the history and practice of the art of photography-第20节

按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！



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　rays　is　to　destroy　the　color；　effecting　a　sort　of　chromatic　analysis；　in　which　two　distinct　elements　of　color　are　separated；　by　destroying　the　one　and　leaving　the　other　outstanding。　The　action　is　confined　within　the　visible　spectrum；　and　thus　a　broad　distinction　is　exhibited　between　the　action　of　the　sun's　rays　on　vegetable　juices　and　on　argentine　compounds；　the　latter　being　most　sensibly　affected　by　the　invisible　rays　beyond　the　violet。

It　may　also　be　observed；　that　the　rays　effective　in　destroying　a　given　tint；　are　in　a　great　many　cases；　those　whose　union　produces　a　color　complementary　to　the　tint　destroyed；　or；　at　least；　one　belonging　to　that　class　of　colors　to　which　such　complementary　tint　may　be　preferred。　For　instance；　yellows　tending　towards　orange　are　destroyed　with　more　energy　by　the　blue　rays；　blues　by　the　red；　orange　and　yellow　rays；　purples　and　pinks　by　yellow　and　green　rays。

V。　AMPHITYPE。

This　process　is　a　discovery　of　Sir　John　Herschel　and　receives　its　name　from　the　fact　that　both　negative　and　positive　photographs　can　be　produced　by　one　process。　　The　positive　pictures　obtained　by　it　have　a　perfect　resemblance　to　impressions　of　engravings　with　common　printer's　ink。　The　process；　although　not　yet　fully　carried　out；　promises　to　be　of　vast　utility。

Paper　proper　for　producing　an　amphitype　picture　may　be　prepared　either　with　the　ferro…tartrate　or　the　ferro…citrate　of　the　protoxide；　or　the　peroxide　of　mercury；　or　of　the　protoxide　of　lead；　by　using　creams　of　these　salts；　or　by　successive　applications　of　the　nitrates　of　the　respective　oxides；　singly　or　in　mixture；　to　the　paper；　alternating　with　solutions　of　the　ammonia…tartrate　or　the　ammonia…citrate　of　iron；　the　latter　solution　being　last　applied；　and　in　more　or　less　excess。　　I　purposely　avoid　stating　proportions；　as　I　have　not　yet　been　able　to　fix　upon　any　which　certainly　succeed。　Paper　so　prepared　and　dried　takes　a　negative　picture；　in　a　time　varying　from　half　an　hour　to　five　or　six　hours；　according　to　the　intensity　of　the　light；　and　the　impression　produced　varies　in　apparent　force　from　a　faint　and　hardly　perceptible　picture　to　one　of　the　highest　conceivable　fulness　and　richness　both　of　tint　and　detail；　the　color　being　in　this　case　a　superb　velvety　brown。　This　extreme　richness　of　effect　is　not　produced　unless　lead　be　present；　either　in　the　ingredients　used；　or　in　the　paper　itself。　It　is　not；　as　I　originally　supposed；　due　to　the　presence　of　free　tartaric　acid。　　The　pictures　in　this　state　are　not　permanent。　They　fade　in　the　dark；　though　with　very　different　degrees　of　rapidity；　some　（especially　if　free　tartaric　or　citric　acid　be　present）　in　a　few　days；　while　others　remain　for　weeks　unimpaired；　and　require　whole　years　for　their　total　obliteration。　But　though　entirely　faded　out　in　appearance；　the　picture　is　only　rendered　dormant；　and　may　be　restored；　changing　its　character　from　negative　to　positive；　and　its　colors　from　brown　to　black；　（in　the　shadows）；　by　the　following　process：A　bath　being　prepared　by　pouring　a　small　quantity　of　solution　of　pernitrate　of　mercury　into　a　large　quantity　of　water；　and　letting　the　subnitrated　precipitates　subside；　the　picture　may　be　immersed　in　it；　（carefully　and　repeatedly　clearing　off　all　air　bubbles；）　and　allowed　to　remain　till　the　picture　（if　any　where　visible；）　is　entirely　destroyed；　or　if　faded；　till　it　is　judged　sufficient　from　previous　experience；　a　term　which　is　often　marked　by　the　appearance　of　a　feeble　positive　picture；　of　a　bright　yellow　hue；　on　the　pale　yellow　ground　of　the　paper。　　A　long　time　（several　weeks）　is　often　required　for　this；　but　heat　accelerates　the　action；　and　it　is　often　completed　in　a　few　hours。　　In　this　state　the　picture　is　to　be　very　thoroughly　rinsed　and　soaked　in　pure　warm　water；　and　then　dried。　　It　is　then　to　be　well　ironed　with　a　smooth　iron；　heated　so　as　barely　not　to　injure　the　paper；　placing　it；　for　greater　security　against　scorching；　between　clean　smooth　paper。　If　then　the　process　have　been　successful；　a　perfectly　black　positive　picture　is　at　once　developed。　　At　first　it　most　commonly　happens　that　the　whole　picture　is　sooty　or　dingy　to　such　a　degree　that　it　is　condemned　as　spoiled；　but　on　keeping　it　between　the　leaves　of　a　book；　especially　in　a　moist　atmosphere；　by　extremely　slow　degrees　this　dinginess　disappears；　and　the　picture　disengages　itself　with　continually　increasing　sharpness　and　clearness；　and　acquires　the　exact　effect　of　a　copper…plate　engraving　on　a　paper　more　or　less　tinted　with　a　pale　yellow。

I　ought　to　observe；　that　the　best　and　most　uniform　specimens　which　I　have　procured　have　been　on　paper　previously　washed　with　certain　preparations　of　uric　acid；　which　is　a　very　remarkable　and　powerful　photographic　element。　　The　intensity　of　the　original　negative　picture　is　no　criterion　of　what　may　be　expected　in　the　positive。　It　is　from　the　production　by　one　and　the　same　action　of　light；　of　either　a　positive　or　negative　picture　according　to　the　subsequent　manipulations；　that　I　have　designated　the　process；　thus　generally　sketched　out；　by　the　term　Amphitype；a　name　suggested　by　Mr。　Talbot；　to　whom　I　communicated　this　singular　result；　and　to　this　process　or　class　of　processes　（which　I　cannot　doubt　when　pursued　will　lead　to　some　very　beautiful　results；）　I　propose　to　restrict　the　name　in　question；　though　it　applies　even　more　appropriately　to　the　following　exceedingly　curious　and　remarkable　one；　in　which　silver　is　concerned：

At　the　last　meeting　I　announced　a　mode　of　producing；　by　means　of　a　solution　of　silver；　in　conjunction　with　ferro…tartaric　acid；　a　dormant　picture　brought　into　a　forcible　negative　impression　by　the　breath　or　moist　air。　　（See　Cyanotype。）　The　solution　then　described；　and　which　had　at　that　time　been　prepared　some　weeks；　I　may　here　incidentally　remark；　has　retained　its　limpidity　and　photogenic　properties；　quite　unimpaired　during　the　whole　year　since　elapsed；　and　is　now　as　sensitive　as　ever；　a　property　of　no　small　value。　　Now；　when　a　picture　（for　example　an　impression　from　an　engraving）　is　taken　on　paper　washed　with　this　solution；　it　shows　no　sign　of　a　picture　on　its　back；　whether　that　on　its　face　is　developed　or　not；　but　if；　while　the　actinic　influence　is　still　fresh　upon　the　face；　（i。e。；　as　soon　as　it　is　removed　from　the　light）；　the　back　be　exposed　for　a　very　few　seconds　to　the　sunshine；　and　then　removed　to　a　gloomy　place；　a　positive　picture；　the　exact　complement　of　the　negative　one　on　the　other　side；　though　wanting　of　course　in　sharpness　if　the　paper　be　thick；　slowly　and　gradually　makes　its　appearance　there；　and　in　half　an　hour　or　an　hour　acquires　a　considerable　intensity。　I　ought　to　mention　that　the　〃ferro…tartaric　acid〃　in　question　is　prepared　by　precipitating　the　ferro…tartrate　of　ammonia　（ammonia…tartrate　of　iron）　by　acetate　of　lead；　and　decomposing　the　precipitate　by　dilute　sulphuric　acid。　When　lead　is　used　in　the　preparation　of　Amphitype　paper；　the　parts　upon　which　the　light　has　acted　are　found　to　be　in　a　very　high　degree　rendered　water　proof。Sir　J。　Herschel。

This　process　is　a　new　invention　of　our　countryman；　J。　A。　Whipple；　Esq。；　of　Boston；　and　has　been　patented　by　M。　A。　Root；　Esq。；　of　Philadelphia。　It　will　be　seen；　however；　from　the　previous　pages　of　my　work　that　Mr。　Root　is　mistaken　in　regard　to　his　being　the　first　improvement　patented　in　this　country；　althongh　it　is　unquestionably　the　first　by　an　American。　Of　this　improvement　Mr。　Root　says：

VI。　〃CRAYON　DAGUERREOTYPE。〃

〃The　improvement　to　which　you　refer　is　denominated　〃The　Crayon　Daguerreotype。〃　　This　invention　made　by　Mr。　J。　A。　Whipple；　is　the　only　improvement　in　Daguerreotyping；　I　believe；　for　which　Letters　Patent　for　the　United　States　were　ever　issued。　The　pictures　produced　by　this　processwhich　is　of　the　simplest　description　imaginablehave　the　appearance　and　effect　of　very　fine　〃Crayon　Drawings；〃　from　which　the　improvement　takes　its　name。　Some　of　our　most　distinguished　artists　have　given　it　their　unqualified　admiration。　　Among　them；　our　Mezzotinto　Engravers；　especially　John　Sartain；　Esq。；　who；　from　his　rich　embellishments　to　most　of　the　leading　Magazines　and　Annuals　of　the　country；　as　well　as　from　the　celebrity　of　the　superb　Magazine　which　bears　his　name；　is　so　well　known　and　so　well　qualified　to　judge　of　its　merits。　　As　an　auxiliary　to　the　artist；　in　furnishing　heads　to　the　Magazines；　or　other　works；　it　is　invaluable；　the　great　object　which　it　accomplishes　being　to　give　a　finer　effect　and　more　distinct　expression　to　all　the　features　the　whole　power　of　the　instrument　being　directed　to；　and　confined　to　the　head。〃

〃The　late　hour　at　which　this　subject　has　been　brought　to　our　notice　prevents　so　full　a　description　as　we　would　otherwise　have　been　glad　to　furnish。　　The　New　England　States　have　been　disposed　of；　negotiations　for　any　of　the　others　can　be　made　through　M。　A。　Root；　140　Chestnut　street；　Philadelphia。〃

〃A　series　of　beautiful　portraits　are　about　being　prepared　by　the　〃Crayton　Process〃　for　the　express　purpose　of　being　placed　on　the　exhibition　at　the　〃Art　Union；〃　when　amateurs；　artists；　and　the　public　generally　will　have　an　opportunity　of　witnessing　its　effect。　We　are　especially