The spectrometer consists of a graduated circle, generally fixed in a horizontal position, to which is attached a collimator and a telescope. The collimator consists of a tube containing an achromatic lens at one end and a vertical slit at the principal focus of the lens. The movable arm carries an astronomical telescope which is always directed towards the center of the graduated circle. The position of the telescope with reference to the fixed circle may be read by means of a vernier. Above the center of the graduated circle is a horizontal table, called the table of the spectrometer, which is capable of rotation about the vertical axis of the circle. The position of the circle table can be determined by means of the vernier.