the history and practice of the art of photography-第19节

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hould　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the　ultimate　effect　less　striking；　but　a　picture　begins　to　be　visibly　produced；　which　darkens　spontaneously　after　it　is　withdrawn。　　But　if　the　exposure　be　discontinued　before　this　effect　comes　on；　an　invisible　impression　is　the　result；　to　develope　which　all　that　is　necessary　is　to　breathe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；　and　require　the　addition　of　alkalies；　others　of　acid；　&c。　Alcohol　has；　however；　been　found　to　enfeeble；　and　in　many　cases　to　discharge　altogether　these　colors；　but　they　are；　in　most　cases；　restored　upon　drying；　when　spread　over　paper。　Papers　tinged　with　vetegable　colors　must　always　be　kept　in　the　dark；　and　perfectly　dry。

The　color　of　a　flower　is　by　no　means　always；　or　usually；　that　which　its　expressed　juice　imparts　to　white　paper。　Sir　John　Herschel　attributes　these　changes　to　the　escape　of　carbonic　acid　in　some　cases；　to　a　chemical　alteration；　depending　upon　the　absorption　of　oxygen；　in　others；　and　again　in　others；　especially　where　the　expressed　juice　coagulates　on　standing；　to　a　loss　of　vitality；　or　disorganization　of　the　molecules。　To　secure　an　eveness　of　tint　on　paper；　the　following　manipulation　is　recommended：The　paper　should　be　moistened　on　the　back　by　sponging　and　blotting　off。　　It　should　then　be　pinned　on　a　board；　the　moist　side　downwards；　so　that　two　of　its　edges　（suppose　the　right…hand　and　lower　ones）　shall　project　a　little　beyond　those　of　the　board。　　The　board　then　being　inclined　twenty　or　thirty　degrees　to　the　horizon；　the　alcoholic　tincture　（mixed　with　a　very　little　water；　if　the　petals　themselves　be　not　very　juicy）　is　to　be　applied　with　a　brush　in　strokes　from　left　to　right；　taking　care　not　to　go　over　the　edges　which　rest　on　the　board；　but　to　pass　clearly　over　those　that　project；　and　observing　also　to　carry　the　tint　from　below　upwards　by　quick　sweeping　strokes；　leaving　no　dry　spaces　between　them；　but　keeping　up　a　continuity　of　wet　spaces。　　When　all　is　wet；　cross　them　by　another　set　of　strokes　from　above　downwards；　so　managing　the　brush　as　to　leave　no　floating　liquid　on　the　paper。　It　must　then　be　dried　as　quickly　as　possible　over　a　stove；　or　in　a　warm　current　of　air；　avoiding；　however；　such　heat　as　may　injure　the　tint。

In　addition　to　the　flowers　already　mentioned　in　my　third　chapter；　the　following　are　among　those　experimented　upon　and　found　to　give　tolerable　good　photographic　sensitives。　　I　can　only　enumerate　them；　referring　the　student；　for　any　further　information　he　may　desire　on　the　subject；　to　Mr。　Hunt's　work；　although　what　I　have　said　above　is　sufficient　for　all　practical　purposes；　and　any　one；　with　the　ambition；　can　readily　experiment　upon　them；　without　further　research；　on　any　other　flower　he　may　choose。

Viola　Odorataor　sweet　sented　violet；　yields　to　alcohol　a　rich　blue　color；　which　it　imparts　in　high　perfection　to　paper

Senecio　Splendensor　double　purple　groundsel；　yields　a　beautiful　color　to　paper。

The　leaves　of　the　laurel；　common　cabbage；　and　the　grasses；　are　found　sufficiently　senstive。

Common　Merrigold　yields　an　invaluable　faecula；　which　appears　identical　with　that　produced　by　the　Wall…flower；　and　Cochorus　japonica　mentioned　before；　and　is　very　sensitive；　but　photographs　procured　upon　it　cannot　be　preserved；　the　color　is　so　fugitive。

From　an　examination　of　the　researches　of　Sir　John　Herschel　on　the　coloring　matter　of　plants；　it　will　be　seen　that　the　action　of　the　sun's　ray