This spectroscope was used to observe the spectrum of light from an astronomical or laboratory source. The dispersion of incident light was achieved with a series of five prisms. The mount consists of three brass tubes and a shaped box containing the prisms. The first tube was connected directly to the telescope. The second, mounted parallel to the first, but not on the same axis, was used to observe the spectrum of the incident radiation by means of a suitable eyepiece. The third tube carried a lamp (now missing)—probably an alcohol lamp—that illuminated a scale used to measure the positions of the absorption bands. The image of the scale was reflected toward the observer by the face of the last prism closest to the eyepiece. The tube with the lamp and the tube carrying the eyepiece can rotate c. 20° on an axis that is perpendicular to the instrument's optical plane and passes through the center of the last prism.