SPECTROMETER (from Lat. spectrum, ap pearance, image, apparition + Gk. Alrpov, metron, measure). An instrument especially de signed to determine the index of refraction of transparent bodies. The ordinary type of instru ment is illustrated in the cut, and, as shown, con sists essentially of two arms which are movable around an axis carrying a platform on which rests a prism of the substance to be investigated. One of the arms carries a telescope, A, while the other supports what is known as the 'colli mator,' B. This consists of a tube at one end of which is a slit made by two movable parallel edges, and at its other end is a lens so placed that the slit is in its principal focus. If a source of light, for instance a sodium flame, is placed in front of the slit, a beam of parallel light will proceed from the collimator lens, fall upon the prism, suffer deviation, enter the telescope and be focused by the objective on the cross-hairs of the eyepiece. The angles made between the tele
scope and collimator may be read on a scale at tached to the platform of the instrument.
To determine the index of refraction of a sub stance in the form of a prism, for radiations of a given wave length, it is necessary to know: First, the angle of the prism. that is, the angle made between the two plane faces of the prism; second, the angle of minimum deviation for the given radiations. Both these quantities can he meas ured with the spectrometer, care being taken to have the axes of the telescope and collimator per pendicular to the axis of the instrument. and the edge of the prism parallel to it. The following formula then gives the index of refraction, in which A represents the angle of the prism and D the angle of minimum deviation: A+ D sin Index of refraction a = sin 7,