. The book of photography; practical, theoretical and applied. ubject. Absorption of Light. It is of importance to understand thatof the light which reaches a photographicplate only a portion of it is absorbed, andonly a fraction of the latter is concernedin the chemical change. A portion of thelight is reflected from the plate, andanother portion passes through un-changed. Of that portion which is ab-sorbed, only a part is used in changingsilver bromide into the condition in whichit can be reduced. If silver bromide isexposed to light it is altered in such a wayas to produce the latent image,
. The book of photography; practical, theoretical and applied. ubject. Absorption of Light. It is of importance to understand thatof the light which reaches a photographicplate only a portion of it is absorbed, andonly a fraction of the latter is concernedin the chemical change. A portion of thelight is reflected from the plate, andanother portion passes through un-changed. Of that portion which is ab-sorbed, only a part is used in changingsilver bromide into the condition in whichit can be reduced. If silver bromide isexposed to light it is altered in such a wayas to produce the latent image, andwhen it is so far altered as to producethe maximum density upon development,any light which subsequently falls uponit is absorbed without in any way addingto that density. Hence the light ab-sorbed by a particle of silver bromide,which has already received sufiicient tobring it into this condition of instability,is useless. The law which connects thedensities with exposuies may be calcu-lated, and the formula given is—D = 7 log. To — (o — 1) /3 Jtn. PICTURE PRINTED BY COMBINATION OF YELLOW, RED, AND BLUE INKS ONLY. DETERMINATION OF PLATE SPEEDS. 417 o being the opacity of the plate to thechemically active rays, b a fraction, thehyperbolic logarithm of which is — ^ ; Itis the exposure, and i a symbol to denotewhat is known as the inertia or slownessof the silver bromide. The densities cal-culated by this formula approximatefairly closely to those obtained bj experi-ment, as will be seen in the followingtable :— but it is too complicated to be regularlyused except as a check upon the simplerone. In the formula— D = ylog (0-(0-l)/3 It) (0—1) may be replaced by the symbol Owhen that represents a large number ( the plate is thickly coated), and as TABLE OF DENSITY CALCULATIONS. Exposure. Density Density 1 Exposure. Density Density FOUXD. Calculated. Found. Calculated. 1 •060 •092 128 D875 1-800 2 •160 •172 256 2-290 2^16o 4 •340 •3
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