ABSTRACT
The brightness of a chromatic surface colour is generally defined in terms of its correspondence to a particular member of the black-white series. Using a scale of greys, we ascertained, in normal illumination, the brightness-values of a large series of chromatic colour-squares differing widely in saturation. Our method was that described by F. Hillebrand. 1 I replaced the colour-wheel carrying disc B2 in the apparatus described above 2 by a holder of white cardboard, with its upper edge somewhat inclined away from the observer but standing essentially in a vertical position ; in each individual case one of the chromatic’ squares and the equally bright achromatic square were exposed upon it. The two squares were now observed through an episcotister and their brightnesses were compared. When we reduced the opening of the episcotister a few degrees to a point at which the squares were just perceptible, we found that the brightness equation made in normal illumination remained practically unchanged. We are led to this generalization : When the intensity of illumination is reduced by means of an episcotister, chromatic surface colours of any saturation and brightness undergo essentially the same losses in brightness as do the members of the black-white series to which they are equal in brightness in normal illumination. If now we increase rather than reduce the intensity of illumination of chromatic surface colours, e.g., by illuminating them with sunlight, we are led to this conclusion: With an increase in the intensity of illumination, chromatic surface colours of any saturation and brightness undergo essentially the same increase in brightness as do the members of the black-white series to which they are equal in brightness in normal illumination.
