is as follows: let there be a light source (see Figure 2) sending parallel rays R, and let, after reflection of the rays on the crystal face, by means of the tube P set at infinity, this source be visible. If the crystal K is rotated so that plane A is replaced by plane B, then from the angle by which the crystal must be rotated (angle β), the angle of the crystal faces α can be easily determined; it is equal to 180 minus the rotation angle β (α=180-β).

Figure 1

Goniometers, remarkable for their precision, were proposed by E.S. Fedorov, who introduced into crystallography the precision necessary for this kind of measurement. The G. also has great significance in the field of physiology, allowing determination of the angles of crystal formations observed in living bodies.