The exponential law of absorption does not hold accurately for various thicknesses of sections on account of reflection, scattering, and fluorescence (under certain conditions). The skin cannot be considered as consisting of a series of layers with identical absorption. The various layers show pronounced absorption differences. A comparison of the absorption curves of the various skin layers suggests that the anti-rachitic effect is produced in the stratum corneum or granulosum and that the erythema originates in the stratum germinativum or corium.