Thc process by which niodcrn photographic cmulsions arc ~nade has been arriwd a t almost entirely by empirical methods. Tlic cniulsion-milker gocs into his dnrkroom and tries thc effect of all sorts of diffcrcnt pliysical trcntmenta nnd clicmical ndditions during manufnctnrc, and after hundreds of cspcrimcnts picks out by trial : l i d error those conditions which give liim an cmulsion of the desired photographic quality. I-Iis work is rendered more diflicult since thc effect of each stngc of thc process is dcpcndcnt upon othcr s t n p , so tli:it the iiitrodiiction of :rny particular motlific:it,ion usually gibws different. rcsiilts with tlifferent cmulsions. Unforcsplairi result,s obtnined by thc practician. Only very occasionnlly can t h theorist forgc ahcad to point tlic \yay to bcttcr photographic emulsions.Thc process of mannfacturc may conrcnicntly be coiisirlcrcd in four stages : (a) cniulsification and ripening, (1) washing, (c) digcstion, and (d) mclting and coating.( { I ) 1i'~nd~iJiccltiott a/td ripciiijig Emulsification is thc nnmc givcn to the precipitation of silvcr halides in n colloid mctlium. Tlic choice of lialides and colloid dcpcnds npon tho type of product required. Negative nintcrinls, cine posit.ive film nnd bromidc papcrs consist of a precipitate of silver broniidc irith about 2-77; silver ioditlc in a gelatin medium. Silver chloridc in gelatin is used for gaslight pnpcrs rind lnntcrn slides, nnd n niisturc of silver cliloride and an t unutcly the dcpeiiclencc of ~~lioto~rapliic qitality 111~on niodc of mcinufncturc is such :I comples relation thiit in thc cvolution of n mctlio$ logic and sciciicc play II c.oniy:irntivcly sinall part. An cspcricnccd cinulsion Iiirikcr is naturally aldc to arrive at a dcsircd result iiftcr firr fewer cspcrimcnts tlian a tyro in the nrt, but his prccepts nJ1d tenets arc sulijcct to so ninny qualificiitions 11s to dcfy siniplc clnssification, nnd to tlic incsperienccd his rcnsoning sccnis to be su1)conscious rittllcr thnn conscious.By this statciucnt I (lo not wish to decry the work of scientists who havc brought ciiiulsion ninking to the surprisingly high standard of tlic present day. Xaturdly tlic upplicntion of logical, sciciitificnlly-trniiictl iiiinil~ even to " hit rind niiss " nicthods has rcsultctl iu coiiipuratively enorinous ndvunccs, but tho fnct is thiit in I hc cinulsio~i-]linking brnnch of photograpliic sciuacc the theorist lrigs beliind in Iris mdcnvour to Fuat iicgutiee etiiiclxiotc ut cird v j the ripeiiiiuJ prim1 X 2600organic silver snlt such :is tlic citrntc in c!itIicr gelritin or collodion for print-out papcr. An nccuriitcly prepared solution of silver nitriite is adjusted to n ccrtnin temperature and added, in nonrictinic light, to n solution of gelritin tind cscess of nllcnli halirlcs, nlso of ndjusted tcmpcraturc. Tlic silvcr nitrate solution is usually nddccl n t n definite slo\v rcitc or in sc\~cral portions rit dcfinitc intcrvals, mid niuy or map not hc rcndcrcd ammoniacul.Tlic rcsulting suspension of silrcr lialidcs in gelatin is...
II'IIIS"lcr&SSITY Olt 'rll I. : ~i~:~~~r i v i~: -l~~s l~~I \~l~1'itock:ss SiiIts o i si~vcr owe t~icir iiiiriviillcntl positioii iis the hnqis of so iiiniiy @mtogriipliic ~~roccsscs to tlic two properties of high sciisitivity to liglit, iiiiil ciisc Of coiivcrsioii into vcry StiiIJlc iiisoliil~lc t1criviit.ivcs. 'i'litw cbrivi~tivcs, of wliicli tlic iiict.iil iiiitl tlic siilpliitlc :ire tlic iiiost iinportiiiit csniitlilcs, iirc dark i i i COIOIIP, iiiitl iiiidcr ortliiiiiry coiiditioiis tlic qiiniititics ~)ro'odiiccd iucrcnsc progrcssivcly with iiicrciisiug light nctiou. For tlicsc rcnsoiis, tlic I~riglitcr tlic light fiilliiig oil it portion of i b iiintcrinl tluriitg csposurc, t,lic blrickcr will bc the deposit, uiid Iiciicc t,lic single ol~criitioii .of ~XIWSIICC, dcvclopinciit, ciud fisiitioii of a silver hnlidc cniiilsion iiiiist protliicc n iicgiitivc iiiinge of iiii ol~jcct, iind thc production of ii p~sitivc iiiiiigc iiiust i i c~c~~i i r i l y com[)risc thc two proccsst!~ of t,lic foriii:ition ot ii iiogiitivc iiungc, iiiid tlic production froiii it of i i jiositive iitiiigo (is., a iicgiitivc of tlic iicgiitivc). Tlic tliscovcry of II stnhlc wliitc tlcriviitivc of silver woiiltl itiiikc it possiblc to produce it dircct positive result wlicii viewed by reflected light, by the proccss of contiiig ii sciisitivc cniulsioii on n bliick support rind coiivcrtiiig tlic csposcd sciisibivo silver coiiipoiiiid iiito the wliitc iiisoliible dcririit,ivc. Agiiiii, it siiiiplc iiictliotl of tlevclopiiig the iiiicsposctl portion of t.hc silver iitiiigc iiistcnd of the c s p s c d portioii would ~iro(Iiicc t i dircct positive, hut iiiitil soiiic siicli ndviiiicc iis t,liis I i i i~ becu iiiiidc, the nc~itivc-positive proecss iiiust coiitiiiuc to bc nliiiost iiuivctrsitlly used.The pvsitice upowrc.-A positive is olitiiiiicd f r w i ii negiit ivc liy csposiiig sciisit.ivc iiiiitcriiil in coiitiict with t,lic coiitcd side of tlic iicgiitivc, or by cxposiiig it iii R positioii coiiicidciit with thc rcnl iiiiiigc of tlic Iiegiit ivo iii soiiic opticril systciii. In citlicr ciiso tlic iicgat ivc is iuiiforiiily illiiiiiiiiiitcd, itiitl light fiills oil thc positivc iiiiitcriiil iittcr traiisiiiissioii by tlic iicgiitivc. All portions of the positive itiittcriiil nrc tliiis csjioscd for tho siiiiic tiiiic, but dilTcrciit portions rcccivc difrcrciit csposiircs, iiccortliiig to tlic viiriirtioiis i n iiitciisity C G U S (~ by dillcrciiccs in opiicitp of corresponding porlioiis of tlic ucgntivc. It lins bccn cspliiiiicd ( C ' i i i . :~~. &S-, h i ) . , 1931, p. 707) thiit density* is iiiciisurcd iis the logirilliiti of thc opncity ; tlmt is, if i i uiiiforiii intensity of 1 iiictrccnudlcs fnlls on tlic negative, the tlciisity Dl \vliicli trnnsinits nil intensity of I, is givcii hy the foriiiiilii : I D, ; = log oliiicity = log-= log Ilog I,. 11If 1, is tlic iiitciiuity trriiisiiiittcd by ii tlciisity 'irud .a,, rc!ccivctl by portioiis of ii psitivc iiiiitcriril oxposcd bcliiiid tlicsc tlciisitics will I)c cquiil to the intcii...
HE remarkable progress which the art of photography has shown over its
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