During the last ten years we have been engaged in making photo-electric measurements of illumination in the open, in woods, and under water. For the most part relative measurements sufficed ; thus, under water the percentage of the blue light in air was given, and later for each colour. In woods, the lighthabitat is best expressed by means of the daylight factor, namely, the illumination, received on a horizontal surface expressed as a percentage of the diffuse illumination, similarly received, in the open. A fuller statement involves giving the daylight factor for each colour separately, or else the “ colour factor.” Thus, if we decide to regard as “ white,” i.e ., as standard, any given type of illumination, by the use of appropriate photo-electric cells and colour filters we can find ratios green/blue, yellow/blue, and red/blue for this illumination, and also for any other illumination whose colour we wish to specify. The ratio of the ratio green/blue for the given illumination to the ratio green/blue for the standard may be called the “ colour factor ” for the green filter, and so on.