Monochromatic irradiation studies of biological objects usually require a large slit area, high intensity, comparable irradiation conditions at all wave‐lengths, and frequently some provision for treating on a horizontal surface. The present instrument provides for the simultaneous irradiation of seventeen different samples, each covering 100 square millimeters of a horizontal surface with the exception of two which cover 75 mm2 each, each sample being exposed to a single wave‐length of the Hg are spectrum. The seventeen wave‐lengths are as follows: 3655, 3341, 3131, 3022, 2967, 2925, 2894, 2804, 2752, 2699, 2652, 2576, 2536, 2483, 2399, 2378 and 2352A. The straight spectral lines are 2×50 mm. The optical system consists of two 60° crystal quartz prisms, Cornu type, 51 mm high and 85 mm length of face; collimator and camera lenses have 70 mm aperture and 418 mm focal length. Exchangeable with the special holder for the biological materials are two other attachments: a photographic film holder and a calibrated traversing slit mechanism designed to carry a thermopile or a photo‐cell. The instrument can be used in either a vertical or a horizontal position.