XII. The diffusion of light

L ITTLE information appears to have been published about the diffusi1~g power of unpolished surfaces.The subject has been studied by optical measurements by ZSllner and others, chiefly for astronomical purposes ; and the radiation from such surfaces has also been investigated with apparatus designed for measuring radiant heat.But the influence of such diffusion in increasing the illumination of rooms and open spaces, although well known, does not appear to be appreciated to the extent that i~s importance de,ryes ; and a few numerical determinations of the coe~clents of reflexion, absorption, and transmission of diffusing surfaces may prove of interest.Terms in light are used vaguely, and it will not be deemed out of place to define those which will be here needed.By the rej~ectinq power of a surface is meant the ratio of the amount of light reflected by it to the total amount of light incident upon it.The illumination of a surface is the amount of incident light per unit area of the surface.The unit quantity of l~ht is the flux of radiation per second across a unit area of a sphere of unit radius at whose centre a unit light (of one candle) is placed.The amount of light radiated by a source of k candle-power, within a solid angle t2, is kl2, and the total quantity of light emitted by it is 4~'k.The brlghtness of a diffusing surface is its candle-power per unit