the history and practice of the art of photography-第13部分

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〃When　they　are　required；　the　above　solution　of　silver　is　to　be　plentifully　applied；　and　the　paper　placed　wet　in　the　camera；　the　greatest　care　being　taken　that　no　day　light　not　even　the　faintest　gleamfalls　upon　it　until　the　moment　when　you　are　prepared；　by　removing　the　dark　slide；　to　permit　the　light；　radiating　from　the　object　you　wish　to　copy；　to　act　in　producing　the　picture。　　After　a　few　seconds　the　light　must　be　again　shut　off；　and　the　camera　removed　into　a　dark　room。〃　The　necessity　of　removing　the　camera　is　now　avoided　by　the　use　of　the　dark　slide；　already　described；　covering　the　picture　in　the　holder；　which　alone　may　be　removed。Amer。　Aut。

〃It　will　be　found　by　taking　the　paper　from　the　holder；　that　there　is　but　a　very　faint　outlineif　anyyet　visible。　　Place　it　aside；　in　perfect　darkness　until　quite　dry；　then　place　it　in　the　mercurial　vapor　box　（meaning　bath）　and　apply　a　very　gentle　heat　to　the　bottom。　The　moment　the　mercury　vaporizes；　the　picture　will　begin　to　develope　itself。　The　spirit　lamp　must　now　be　removed　for　a　short　time；　and　when　the　action　of　the　mercury　appears　to　cease；　it　is　to　be　very　carefully　applied　again；　until　a　well　defined　picture　is　visible。　　The　vaporization　must　then　be　suddenly　stopped；　and　the　photograph　removed　from　the　box。　The　drawing　will　then　be　very　beautiful　and　distinct；　but　much　detail　is　still　clouded；　for　the　developement　of　which　it　is　only　necessary　to　place　it　in　the　dark　and　suffer　it　to　remain　undisturbed　for　some　hours。　　There　is　now　an　inexpressible　charm　about　the　pictures；　equaling　the　delicate　beauty　of　the　daguerreotype；　but　being　very　susceptible　of　change；　it　must　be　viewed　by　the　light　of　a　taper　only。　The　nitrate　of　silver　must　now　be　removed　from　the　paper；　by　well　washing　it　in　soft　water；　to　which　a　small　quantity　of　salt　has　been　added；　and　it　should　afterwards　be　soaked　in　water　only。　　When　the　picture　has　been　dried；　wash　it　quickly　over　with　a　soft　brush　dipped　in　a　warm　solution　of　hyposulphite　of　soda；　and　then　wash　it　for　some　time　in　distilled　water；　in　order　that　all　the　hyposulphite　may　be　removed。　The　drawing　is　now　fixed　and　we　may　use　it　to　procure　positive　copies；　（the　original　being　termed　a　negative；）　many　of　which　may　be　taken　from　one　original。〃

〃The　action　of　light　on　this　preparation；　does　indeed　appear　to　be　instantaneous。　　The　exquisite　delicacy　of　this　preparation　may　be　imagined；　when　I　state　that　in　five　seconds　in　the　camera；　I　have；　during　sunshine；　obtained　perfect　pictures；　and　that　when　the　sky　is　overcast；　one　minute　is　quite　sufficient　to　produce　a　most　decided　effect。〃

〃This　very　beautiful　process　is　not　without　its　difficulties；　and　the　author　cannot　promise　that；　even　with　the　closest　attention　to　the　above　directions；　annoying　failures　will　not　occur。　It　often　happens　that　some　accidental　circumstancegenerally　a　projecting　film　or　a　little　dustwill　occasion　the　mercurial　vapor　to　act　with　great　energy　on　one　part　of　the　paper；　and　blacken　it　before　the　other　portions　are　at　all　effected。　　Again；　the　mercury　will　sometimes　accumulate　along　the　lines　made　by　the　brush；　and　give　a　streaky　appearance　to　the　picture；　although　these　lines　are　not　at　all　evident　before　the　mercurial　vapor　was　applied。　（A　brush　sufficiently　largeand　they　may　be　easily　obtained　will；　in　a　measure；　prevent　this　difficulty。Amer　Au。）　I　have　stated　that　the　paper　should　be　placed　wet　in　the　camera；　the　same　paper　may　be　used　dry；　which　often　is　a　great　convenience。　When　in　the　dry　state　a　little　longer　exposure　is　required；　and　instead　of　taking　a　picture　in　four　or　five　seconds；　two　or　three　minutes　are　necessary。〃

The　durability　of　daguerreotypes　has　been；　and　is　still；　doubted　by　many；　but　experiment　has　proved　that　they　are　more　permanent　than　oil　paintings　or　engravings。

ETCHING　DAGUERREOTYPES。There　are　several　methods　of　accomplishing　this　object；　discovered　and　applied　by　different　individuals。

The　first　process　was　published　at　Vienna　by　Dr。　Berres；　and　consisted　in　covering　the　plate　with　the　mucilage　of　gum　arabic；　and　then　immersing　the　plate　in　nitric　acid　of　different　strengths。

Mr。　Figeau；　of　whom　I　have　already　spoken；　likewise　discovered　a　process　for　the　engraving　of　Daguerreotypes；　and　founded　on　the　belief　that　the　lights　of　a　Daguerreotype　plate　consists　of　unaltered　silver；　while　the　dark　or　shadows　consists　of　mercury　or　an　amalgam　of　mercury　with　silver。　He　finds　that　a　compound　acid；　consisting　of　a　mixture　of　nitric；　nitrous；　and　muriatic　acids；　or　of　nitric　mixed　with　nitrate　of　potass　and　common　salt；　has　the　property　of　attaching　the　silver　in　presence　of　the　mercury　without　acting　upon　the　latter。　　Bi…chloride　of　copper　answers　the　purpose　also；　but　less　completely。

〃When　the　clean　surface　of　a　Daguerreotype　plate　is　exposed　to　the　action　of　this　menstruum；　particularly　if　warm；　the　white　parts；　or　lights　are　not　altered；　but　the　dark　parts　are　attacked；　and　chloride　of　silver　is　formed；　of　which　an　insoluable　coating　is　soon　deposited；　and　the　action　of　the　acid　soon　ceases。　This　coat　of　chloride　of　silver　is　removed　by　a　solution　of　ammonia；　and　then　the　acid　applied　again；　and　so　on；　until　the　depth　of　biting　in　is　sufficient。　　However；　it　is　not　possible；　by　repeating　this　process；　to　get　a　sufficient　force　of　impression；　a　second　operation　is　required；　in　order　to　obtain　such　a　depth　as　will　hold　the　ink；　to　give　a　dark　impression；　for　this　purpose　the　whole　plate　is　covered　with　drying　oil；　this　is　cleared　off　with　the　hand；　exactly　in　the　way　a　copper　plate　printer　cleans　his　plate。　The　oil　is　thus　left　in　the　sinkings；　or　dark　bitten　in　parts　only。　The　whole　plate　is　now　placed　in　a　suitable　apparatus；　and　the　lights　or　prominent　parts　of　the　face　are　gilt　by　the　electrotype　process。　The　whole　surface　is　now　touched　with　what　the　French　engravers　call　the　〃Resin　Grain；〃　（grain　de　resine）；　a　species　of　partial　stopping　out；　and　it　is　at　once　bitten　in　to　a　sufficient　depth　with　nitric　acid；　the　gilding　preserving　the　lights　from　all　action　of　the　acid。　The　resin　grain　gives　a　surface　to　the　corroded　parts　suitable　for　holding　the　ink；　and　the　plate　is　now　finished　and　fit　to　give　impressions　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　shou