Grating spectrometer, 1882-1905.
© Science Museum / Science & Society Picture Library
The spectrometer measures the angle light is refracted (bent) after pasing through a prism or a grating. Unlike a simple spectroscope, where the wavelength of light is determined by comparison with a known light source, the spectrometer only measures the angle of deflection. Like its counterpart, it is used to examine the chemical nature of matials that has been vaporised in a hot flame. In operation, light enters through the slit on the right-hand-side tube, is deflected by the transmision grating and is measured using the telescope on the left. This sighting telescope pivots around circular scale below that gives the angular displacement of the light.