A prism/grating double monochromator has been made in which the grating monochromator has been ganged with a rocksalt monochromator on a linear frequency basis. Two replica gratings, each used in a single order, cover the range 650 cm<sup>-1</sup> to 3650 cm<sup>-1</sup>. Resolution is such that peaks with a separation of 0.3 cm<sup>-1</sup> at 950 cm<sup>-1</sup> may be resolved. Gratings and cams and also prisms may be interchanged by operation of press buttons. Similarly the instrument may be converted to a single prism monochromator to extend the frequency range. A constant energy background is provided by magnetically operated slits programed by a tapped potentiometer. The double beam photometer uses a starwheel beam attenuator and is capable of good quantitative accuracy. The whole optical system is evacuable and a pressure of 10<sup>-3</sup> mm Hg within the vacuum casing is satisfactory for accurate work at grating resolution.
C. S. C. TARBET and E. F. DALY, "Infrared Vacuum Spectrometer with Prism/Grating Double Monochromator," J. Opt. Soc. Am. 49, 603-606 (1959)