A simple monochromator with a 4×6 inch replica grating f: 3.4, which covers the range 365–1000 mμ is described. The dispersion is 3.55 mμ/mm in the second order and 7.1 mμ/mm in the first order. The slits are fixed in position and the wave-length of the emergent beam is varied by rotating a mirror located between the telescope lens and the exit slit. It is useful for the production of monochromatic light with band widths of 10 mμ or more. With a 2×34-mm entrance slit it produces 4×10−2 cal./min. of red light with a band width of about 28 mμ from a 350-w tungsten lamp. The total energy output, spectral distribution and percent of stray light are given for various wave-lengths and slit settings. The transmission of the instrument is given for the wave-length range 365–800 mμ.