the history and practice of the art of photography-第14部分

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Nitrate　of　Silver。In　the　practice　of　the　photographic　art；　much　depends　on　the　nitrate　of　silver。　　Care　should　be　taken　to　procure　the　best；　the　crystalized　salt　is　most　suitable　for　the　purpose。　　While　in　the　form　of　crystal　it　is　not　injured　by　exposure　to　light；　but　the　bottles　containing　the　solutions　of　this　salt　should　at　all　times　be　kept　wrapped　in　dark　paper；　and　excluded　from　daylight。

II。　DIFFERENT　METHODS　OF　PREPARING　THE　PAPER。Preparation　of　the　Paper。　Dip　the　paper　to　be　prepared　into　a　weak　solution　of　common　salt。　The　solution　should　not　be　saturated；　but　six　or　eight　times　diluted　with　water。　　When　perfectly　moistened；　wipe　it　dry　with　a　towel；　or　press　it　between　bibulous　paper；　by　which　operation　the　salt　is　uniformly　dispersed　through　its　substance。　　Then　brush　over　it；　on　one　side　only；　a　solution　of　nitrate　of　silver。　　The　strength　of　this　solution　must　vary　according　to　the　color　and　sensitiveness　required。　Mr。　Talbot　recommends　about　fifty　grains　of　the　salt　to　an　ounce　of　distilled　water。　　Some　advise　twenty　grains　only；　while　others　say　eighty　grains　to　the　ounce。　　When　dried　in　a　dark　room；　the　paper　is　fit　for　use。　　To　render　this　paper　still　more　sensitive；　it　must　again　be　washed　with　salt　and　water；　and　afterwards　with　the　same　solution　of　nitrate　of　silver；　drying　it　between　times。　　This　paper；　if　carefully　made；　is　very　useful　for　all　ordinary　photographic　purposes。　For　example；　nothing　can　be　more　perfect　than　the　images　it　gives　of　leaves　and　flowers；　especially　with　a　summer's　sun；　the　light；　passing　through　the　leaves；　delineates　every　ramification　of　their　fibres。　In　conducting　this　operation；　however；　it　will　be　found　that　the　results　are　sometimes　more　and　sometimes　less　satisfactory；　in　consequence　of　small　and　accidental　variations　in　the　proportions　employed。　It　happens　sometimes　that　the　chloride　of　silver　formed　on　the　surface　of　the　paper　is　disposed　to　blacken　of　itself；　without　any　exposure　to　light。　This　shows　that　the　attempt　to　give　it　sensibility　has　been　carried　too　far。　The　object　is；　to　approach　as　nearly　to　this　condition　as　possible　without　reaching　it；　so　that　the　preparation　may　be　in　a　state　ready　to　yield　to　the　slightest　extraneous　force；　such　as　the　feeblest　effect　of　light。

Cooper's　Method。Soak　the　paper　in　a　boiling　hot　solution　of　chlorate　of　potash　（the　strength　matters　not）　for　a　few　minutes；　then　take　it　out；　dry　it；　and　wet　it　with　a　brush；　on　one　side　only；　dipped　in　a　solution　of　nitrate　of　silver；　sixty　grains　to　an　ounce　of　distilled　water；　or；　if　not　required　to　be　so　sensitive；　thirty　grains　to　the　ounce　will　do。　　This　paper　possesses　a　great　advantage　over　any　other；　for　the　image　can　be　fixed　by　mere　washing。　It　is；　however；　very　apt　to　become　discolored　even　in　the　washing；　or　shortly　afterwards；　and　is；　besides；　not　so　sensitive；　nor　does　it　become　so　dark　as　that　made　according　to　Mr。　Talbot's　method。

Daguerre's　Method。Immerse　the　paper　in　hydrochloric　（or　as　it　is　more　commonly　called；　muriatic）　ether；　which　has　been　kept　sufficiently　long　to　become　acid；　the　paper　is　then　carefully　and　completely　dried；　as　this　is　essential　to　its　proper　preparation。　It　is　then　dipped　into　a　solution　of　nitrate　of　silver；　and　dried　without　artificial　heat　in　a　room　from　which　every　ray　of　light　is　carefully　excluded。　　By　this　process　it　acquires　a　very　remarkable　facility　in　being　blackened　on　a　very　slight　exposure　to　light；　even　when　the　latter　is　by　no　means　intense。　The　paper；　however；　rapidly　loses　its　extreme　sensitiveness　to　light；　and　finally　becomes　no　more　impressionable　by　the　solar　beams　than　common　nitrate　paper。

Bromide　Paper。Of　all　common　photographic　paper；　the　best；　because　the　least　troublesome　in　making；　and　the　most　satisfactory　in　result；　is　that　which　is　termed　bromine　paper；　and　which　is　thus　prepared：Dissolve　one　hundred　grains　of　bromide　of　potassium　in　one　ounce　of　distilled　water；　and　soak　the　paper　in　this　solution。　Take　off　the　superfluous　moisture；　by　means　of　your　bibulous　paper；　and　when　nearly　dry；　brush　it　over　on　one　side　only；　with　a　solution　of　one　hundred　grains　of　nitrate　of　silver　to　an　ounce　of　distilled　water。　The　paper　should　then　be　dried　in　a　dark　room；　and；　if　required　to　be　very　sensitive；　should　a　second　time　be　brushed　over　with　the　nitrate　of　silver　solution。

In　preparing　the　papers　mentioned　above；　there　are　two　circumstances　which　require　particular　attention。　　In　the　first　place；　it　is　necessary　to　mark　the　paper　on　the　side　spread　with　the　solutions　of　nitrate　of　silver；　near　one　of　the　extreme　corners。　This　answers　two　purposes：　　in　the　first　place　it　serves　to　inform　the　experimemtalist　of　the　sensitive　surface；　and　secondly；　it　will　be　a　guide　as　to　which　portion　of　the　papers　has　been　handled　during　the　application　of　the　solution；　as　the　impress　of　the　fingers　will　probably　come　out　upon　the　photograph。　The　second　caution　is；　that　the　application　of　the　sensitive　solution　（nitrate　of　silver；）　and　the　subsequent　drying　of　the　paper；　must　be　always　conducted　in　a　perfectly　dark　room；　the　light　of　a　candle　alone　being　used。

III。　PHOTOGENIC　PROCESS　ON　PAPER。Method。The　simplest　mode　is　to　procure　a　flat　board　and　a　square　of　glass；　larger　in　size　than　the　object　intended　to　be　copied。　On　the　board　place　the　photographic　paper　　＇hipho_29。gif＇　with　the　prepared　side　upwards；　and　upon　it　the　object　to　be　copied；　over　both　lay　the　glass　and　secure　them　so　that　they　are　in　close　connection　by　means　of　binding　screws　or　clamps；　similar　to　g。　g。　fig。　29。　　Should　the　object　to　be　copied　be　of　unequal　thickness；　such　as　a　leaf；　grass；　&c。；　it　will　be　necessary　to　place　on　the　board；　first；　a　soft　cushion；　which　may　be　made　of　a　piece　of　fine　flannel　and　cotton　wool。　By　this　means　the　object　is　brought　into　closer　contact　with　the　paper；　which　is　of　great　consequence；　and　adds　materially　to　the　clearness　of　the　copy。　The　paper　is　now　exposed　to　diffused　daylight；　or；　still　better；　to　the　direct　rays　of　the　sun；　when　that　part　of　the　paper　not　covered　by　the　object　will　become　tinged　with　a　violet　color；　and　if　the　paper　be　well　prepared；　it　will　in　a　short　time　pass　to　a　deep　brown　or　bronze　color。　　It　must　then　be　removed；　as　no　advantage　will　be　obtained　by　keeping　it　longer　exposed；　on　the　contrary；　the　delicate　parts　yet　uncolored　will　become　in　some　degree　affected。　　The　photogenic　paper　will　now　show　a　more　or　less　white　and　distinct　representation　of　the　object。　The　apparatus　figured　at　29　consists　of　a　wooden　frame　similar　to　a　picture　frame；　a　piece　of　plate　glass　is　fixed　in　front；　and　it　is　provided　with　a　sliding　cover　of　wood；　c。；　which　is　removed　when　the　paper　is　ready　to　be　exposed　to　the　action　of　the　light。　The　back；　d。；　which　is　furnished　with　a　cushion；　as　just　described；　is　made　to　remove　for　the　purpose　of　introducing　the　object　to　be　copied；　and　upon　it　the　prepared　paper；　the　back　is　then　replaced；　and；　by　aid　of　the　cross　piece　and　screw；　e。；　the　whole　is　brought　into　close　contact　with　the　glass。

The　objects　best　delineated　on　these　photographic　papers；　are　lace；　feathers；　dried　plants；　particularly　the　ferns；　sea…weeds　and　the　light　grasses；　impressions　of　copper　plate　and　wood　engravings；　particularly　if　they　have　considerable　contrast　of　light　and　shade（these　should　be　placed　with　the　face　downwards；　having　been　previously　prepared　as　hereafter　directed）　paintings　on　glass；　etchings；　&c。

To　fix　the　Drawings。Mr。　Talbot　recommends　that　the　drawings　should　be　dipped　in　salt　and　water；　and　in　many　instances　this　method　will　succeed；　but　at　times　it　is　equally　unsuccessful。　Iodide　of　potassium；　or；　as　it　is　frequently　called；　hydriodate　of　potash；　dissolved　in　water；　and　very　much　diluted；　（twenty…five　grains　to　one　ounce　of　water；）　is　a　more　useful　preparation　to　wash　the　drawings　with；　it　must　be　used　very　weak　or　it　will　not　dissolve　the　unchanged　muriate　only；　as　is　intended　but　the　black　oxide　also；　and　the　drawing　be　thereby　spoiled。

But　the　most　certain　material　to　be　used　is　the　hyposulphite　of　soda。　One　ounce　of　this　salt　should　be　dissolved　in　about　a　pint　of　distilled　water。　　Having　previously　washed　the　drawing　in　a　little　lukewarm　water；　which　of　itself　removes　a　large　portion　of　the　muriate　of　silver　which　is　to　be　got　rid　of；　it　should　be　dipped　once　or　twice　in　the　hyposulphite　solution。　By　this　operation　the　muriate　which　lies　upon　the　lighter　parts　will　become　so　altered　in　its　nature　as　to　be　unchanged　by　light；　while　the　rest　remains　dark　as　before。

It　will　be　evident　from　the　nature　of　the　process；　that　the　lights　and　shadows　of　an　object　are　reversed。　　That　which　is　originally　opaque　will　intercept　the　light；　and　consequently　those　parts　of　the　photogenic　paper　will　be　least　influenced　by　light；　while　any　part　of　the　object　which　is　transparent；　by　admitting　the　light　through　it；　will　suffer　the　effect　to　be　greater　or　less　in　exact　proportion　to　its　degree　of　transparency。　The　object　wholly　intercepting　the　light　will　show　a　white　impression；　in　selecting；　for　example；　a　butterfly　for　an　object；　the　insect；　being　more　or　less　transparent；　leaves　a　proportionate　gradation　of　light　and　shade；　the　most　opaque　parts　showing　the　whitest。　It　may　be　said；　therefore；　that　this　is　not　natural；　and　in　order　to　obtain　a　true　pictureor；　as　it　is　termed；　a　positive　picture　we　must　place　our　first　acquired　photograph　upon　a　second　piece　of　photogenic　paper。　　Before　we　do　this；　however；　we　must　render　our　photograph　transparent；　otherwise　the　opacity　of　the　paper　will　mar　our　efforts。

To　accomplish　this　object；　the　back　of　the　paper　containing　the　negative；　or　first　acquired　photograph；　should　be　covered　with　white　or　virgin　wax。　This　may　be　done　by　scraping　the　wax　upon　the　paper；　and　then；　after　placing　it　between　two　other　pieces　of　paper；　passing　a　heated　iron　over　it。　The　picture；　being　thus　rendered　transparent；　should　now　be　applied　to　a　second　piece　of　photogenic　paper；　and　exposed；　in　the　manner　before　directed；　either　to　diffused　day…light　or　to　the　direct　rays　of　the　sun。　The　light　will　now　penetrate　the　white　parts；　and　the　second　photograph　be　the　reverse　of　the　first；　or　a　true　picture　of　the　original。

Instead　of　wax；　boiled　linseed　oilit　must　be　the　b