the history and practice of the art of photography-第15节

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essions　resembling　aquatint。　　But　as　silver　is　so　soft　a　metal　that　the　surface　of　the　plate　might　be　expected　to　wear　rapidly；　the　discoverer　proposes　to　shield　it　by　depositing　over　its　whole　surface　a　very　thin　coat　of　copper　by　the　electrotype　process；　which　when　worn　may　be　removed　at　pleasure　down　to　the　surface　of　the　noble　metal　beneath；　and　again　a　fresh　coat　of　copper　deposited；　and　so　an　unlimited　number　of　impressions　obtained　without　injuring　the　plate　itself。〃

If；　as　has　been　asserted；　steel　may　be　rendered　sufficiently　sensitive；　to　take　photographic　impressions；　to　what　a　revolution　will　the　art　of　engraving　be　subject　by　the　discovery　of　this　process。



CHAP。　　VIII。

PHOTOGENIC　DRAWING　ON　PAPER。


We　shall　now　proceed　to　describe　the　various　processes　for　Photogenic　drawing　on　paper；　first；　however；　impressing　on　the　mind　of　the　experimenter；　the　necessity　which　exists　for　extreme　care　in　every　stage　of　the　manipulation。　In　this　portion　of　my　work　I　am　entirely　indebted　to　the　works　of　Professors　Hunt；　Fisher　and　others。

I。　APPARATUS　AND　MATERIALS。Paper。The　principal　difficulty　to　be　contended　with　in　using　paper；　is　the　different　power　of　imbibition　which　we　often　find　possessed　in　the　same　sheet；　owing　to　trifling　inequalities　in　its　texture。　　This　is；　to　a　certain　extent；　to　be　overcome　by　a　careful　examination　of　each　sheet；　by　the　light　of　a　candle　or　lamp　at　night；　or　in　the　dark。　By　extending　each　sheet　between　the　light　and　the　eye；　and　slowly　moving　it　up　and　down；　and　from　left　to　right；　the　variations　in　its　texture　will　be　seen　by　the　different　quantities　of　light　which　pass　through　it　in　different　parts；　and　it　is　always　the　safest　course　to　reject　every　sheet　in　which　inequalities　exist。　　Paper　sometimes　contains　minute　portions　of　thread；　black　or　brown　specks；　and　other　imperfections；　all　of　which　materially　interfere　with　the　process。　　Some　paper　has　an　artificial　substance　given　to　it　by　sulphate　of　lime　（Plaster　of　Paris）；　this　defect　only　exists；　however；　in　the　cheaper　sorts　of　demy；　and　therefore　can　be　easily　avoided。　In　all　cases　such　paper　should　be　rejected；　as　no　really　sensitive　material　can　be　obtained　with　it。　　Paper…makers；　as　is　well　known；　often　affix　their　name　to　one　half　the　sheet；　this　moiety　should　also　be　placed　aside；　as　the　letters　must　frequently　come　out　with　annoying　distinctness。　Well　sized　paper　is　by　no　means　objectionable；　indeed；　is　rather　to　be　preferred；　since　the　size　tends　to　exalt　the　sensitive　powers　of　the　silver。　　The　principal　thing　to　be　avoided；　is　the　absorption　of　the　sensitive　solution　into　the　pores；　and　it　must　be　evident　that　this　desideratum　cannot　be　obtained　by　unsized　paper。　　Taking　all　things　into　consideration；　the　paper　known　as　satin　post　would　appear　to　be　preferable；　although　the　precautions　already　recommended　should　be　taken　in　its　selection。

Brushes。The　necessary　solutions　are　to　be　laid　upon　the　paper　by　brushes。　Some　persons　pass　the　paper　over　the　surface　of　the　solutions；　thus　licking　up；　as　it　were；　a　portion　of　the　fluid；　but　this　method　is　apt　to　give　an　uneven　surface；　it　also　rapidly　spoils　the　solutions。　At　all　events；　the　brush　is　the　most　ready　and　the　most　effectual　means。

Distilled　Water。All　the　water　used；　both　for　mixing　the　solutions；　washing　the　paper；　or　cleaning　the　brushes；　must　be　distilled；　to　obtain　good　results；　for　reasons　before　specified。

Blotting　Paper。In　many　instances；　the　prepared　paper　requires　to　be　lightly　dried　with　bibulous　paper。　　The　best　description　is　the　white　sort。　In　each　stage　of　the　preparation　distinct　portions　of　bibulous　paper　must　be　used。　　If　these　be　kept　seperate　and　marked；　they　can　be　again　employed　for　the　same　stage；　but　it　would　not　do；　for　example；　to　dry　the　finished　picture　in　the　same　folds　in　which　the　sensitive　paper　had　been　pressed。　A　very　convenient　method　is　to　have　two　or　three　quarto　size　books　of　bibulous　paper；　one　for　each　seperate　process。

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