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

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In　the　year　1841；　while　practicing　the　art　in　St。　Louis；　Mo。；　I　was　at　times；　during　the　summer；　much　troubled　with　the　electric　influence　of　the　atmosphere；　especially　on　the　approach　of　a　thunder…storm。　At　such　times　I　found　the　coating　of　my　plates　much　more　sensitive　than　when　the　atmosphere　was　comparatively　free　from　the　electric　fluid；　and　the　effect　was　so　irregular　that　no　calculation　could　counteract　the　difficulty。　This　satisfied　me　that　electricity　was　in　some　measure　an　important　agent　in　the　chemical　process；　and　it　occurred　to　me　that　the　element　might　be　turned　to　advantage。　I　determined；　therefore；　to　enter　on　a　series　of　experiments　to　test　my　theory。　　Finding　it　impossible　to　obtain　an　electric　machine；　and　unwilling　to　abandon　the　examination；　it　occurred　to　me；　that　the　galvanic　influence　might　answer　the　same　purpose。　I　therefore　proceeded　to　make　a　galvanic　battery　in　the　following　simple　manner。　　I　obtained　a　piece　of　zinc　about　two　inches　long；　one　inch　wide；　and　an　eighth　of　an　inch　thick。　On　this　I　soldered　a　narrow　strip　of　copper；　about　six　inches　long；　the　soldered　end　laid　on　one　side　of　the　zinc；　and　extending　its　whole　length。　　The　battery　was　completed　by　placing　the　zinc　in　a　glass　tumbler；　two…thirds　full　of　dilute　sulphuric　acid；　strong　enough　to　produce　a　free　action　of　the　metals。　The　upper　end　of　the　copper　slip　extending　above　the　tumbler　was　sharpened　to　a　point；　and　bent　a　little　over　the　glass。

The　method　of　using；　was　thus：After　preparing　the　plate　in　the　usual　manner　and　placing　it　in　the　camera；　in　such　manner　as　to　expose　the　back　of　the　plate　to　view；　the　battery　was　prepared　by　placing　the　zinc　in　the　acid；　and　as　soon　as　the　galvanic　fluid　began　to　traverse　（as　could　be　known　by　the　effervessence　of　the　acid；　operating　on　the　zinc　and　copper）　the　cap　of　the　camera　was　removed；　and　the　plate　exposed　to　the　sitter；　at　the　same　instant　the　point　of　the　battery　was　brought　quickly　against　the　back　of　the　plate；　and　the　cap　replaced　instantly。　　If　the　plate　is　exposed　more　than　an　instant　after　the　contact　the　picture　will　generally　be　found　solarized。　By　this　process　I　have　taken　pictures　of　persons　in　the　act　of　walking；　and　in　taking　the　pictures　of　infants　and　young　children　I　found　it　very　useful。　　Very　respectfully　yours；　　THOMAS　R。　WHITNEY。



CHAP。　VII

PAPER　DAGUERREOTYPES。ETCHING　DAGUERREOTYPES。


Mr。　Hunt　describes　a　process；　discovered　by　himself　by　which　the　Daguerrean　art　may　be　applied　to　paper。　His　description　is　as　follows：

〃Placing　the　paper　on　some　hard　body；　wash　it　over　on　one　side　by　means　of　a　very　soft　camel's　hair　pencilwith　a　solution　of　sixty　grains　of　bromide　of　potassium；　in　two　fluid　ounces　of　distilled　water；　and　then　dry　it　quickly　by　the　fire。　Being　dry；　it　is　again　washed　over　with　the　same　solution；　and　dried　as　before。　　A　solution　of　nitrate　of　silver　one　hundred　grains　to　an　ounce　of　distilled　wateris　to　be　applied　over　the　same　surface；　and　the　paper　quickly　dried　in　the　dark。　In　this　state　the　papers　may　be　kept　for　use。

〃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　resemb