the history and practice of the art of photography-第21章

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；　Esq。；　of　Boston

I。　CYANOTYPE；

So　called　from　the　circumstance　of　cyanogen　in　its　combinations　with　iron　performing　a　leading　part　in　the　process。　It　was　discovered　by　Sir　John　Herschel。　　The　process　is　a　simple　one；　and　the　resulting　pictures　are　blue。

Brush　the　paper　over　with　a　solution　of　the　ammonio…citrate　of　iron。　This　solution　should　be　sufficiently　strong　to　resemble　sherry　wine　in　color。　Expose　the　paper　in　the　usual　way；　and　pass　over　it　very　sparingly　and　evenly　a　wash　of　the　common　yellow　ferro…cyanate　of　potass。　As　soon　as　the　liquid　is　applied；　the　negative　picture　vanishes；　and　is　replaced　by　a　positive　one；　of　a　violet　blue　color；　on　a　greenish　yellow　ground；　which　at　a　certain　time　possesses　a　high　degree　of　sharpness；　and　singular　beauty　of　tint。

A　curious　process　was　discovered　by　Sir　John　Herschel；　by　which　dormant　pictures　are　produced　capable　of　developement　by　the　breath；　or　by　keeping　in　a　moist　atmosphere。　It　is　as　follows。

If　nitrate　of　silver；　specific　gravity　1。200　be　added　to　ferro…tartaric　acid；　specific　gravity　1。023；　a　precipitate　falls；　which　is　in　a　great　measure　redissolved　by　a　gentle　heat；　leaving　a　black　sediment；　which；　being　cleared　by　subsidence；　a　liquid　of　a　pale　yellow　color　is　obtained；　in　which　the　further　addition　of　the　nitrate　causes　no　turbidness。　When　the　total　quantity　of　the　nitrated　solution　added　amounts　to　about　half　the　bulk　of　the　ferro…tartaric　acid；　it　is　enough。　The　liquid　so　prepared　does　not　alter　if　kept　in　the　dark。　Spread　on　paper；　and　exposed　wet　to　the　sunshine　（partly　shaded）　for　a　few　seconds；　no　impression　seems　to　be　made；　but　by　degrees；　although　withdrawn　from　the　action　of　light；　it　developes　itself　spontaneously；　and　at　length　becomes　very　intense。　But　if　the　paper　be　thoroughly　dried　in　the　dark；　（in　which　state　it　is　of　a　very　pale　greenish　yellow　color；）　it　possesses　the　singular　property　of　receiving　a　dormant　or　invisible　picture；　to　produce　which　from　thirty　to　sixty　seconds'　exposure　to　sunshine　is　requisite。　　It　should　not　be　exposed　too　long；　as　not　only　is　the　ultimate　effect　less　striking；　but　a　picture　begins　to　be　visibly　produced；　which　darkens　spontaneously　after　it　is　withdrawn。　　But　if　the　exposure　be　discontinued　before　this　effect　comes　on；　an　invisible　impression　is　the　result；　to　develope　which　all　that　is　necessary　is　to　breathe　upon　it；　when　it　immediately　appears；　and　very　speedily　acquires　an　extraordinary　intensity　and　sharpness；　as　if　by　magic。　Instead　of　the　breath；　it　may　be　subject　to　the　regular　action　of　aqueous　vapor；　by　laying　it　in　a　blotting　paper　book；　of　which　some　of　the　outer　leaves　on　both　sides　have　been　dampened；　or　by　holding　over　warm　water。

II。　ENERGIATYPE。

Under　this　title　a　process　has　been　brought　forward　by　Mr。　Hunt。　It　consists　of　the　application　of　a　solution　of　succinic　acid　to　paper；　which　is　subsequently　washed　over　with　nitrate　of　silver。　The　image　is　then　to　be　taken　either　in　the　camera　or　otherwise；　as　required；　and　is　brought　out　by　the　application　of　the　sulphate　of　iron　in　solution。　　Although　this　process　has　not　come　into　general　use；　its　exact　description　may　be　interesting　to　the　general　reader；　and　we　therefore　subjoin　it。

The　solution　with　which　the　paper　is　first　washed　is　to　be　prepared　as　follows：　　succinic　acid；　two　drachms；　common　salt；　five　grains；　mucilage　of　gum　arabic；　half　a　fluid　drachm；　distilled　water；　one　fluid　drachm　and　a　half。　　When　the　paper　is　nearly　dry；　it　is　to　be　brushed　over　with　a　solution　of　nitrate　of　silver；　containing　a　drachm　of　the　salt；　to　an　ounce　of　distilled　water。　It　is　now　ready　for　exposure　in　the　camera。　　To　bring　out　the　dormant　picture　it　is　necessary　to　wash　it　with　a　mixture　of　a　drachm　of　concentrated　solution　of　the　green　sulphate　of　iron　and　two　drachms　and　a　half　of　mucilage　of　gum　arabic。

Subsequently；　however；　it　has　been　found　that　the　sulphate　of　iron　produces　upon　all　the　salts　of　silver　effects　quite　as　beautiful　as　in　the　succinate。　　On　the　iodide；　bromide；　acetate；　and　benzoate；　the　effects　are　far　more　pleasing　and　striking。　　When　pictures　are　produced；　or　the　dormant　camera　image　brought　out；　by　the　agency　of　sulphate　of　iron；　it　is　remarkable　how　rapidly　the　effect　takes　place。　Engravings　can　be　thus　copied　almost　instantaneously；　and　camera　views　obtained　in　one　or　two　minutes　on　almost　any　preparation　of　silver。　The　common　sulphate　of　copper　solution　has　the　same　property。

III。　CHROMATYPE。

Many　efforts　have　been　made　to　render　chromatic　acid　an　active　agent　in　the　production　of　photographs。　　M。　Ponton　used　a　paper　saturated　with　bichromate　of　potash；　and　this　was　one　of　the　earliest　photogenic　processes。　M。　Becquerel　improved　upon　this　process　by　sizing　the　paper　with　starch　previous　to　the　application　of　the　bichromate　of　potash　solution；　which　enabled　him　to　convert　the　negative　picture　into　a　positive　one；　by　the　use　of　a　solution　of　iodine；　which　combined　with　that　portion　of　the　starch　on　which　the　light　had　not　acted。　　But　by　neither　of　these　processes　could　clear　and　distinct　pictures　be　formed。　Mr。　Hunt　has；　however；　discovered　a　process　which　is　so　exceedingly　simple；　and　the　resulting　pictures　of　so　pleasing　a　character；　that；　although　it　is　not　sufficiently　sensitive　for　use　in　the　camera；　it　will　be　found　of　the　greatest　value　for　copying　botanical　specimens；　engravings；　or　the　like。

The　paper　to　be　prepared　is　washed　over　with　a　solution　of　sulphate　of　copperabout　one　drachm　to　an　ounce　of　waterand　partially　dried；　it　is　then　washed　with　a　moderately　strong　solution　of　bichromate　of　potash；　and　dried　at　a　little　distance　from　the　fire。　Paper　thus　prepared　may　be　kept　any　length　of　time；　in　a　portfolio；　and　are　always　ready　for　use。

When　exposed　to　the　sunshine　for　a　time；　varying　with　the　intensity　of　the　light；　from　five　to　fifteen　or　twenty　minutes；　the　result　is　generally　a　negative　picture。　It　is　now　to　be　washed　over　with　a　solution　of　nitrate　of　silver；　which　immediately　produces　a　very　beautiful　deep　orange　picture　upon　a　light　dim　colored；　or　sometimes　perfectly　white　ground。　This　picture　must　be　quickly　fixed；　by　being　washed　in　pure　water；　and　dried。　　With　regard　to　the　strength　of　the　solutions；　it　is　a　remarkable　fact；　that；　if　saturated　solutions　be　employed；　a　negative　picture　is　first　produced；　but　if　the　solutions　be　three　or　four　times　their　bulk　of　water；　the　first　action　of　the　sun's　rays　darkens　the　picture；　and　then　a　very　bleaching　effect　follows；　giving　an　exceedingly　faint　positive　picture；　which　is　brought　out　with　great　delicacy　by　the　silver　solution。

It　is　necessary　that　pure　water　should　be　used　for　the　fixing；　as　the　presence　of　any　muriate　damages　the　picture；　and　here　arises　another　pleasing　variation　of　the　Chromatype。　If　the　positive　picture　be　placed　in　a　very　weak　solution　of　common　salt　the　image　slowly　fades　out；　leaving　a　faint　negative　outline。　　If　it　now　be　removed　from　the　saline　solution；　dried；　and　again　exposed　to　sunshine；　a　positive　picture　of　a　lilac　color　will　be　produced　by　a　few　minutes　exposure。　Several　other　of　the　chromates　may　be　used　in　this　process；　but　none　is　so　successful　as　the　chromate　of　copper。

IV。　ANTHOTYPE。

The　expressed　juice；　alcholic；　or　watery　infusion　of　flowers；　or　vegetable　substances；　may　be　made　the　media　of　photogenic　action。　This　fact　was　first　discovered　by　Sir　John　Herschel。　We　have　already　given　a　few　examples　of　this　in　the　third　chapter。

Certain　precautions　are　necessary　in　extracting　the　coloring　matter　of　flowers。　　The　petals　of　fresh　flowers　are　carefully　selected；　and　crushed　to　a　pulp　in　a　marble　mortar；　either　alone　or　with　the　addition　of　a　little　alcohol；　and　the　juice　expressed　by　squeezing　the　pulp　in　a　clean　linen　or　cotton　cloth。　It　is　then　to　be　spread　upon　paper　with　a　flat　brush；　and　dried　in　the　air　without　artificial　heat。　　If　alcohol　be　not　added；　the　application　on　paper　must　be　performed　immediately；　as　the　air　（even　in　a　few　minutes）；　irrecoverably　changes　or　destroys　their　color。　If　alcohol　be　present　this　change　is　much　retarded；　and　in　some　cases　is　entirely　prevented。

Most　flowers　give　out　their　coloring　matter　to　alcohol　or　water。　Some；　however；　refuse　to　do　so；