Two beams of light from opposite sides of an electric lamp, after being diffused by opal screens, pass through suitable optical systems and a specially designed shutter which enables them to fall alternately in quick succession on a selenium cell connected in series with a galvanometer and a source of E.M.F. The density to be measured is placed in the path of one beam and a standard optical wedge, which can be moved along a scale graduated in density units, in the path of the other. The wedge is adjusted until the deflection of the galvanometer remains constant. The intensities of the beams falling on the cell are then equal, and the value of the density may be read on the wedge scale. The instrument is essentially a density comparator and is calibrated by means of standard densities.