The Spectrometer

This instrument (fig. 38) consists of a graduated circle, generally fixed in a horizontal position. A collimator is rigidly attached to the circle. The axis of the collimator is in a plane parallel to that of the circle, and is directed to a point vertically above the centre of the circle. A movable arm, fitted with a clamp and tangent screw, carries an astronomical telescope which is generally provided with a Ramsden's eye-piece and cross-wires. The position of the telescope with reference to the circle is read by means of a vernier.⁽¹⁾ Above the centre of the circle there is a horizontal table, which is generally capable of rotation about the vertical axis of the circle; and this table has also a vernier attached to it, so that its position can be determined. The whole instrument rests on levelling screws, and the telescope and collimator are held in their positions by movable screws, so that their axes can be adjusted till they are parallel to the circle.