The spectrometer, also known as a reflection goniometer, is an instrument used in measuring the refractivity index with respect to the air of a substance of prismatic form.
The Fuess spectrometer presented here is made entirely of brass except for the heavy tripod, which is cast iron painted black, and is composed essentially of:

• a collimator attached to the tripod;
• a round disk on the silver-plated edge of which is a scale divided into degrees and quarters of a degree, which can rotate around a vertical axis passing through its centre to which it can be attached;
• a prism-bearing platform which can be rotated around the axis of the instrument independently of, or together with, the graduated round disk;
• a telescope which can revolve around the axis, to which it can be blocked;
• a double alidade joined to the telescope support, consisting of two indices with verniers and eyepieces for readings, the edges of which slide on the edge of the graduated disk on opposite sides.