. The book of photography; practical, theoretical and applied. d (on the principle ex-plained in the section on Exposure of thePhotographic Plate, where it was stated 442 THE BOOK OF PHOTOGRAPHY. that only a certain pioportion of lightintensities can be represented on theplate, after which a reversal occurred).It will be found that this falsity ofrendering may be overcome to someextent, but only by the alteration of thegradations. Suppose an ordinary plateis taken and bathed in the dark with asolution of erythrosin dried, and after-wards submitted to the same test exposurein two sections to en
. The book of photography; practical, theoretical and applied. d (on the principle ex-plained in the section on Exposure of thePhotographic Plate, where it was stated 442 THE BOOK OF PHOTOGRAPHY. that only a certain pioportion of lightintensities can be represented on theplate, after which a reversal occurred).It will be found that this falsity ofrendering may be overcome to someextent, but only by the alteration of thegradations. Suppose an ordinary plateis taken and bathed in the dark with asolution of erythrosin dried, and after-wards submitted to the same test exposurein two sections to ensure accuracy in posi-tion. The curve given by such a plate will screen, which lessens the activity of theenergetic vibrations. Theories Regarding Orthochromatism. The colouring matters most effective forrendering films orthoehromatic are, Eosin, C20Hg Br4 O5 K2 ; Erythrosin, C20 Hg I^ O,, K2 ;and Rose Bengal, C20 H2 CI4 I4 O5 K2) be-longing to the family of the phthaleins ; andQuinoline Blue or Cyanin, C29 H^,, , it will be noticed, are compounds of. Fiff. 593.—Spectrum Curve on Orthochromatic Tlate. be approximately that shown in Fig. 593,which more nearly approaches to Fig. the plate in two sections, thefirst one should show the locality let a further experiment be made ofplacing a piece of lemon-yellow glass in thepath of the rays, the object of this beingto entirely cut out the ultra-violet, whichin any circumstances must give a falseeffect, and to tone down the excessive ac-tivity of the blue. Thus, it will be seen,there are two influences at work: (1) thedye, which raises the sensitiveness of theplate to the less active rays ; and (2) theyellow glass or light filter, also called a great molecular complexity. As dyes they arevery fugitive, and are readily oxidised in thepresence of light to colourless it has been shown that their sensitisingaction is considerably increased in the presenceof ammonia. Abney found that it was
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