the history and practice of the art of photography-第6部分

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In　photographic　drawing　this　salt　is　of　the　utmost　importance。　Mr。　Talbot's　application　of　it　will　be　given　hereafter　in　another　portion　of　this　work。

IODIDE　OF　SILVERPerfectly　pure；　undergoes　very　little　change　under　the　influence　of　light　or　heat；　but　if　a　very　slight　excess　of　the　nitrate　of　silver　be　added　it　becomes　infinitely　more　senitive　than　the　chloride

The　spectrum　impressed　upon　paper　prepared　with　a　weak　solution　of　the　hydriodate　of　potash　presents　some　very　remarkable　peculiarities。　The　maximum　of　intensity　is　found　at　the　edge　of　the　most　refrangible　violet　rays；　or　a　little　beyond　it；　varying　slightly　according　to　the　kind　of　paper　used；　and　the　quantity　of　free　nitrate　of　silver　present。　The　action　commences　at　a　point　nearly　coincident　with　the　mean　red　of　the　luminous　spectrum；　where　it　gives　a　dull　ash　or　lead　color；　while　the　most　refrangible　rays　impress　a　ruddy　snuff…brown；　the　change　of　tint　coming　on　rather　suddenly　about　the　end　of　the　blue　or　beginning　of　the　violet　rays　of　the　luminous　spectrum。　Beyond　the　extreme　violet　rays；　the　action　rapidly　diminishes；　but　the　darkening　produced　by　these　invisible　rays；　extends　a　very　small　space　beyond　the　point　at　which　they　cease　to　act　on　the　chloride　of　silver。

In　its　photographic　application；　it　is；　alone；　of　very　little　use；　but　in　combination　with　other　reagents　it　becomes　exquisitely　sensitive。　With　gallic　acid　and　the　ferrocyanate　of　potash　it　forms　two　of　the　most　sensitive　photographic　solutions　with　which　we　are　acquainted。　These　are　used　in　the　calotype　process。

IODURET　OF　SILVER。If　upon　a　plate　of　polished　silver　we　place　a　small　piece　of　iodine；　and　apply　the　heat　of　a　lamp　beneath　the　plate　for　a　moment；　a　system　of　rings　is　speedily　formed。　The　first　ring；　which　spreading　constantly　forms　the　exterior　of　the　circle；　is　of　a　bright　yellow　color；　within　this；　there　arises；　sucessively；　rings　of　green；　red　and　blue　colors；　and　then　again　a　fine　yellow　circle；　centred　by　a　greyish　spot　on　the　place　occupied　by　the　iodine。　　On　exposing　these　to　the　light；　the　outer　yellow　circle　almost　instantly　changes　color；　the　others　slowly；　in　the　order　of　their　position；　the　interior　yellow　circle　resisting　for　a　long　time　the　solar　influence。　These　rings　must　be　regarded　as　films　of　the　ioduret　of　silver；　varying；　not　only　in　thickness；　but　in　the　more　or　less　perfect　states　of　combination　in　which　the　iodine　and　metal　are。　The　exterior　circle　is　an　ioduret　in　a　very　loose　state　of　chemical　agregation；　the　attractive　forces　increase　as　we　proceed　towards　the　centre；　where　a　well　formed　ioduret；　or　probably　a　true　iodide　of　silver；　is　formed；　which　is　acted　upon　by　sunlight　with　difficulty。　　The　exterior　and　most　sensitive　film　constitutes　the　surface　of　Daguerreotype　plates。　The　changes　which　these　colored　rings　undergo　are　remarkable；　by　a　few　minutes　exposure　to　sunlight；　an　inversion　of　nearly　all　the　colors　takes　place；　the　two　first　rings　becoming　a　deep　olive　green；　and　a　deep　blue　inclining　to　black。

The　nature　of　the　change　which　the　ioduret　of　silver　undergoes　on　Daguerreotype　plates；　through　the　action　of　light；　Mr。　Hunt　considers　to　be　a　decided　case　of　decomposition；　and　cites　several　circumstances　in　proof　of　his　position。　These　with　other　facts　given　by　Mr。　Hunt　in　his　great　work　on　the　Photographic　art；　but　to　volumnious　to　include　in　a　volume　of　the　size　to　which　I　am　obliged　to　cofine　myself；　should　be　thoroughly　studied　by　all　Daguerreotypists。

PRISMATIC　ANALYSIS。The　most　refrangible　portion　of　the　spectrum；　（on　a　Daguerreotype　plate）　appears；　after　the　plate　has　been　exposed　to　the　vapor　of　mercury；　to　have　impressed　its　colors；　the　light　and　delicate　film　of　mercury；　which　covers　that　portion；　assuming　a　fine　blue　tint　about　the　central　parts；　which　are　gradually　shaded　off　into　a　pale　grey；　and　this　is　again　surrounded　by　a　very　delicate　rose　hue；　which　is　lost　in　a　band　of　pure　white。　Beyond　this　a　protecting　influence　is　powerfully　exerted；　and　notwithstanding　the　action　of　the　dispersed　light；　which　is　very　evident　over　the　plate；　a　line　is　left；　perfectly　free　from　mercurial　vapor；　and　which；　consequently；　when　viewed　by　a　side　light；　appears　quite　dark。　The　green　rays　are　represented　by　a　line　of　a　corresponding　tint；　considerably　less　in　size　than　the　luminous　green　rays。　The　yellow　rays　appear　to　be　without　action；　or　to　act　negatively；　the　space　upon　which　they　fall　being　protected　from　the　mercurial　vapor；　and　it　consequently　is　seen　as　a　dark　band。　A　white　line　of　vapor　marks　the　place　of　the　orange　rays。　The　red　rays　effect　the　sensitive　surface　in　a　peculiar　manner；　and　we　have　the　mercurial　vapor；　assuming　a　molecular　arrangement　which　gives　to　it　a　fine　rose　hue；　this　tint　is　surrounded　by　a　line　of　white　vapor；　shaded　at　the　lowest　extremity　with　a　very　soft　green。　　Over　the　space　occupied　by　the　extreme　red　rays；　a　protecting　influence　is　again　exerted；　the　space　is　retained　free　from　mercurial　vapor　and　the　band　is　found　to　surround　the　whole　of　the　least　refrangible　rays；　and　to　unite　itself　with　the　band　which　surrounds　the　rays　of　greatest　refrangibility。　This　band　is　not　equally　well　defined　throughout　its　whole　extent。　It　is　most　evident　from　the　extreme　red　to　the　green；　it　fades　in　passing　through　the　blue；　and　increases　again；　as　it　leaves　the　indigo；　until　beyond　the　invisible　chemical　rays　it　is　nearly　as　strong　as　it　is　at　the　calorific　end　of　the　spectrum。

Images　on　Daguerreotype　plates　which　have　been　completely　obliterated　by　rubbing　may　be　restored；　by　placing　it　in　a　tolerably　strong　solution　of　iodine　in　water。

BROMIDE　OF　SILVER。This　salt；　like　the　iodide；　does　not　appear　to　be　readily　changed　by　the　action　of　light；　but　when　combined　with　the　nitrate　of　silver　it　forms　a　very　sensitive　photographic　preparation。

Paper　prepared　with　this　salt；　blackens　over　its　whole　extent　with　nearly　equal　intensity；　when　submitted　to　the　prismatic　spectrum。　　The　most　characteristic　peculiarity　of　the　spectrum　is　its　extravagant　length。　Instead　of　terminating　at　the　mean　yellow　ray；　the　darkened　portion　extends　down　to　the　very　extremity　of　the　visible　red　rays。　In　tint　it　is　pretty　uniformly　of　a　grey…black　over　its　whole　extent；　except　that　a　slight　fringe　of　redness　is　perceptible　at　the　least　refracted　end。　　Beyond　the　red　ray；　an　extended　space　is　protected　from　the　agency　of　the　dispersed　light；　and　its　whiteness　maintained；　thus　confirming　the　evidence　of　some　chemical　power　in　action；　over　a　space　beyond　the　luminous　spectrum；　which　corresponds　with　the　rays　of　the　least　refrangibility。

This　salt　is　extensively　used　in　photographic　drawing。

PREPARATIONS　OF　GOLD。Chloride　of　Gold；　freed　from　an　excess　of　acid　is　slowly　changed　under　the　action　of　light；　a　regularly　increasing　darkness　taking　place　until　it　becomes　purple；　the　first　action　of　the　light　being　to　whiten　the　paper；　which；　if　removed　from　the　light　at　this　stage；　will　gradually　darken　and　eventually　develope　the　picture。　This　process　may　be　quickened　by　placing　the　paper　in　cold　water。

Chloride　of　gold　with　nitrate　of　silver　gives　a　precipitate　of　a　yellow　brown　color。　　Paper　impregnated　with　the　acetate　of　lead；　when　washed　with　perfectly　neutral　chloride　of　gold；　acquires　a　brownish…yellow　hue。　The　first　impression　of　light　seems　rather　to　whiten　than　darken　the　paper；　by　discharging　the　original　color；　and　substituting　for　it　a　pale　greyish　tint；　which　by　slow　degrees　increases　to　a　dark　slate　color；　but　if　arrested；　while　yet；　not　more　than　a　moderate　ash　grey；　and　held　in　a　current　of　steam；　the　color　of　the　parts　acted　upon　by　light　and　of　that　onlydarkens　immediately　to　a　deep　purple。

Here　I　must　leave　the　subject　of　the　action　of　light　upon　metalic　compounds　referring　to　Mr。　Hunts　work　for　any　further　information　the　student　may　desire　on　the　other　metalsas　I　find　myself　going　beyond　my　limits。　I　cannot；　however；　entirely　dismiss　the　subject　without　giving　a　few　examples　of　the　action　of　light　on　the　juices　of　plants；　some　of　which　produce　very　good　photographic　effect。

CORCHORUS　JAPONICAThe　juice　of　the　flowers　of　this　plant　impart　a　fine　yellow　color　to　paper；　and；　so　far　as　ascertained；　is　the　most　sensitive　of　any　vegetable　preparation；　but　owing　to　its　continuing　to　change　color　even　in　the　dark；　photographic　images　taken　on　paper　prepared　with　it　soon　fade　out。

WALL　FLOWER。This　flower　yields　a　juice；　when　expressed　with　alcohol；　from　which　subsides；　on　standing；　a　bright　yellow　finely　divided　faecula；　leaving　a　greenish…yellow　transparent　liquid；　only　slightly　colored　supernatant。　The　faecula　spreads　well　on　paper；　and　is　very　sensitive　to　light；　but　appears　at　the　same　time　to　undergo　a　sort　of　chromatic　analysis；　and　to　comport　itself　as　if　composed　of　two　very　distinct　coloring　principles；　very　differently　affected。　　The　one　on　which　the　intensity　and　sub…orange　tint　of　the　color　depends；　is　speedily　destroyed；　but　the　paper　is　not　thereby　fully　whitened。　A　paler　yellow　remains　as　a　residual　tint；　and　this　on　continued　exposure　to　the　light；　slowly　darkens　to　brown。　Exposed　to　the　spectrum；　the　paper　is　first　reduced　nearly　to　whiteness　in　the　region　of　the　blue　and　violet　rays。　More　slowly；　an　insulated　solar　image　is　whitened　in　the　less　refrangible　portion　of　the　red。　　Continue　the　exposure；　and　a　brown　impression　begins　to　be　percieved　in　the　midst　of　the　white　streak；　which　darkens　slowly　over　the　region　between　the　lower　blue　and　extreme　violet　rays。

THE　RED　POPPY　yields　a　very　beautiful　red　color；　which　is　entirely　destroyed　by　light。　　When　perfectly　dried　on　paper　the　color　becomes　blue。　This　blue　color　is　speedily　discharged　by　exposure　to　the　sun's　rays；　and　papers　prepared　with　it　afford　very　interesting　photographs。　Future　experiments　will　undoubtedly　more　fully　develope　the　photogenic　properties　of　flowers；　and　practically　apply　them。

Certain　precautions　are　necessary　in　extracting　the　coloring　matter　of　flowers。　　The　petals　of　fresh　flowers；　carefully　selected；　are　crushed　to　a　pulp　in　a　mortar；　either　alone　or　with　the　addition　of　a　litte　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