Several types of large ultraviolet monochromatic illuminator are described, in which water or other transparent liquid is used as the dispersing medium. The most readily made consists merely of a spherical concave mirror immersed in a basin of distilled water at an angle of about 30° from the horizontal; this acts as a crude prism somewhat on the principle of the Féry quartz prism, and gives monochromatic though distorted images of a high power horizontal mercury arc used as a source. By placing an exit slit of suitable shape in the focal plane these images can be isolated one at a time, giving surprisingly pure monochromatic radiation. Such an instrument of large size can be constructed at small expense and should be useful for many types of biophysical and photochemical work. A somewhat more expensive but more distortion‐free instrument can be made by using a plane mirror under water with one or two large lenses of fused quartz. The horizontal position of the input face of the water prism greatly reduces convection currents, and renders large sheets of expensive optical material unnecessary. Instruments of both types with aluminum, chromium and platinum‐coated glass mirrors over eight inches in diameter have been constructed and found to operate satisfactorily. Typical spectrum photographs taken with one of the concave mirror instruments show the degree of spectrum resolution obtained.