Prior to use, the instrument must be calibrated by focusing the telescope to infinity by placing the vertical slit on the front focal plane of the collimating lens, aligning the telescope with the collimator and placing its axis perpendicular to the axis of rotation of the instrument and the edge of the prism parallel to the same axis.
Once the spectrometer has been prepared, it can be used to measure the refractivity angle of a prism and, by using a monochromatic light source, also to measure the angle of minimum deviation.
Knowing these two angles, it is possible to determine the refractivity index of the substance of which the prism is made by the wavelength of the monochromatic light used in measuring the angle of minimum deviation; in fact, according to the principle of the reversibility of the path of light rays, in conditions of minimum deviation the angle of incidence and the angle of emergence are equal and, if we take into account this condition, by the laws of prisms it is found that the refractivity index, for the monochromatic light used in measuring the angle of minimum deviation, is given by the ratio between the sine of the half-sum of the refractivity angle and the angle of minimum deviation and the sine of half the refractivity angle. ...continues