In this paper instruments are described for the measurement of luminous compound of brightness of the order of 0.02 candle per sq. ft. One instrument is used for surfaces larger than about 6 mm. diameter, and the other, embodying the Maxwellian view, is designed for measuring smaller surfaces than this. In both instruments opal glass, illuminated through a green colour screen, by a standard lamp running along a photometer bench is used as the comparison surface. One instrument has a Lummer-Brodhun "brightness" type cube, and the compound is viewed through the central aperture. It is thus compared with the opal surface, which is seen by reflection in the interface of the prisms. In the other the compound is placed behind a small aperture. A lens forms an image of this aperture at the eye of the observer, who thus sees the lens as a surface of uniform brightness. This is viewed through a rectangular hole in a piece of silvered mirror set at 45° with the axis, and is compared with the opal surface which is seen by reflection in the surrounding portions of the mirror. The instruments are calibrated by an absolute method over a range of brightness from 0.002 to 0.03 candle per sq. ft., using rotating sectors to vary the brightness. Over this range the reproducibility obtainable is about 2 per cent. for one instrument and within 10 per cent. for that employing the Maxwellian view.