the history and practice of the art of photography-第17节

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dry　dip　it　into　a　solution　of　iodide　of　potassium；　containing　five　hundred　grains　of　that　salt　dissolved　in　a　pint　of　water。　When　perfectly　saturated　with　this　solution；　it　should　be　washed　in　distilled　water；　drained　and　allowed　to　dry。　　This　is　the　first　part　of　the　process；　and　the　paper　so　prepared　is　called　iodized　paper。　It　should　be　kept　in　a　port…folio　or　drawer　until　required：　with　this　care　it　may　be　preserved　for　any　length　of　time　without　spoiling　or　undergoing　any　change。

Mr。　Cundell　finds　a　stronger　solution　of　nitrate　of　silver　preferable；　and　employs　thirty　grains　to　the　ounce　of　distilled　water：　he　also　adds　fifty　grains　of　common　salt　to　the　iodide　of　potassium；　which　he　applies　to　the　marked　side　of　the　paper　only。　This　is　the　first　process。

Preparation　of　the　paper　for　the　Camera。The　second　process　consists　in　applying　to　the　above　a　solution　which　has　been　named　by　Mr。　Talbot　the　〃Gallo…Nitrate　of　Silver；〃　it　is　prepared　in　the　following　manner：　Dissolve　one　hundred　grains　of　crystalized　nitrate　of　silver　in　two　ounces　of　distilled　water；　to　which　is　added　two　and　two…third　drachms　of　strong　acetic　acid。　　This　solution　should　be　kept　in　a　bottle　carefully　excluded　from　the　light。　　Now；　make　a　solution　of　gallic　acid　in　cold　distilled　water：　　the　quantity　dissolved　is　very　small。　When　it　is　required　to　take　a　picture；　the　two　liquids　above　described　should　be　mixed　together　in　equal　quantities；　but　as　it　speedily　undergoe　decomposition；　and　will　not　keep　good　for　many　minutes；　only　just　sufficient　for　the　time　should　be　prepared；　and　that　used　without　delay。　It　is　also　well　not　to　make　much　of　the　gallic　acid　solution；　as　it　will　not　keep　for　more　than　a　few　days　without　spoiling。　A　sheet　of　the　iodized　paper　should　be　washed　over　with　a　brush　with　this　mixed　solution；　care　being　taken　that　it　be　applied　to　the　marked　side。　This　operation　must　be　performed　by　candle　light。　　Let　the　paper　rest　half　a　minute；　then　dip　it　into　one　of　the　dishes　of　water；　passing　it　beneath　the　surface　several　times；　it　is　now　allowed　to　drain；　and　dried　by　placing　its　marked　side　upwards；　on　the　drying　apparatus。　It　is　better　not　to　touch　the　surface　with　bibulous　paper。　It　is　now　highly　sensitive；　and　ready　to　receive　the　impression。　In　practice　it　is　found　better　and　more　economical　not　to　mix　the　nitrate　of　silver　and　gallic　acid；　but　only　to　brush　the　paper　with　the　solution　of　the　nitrate。

Mr。　Talbot　has　recently　proposed　some　modifications　in　his　method　of　preparing　the　calotype　paper。　　The　paper　is　first　iodized　in　the　usual　way；　it　is　then　washed　over　with　a　saturated　solution　of　gallic　acid　in　distilled　water　and　dried。　　Thus　prepared　he　calls　it　the　io…gallic　paper：　it　will　remain　good　for　a　considerable　time　if　kept　in　a　press　or　portfolio。　When　required　for　use；　it　is　washed　with　a　solution　of　nitrate　of　silver　（fifty　grains　to　the　ounce　of　distilled　water）；　and　it　is　then　fit　for　the　camera。

Exposure　in　the　Camera。The　calotype　paper　thus　prepared　possesses　a　very　high　degree　of　sensibility　when　exposed　to　light；　and　we　are　thus　provided　with　a　medium　by　which；　with　the　aid　of　the　photogenic　camera；　we　may　effectually　copy　views　from　nature；　figures；　buildings；　and　even　take　portraits　from　the　shadows　thrown　on　the　paper　by　the　living　face。　The　paper　may　be　used　somewhat　damp。　　The　best　plan　for　fixing　it　in　the　camera　is　to　place　it　between　a　piece　of　plate　glass　and　some　other　material　with　a　flat　surface；　as　a　piece　of　smooth　slate　or　an　iron　plate；　which　latter；　if　made　warm；　renders　the　paper　more　sensitive；　and　consequently　the　picture　is　obtained　more　rapidly。

Time　of　Exposure。With　regard　to　the　time　which　should　be　allowed　for　the　paper　to　remain　in　the　camera；　no　direct　rules　can　be　laid　down；　this　will　depend　altogether　upon　the　nature　of　the　object　to　be　copied；　and　the　light　which　prevails。　All　that　can　be　said　is；　that　the　time　necessary　for　forming　a　good　picture　varies　from　thirty　seconds　to　five　minutes；　and　it　will　be　naturally　the　first　object　of　the　operator　to　gain　by　experience　this　important　knowledge。

Bringing　Out　the　Picture。The　paper　when　taken　from　the　camera；　which　should　be　done　so　as　to　exclude　every　ray　of　lightand　here　the　dark　slide　of　the　camera　plate　holder　becomes　of　great　use　bears　no　resemblance　to　the　picture　which　in　reality　is　formed。　The　impression　is　latent　and　invisible；　and　its　existence　would　not　be　suspected　by　any　one　not　acquainted　with　the　process　by　previous　experiment。　　The　method　of　bringing　out　the　image　is　very　simple。　It　consists　in　washing　the　paper　with　the　gallo…nitrate　of　silver；　prepared　in　the　way　already　described；　and　then　warming　it　gently；　being　careful　at　the　same　time　not　to　let　any　portion　become　perfectly　dry。　　In　a　few　seconds　the　part　of　the　paper　upon　which　the　light　has　acted　will　begin　to　darken；　and　finally　grow　entirely　black；　while　the　other　parts　retain　their　original　color。　Even　a　weak　impression　may　be　brought　out　by　again　washing　the　paper　in　the　gallo…nitrate；　and　once　more　gently　warming　it。　When　the　paper　is　quite　black；　as　is　generally　the　case；　it　is　a　highly　curious　and　beautiful　phenomenon　to　witness　the　commencement　of　the　picture；　first　tracing　out　the　stronger　outlines；　and　then　gradually　filling　up　all　the　numerous　and　complicated　details。　The　artist　should　watch　the　picture　as　it　developes　itself；　and　when　in　his　judgment　it　has　attained　the　greatest　degree　of　strength　and　clearness；　he　shall　stop　further　proceedings　by　washing　it　with　the　fixing　liquid。　Here　again　the　mixed　solution　need　not　be　used；　but　the　picture　simply　brushed　over　with　the　gallic　acid。

The　Fixing　Process。In　order　to　fix　the　picture　thus　obtained；　first　dip　it　into　water；　then　partly　dry　it　with　bibulous　paper；　and　wash　it　with　a　solution　of　bromide　of　potassiumcontaining　one　hundred　grains　of　that　salt　dissolved　in　eight　or　ten　ounces　of　distilled　water。　The　picture　is　again　washed　with　distilled　water；　and　then　finally　dried。　Instead　of　bromide　of　potassium；　a　solution　of　hyposulphite　of　soda；　as　before　directed；　may　be　used　with　equal　advantage。

The　original　calotype　picture；　like　the　photographic　one　described　in　the　last　chapter；　is　negative；　that　is　to　say；　it　has　its　lights　and　shades　reversed；　giving　the　whole　an　appearance　not　conformable　to　nature。　　But　it　is　easy　from　this　picture　to　obtain　another　which　shall　be　conformable　to　nature；　viz。；　in　which　the　lights　shall　be　represented　by　lights；　and　the　shades　by　shades。　It　is　only　necessary　to　take　a　sheet　of　photographic　paper　（the　bromide　paper　is　the　best）；　and　place　it　in　contact　with　a　calotype　picture　previously　rendered　transparent　by　wax　or　oil　as　before　directed。　Fix　it　in　the　frame；　Fig。　29；　expose　it　in　the　sunshine　for　a　short　time；　and　an　image　or　copy　will　be　formed　on　the　photogenic　paper。　The　calotype　paper　itself　may　be　used　to　take　the　second；　or　positive；　picture；　but　this　Mr。　Talbot　does　not　recommend；　for　although　it　takes　a　much　longer　time　to　take　a　copy　on　the　photogenic　paper；　yet　the　tints　of　such　copy　are　generally　more　harmonious　and　agreeable。　After　a　calotype　picture　has　furnished　a　number　of　copies　it　sometimes　grows　faint；　and　the　subsequent　copies　are　inferior。　This　may　be　prevented　by　means　of　a　process　which　revives　the　strength　of　the　calotype　pictures。　　In　order　to　do　this；　it　is　only　nesessary　to　wash　them　by　candlelight　with　gallo…nitrate　of　silver；　and　then　warm　them。　　This　causes　all　the　shades　of　the　picture　to　darken　considerably；　while　the　white　parts　are　unaffected。　After　this　the　picture　is　of　course　to　be　fixed　a　second　time。　It　will　then　yield　a　second　series　of　copies；　and；　in　this　way；　a　great　number　may　frequently　be　made。

The　calotype　pictures　when　prepared　as　we　have　stated；　possess　a　yellowish　tint；　which　impedes　the　process　of　taking　copies　from　them。　In　order　to　remedy　this　defect；　Mr。　Talbot　has　devised　the　following　method。　　The　calotype　picture　is　plunged　into　a　solution　consisting　of　hyposulphite　of　soda　dissolved　in　about　ten　times　its　weight　of　water；　and　heated　nearly　to　the　boiling　point。　The　picture　should　remain　in　about　ten　minutes；　it　must　then　be　removed；　washed　and　dried。　　By　this　process　the　picture　is　rendered　more　transparent；　and　its　lights　become　whiter。　It　is　also　rendered　exceedingly　permanent。　　After　this　process　the　picture　may　be　waxed；　and　thus　its　transparency　increased。　This　process　is　applicable　to　all　photographic　papers　prepared　with　solutions　of　silver。

Having　thus　fully；　and　it　is　hoped　clearly；　considered　the　process；　it　may　be　necessary　before　dismissing　the　calotype　from　notice；　to　add　one　or　two　remarks　from　the　observations　and　labors　of　some　who　have　experimented　in　this　art。　　Dr。　Ryan　in　his　lectures　before　the　Royal　Polytechnic　Institution；　has　observed；　that　in　the　iodizing　process　the　sensitiveness　of　the　paper　is　materially　injured　by　keeping　it　too　long　in　the　solution　of　iodide　of　potassium；　owing　to　the　newly　formed　iodide　of　silver　being　so　exceedingly　solvable　in　excess　of　iodide　of　potassium　as　in　a　few　minutes　to　be　completely　removed。　The　paper　should　he　dipped　in　the　solution　and　instantly　removed。　There　is　another　point；　too；　in　the　preparation　of　the　iodized　paper　in