On the variation in gradation of a developed photographic image when impressed by monochromatic light of different wave-lengths

When a series of small spaces on a photographic plate are exposed to a constant light for geometrically increasing times, or for a constant time to geometrically increasing intensity of illumination, the spaces so exposed will on development show deposits of silver of different opacities. These opacities may be measured and noted as “transparencies,” “opacities,” or “densities,” the last being the - log transparencies and the opacity 1/transparency. (These definitions of opacity and density are those given by Hurter and Driffield, and are generally understood as such in photographic literature.)