Once the telescope (or graduated disk) has been blocked on the axis of the instrument, the sighting is adjusted by means of a micrometric screw prior to reading on the two verniers to improve accuracy of the measurements; the combination of the readings made on the two verniers, since these are diametrically opposite, any alidade eccentricity error is eliminated.
The collimator is composed of a vertical slit, with an adjusting screw, placed on the front focal plane of a converging collimating lens.
By illuminating the slit with a light source placed along the axis of the collimator, the collimating lens generates a parallel beam of light which the objective of the telescope makes converge on its own back focal plane, thus producing an image of the slit which the eyepiece, which lies on that plane, allows us to view.
The platform consists of two superimposed disks and can be fixed to the axis of the instrument; one of the two disks lies on the other by means of two projecting screws and a pin with a spring. They are equidistant and can thus be arranged perpendicular to the instrument axis.
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
The prism method can also be used to measure the refraction index of a liquid by using special prismatic recipients having two faces made of plates of flat, parallel glass.
Since the verniers have fifteen lines and allow appreciation to the minute, in the case of a glass prism having a refractivity index of about 1.5, homogenous and cut with precision, with this spectrometer it is possible to measure the refraction index to three exact decimal places.