ABSORPTION OF LIGHT.
it is of importance to understand that of the light which reaches a photographic plate only a portion of it is absorbed, and only a fraction of the latter is concerned in the chemical change. A portion of the light is reflected from the plate, and another portion passes through un changed. Of that portion which is ab sorbed, only a part is used in changing silver bromide into the condition in which it can be reduced. If silver bromide is . exposed to light it is altered in such a way as to produce the " latent image," and when it is so far altered as to produce the maximum density upon development; any light which subsequently falls upon it is absorbed without in any way adding to that density. Hence the light ab sorbed by a particle of silver bromide, which has already received sufficient to bring it into this condition of instability, is useless. The law which connects the densities with exposures may be calcu lated, and the formula given is = 10g. [0 - (0 - $ It] i o being the opacity of the plate to the chemically active rays, B a fraction, the hyperbolic logarithm of which is ; It is the exposure, and i a symbol to denote what is known as the inertia or slowness of the silver bromide. The densities cal
culated by this formula approximate fairly closely to those obtained by experi ment. as will be seen in the following table :— but it is too complicated to be regularly used except as a cheek upon the simpler one. In the formula D = y log (0— (0 — 1) /3 ) (0-1) may be replaced by the symbol U when that represents a large number (i.e. when the plate is thickly coated), and as The short exposures show that the densities are proportioned to the ex posure, while the exposures from 16 up to 1,000 (candlemeter seconds) give very nearly equal increments of density for every successive double exposure ; while above 1,000 there is a rapid decline in the production of density. The figures from 16 to 1,000 C.M.S. therefore belong to the period of correct representation, and are very similar to those obtained by the simple linear equation D = y [log It —