the history and practice of the art of photography-第7章

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variety　in　combination　with　the　nitrate；　becomes　at　last　of　the　same　olive　color；　the　following　examples；　therefore；　have　reference　to　a　few　minutes　exposure；　only；　to　good　sunshine；　it　must　also　be　recollected　that　the　chloride　of　silver　in　these　cases　is　contaminated　with　the　precipitant。

Muriate　of　ammonia　precipitates　chloride　to　darken　to　a　fine　chocolate　brown；　whilst　muriate　of　lime　produces　a　brick…red　color。　Muriates　of　potash　and　soda　afford　a　precipitate；　which　darkens　speedly　to　a　pure　dark　brown；　and　muriatic　acid；　or　aqueous　chlorine；　do　not　appear　to　increase　the　darkening　power　beyond　the　lilac　to　which　the　pure　chloride　of　silver　changes　by　exposure。　This　difference　of　color　appears　to　be　owing　to　the　admixture　of　the　earth　or　alkali　used　with　the　silver　salt。

The　prismatic　impression　on　paper　spread　with　the　chloride　of　silver　is　often　very　beautifully　tinted；　the　intensity　of　color　varying　with　the　kind　of　muriate　used。　　Spread　paper　with　muriate　of　ammonia　or　baryta　and　you　obtain　a　range　of　colors　nearly　corresponding　with　the　natural　hues　of　the　prismatic　spectrum。　Under　favorable　circumstances　the　mean　red　ray；　leaves　a　red　impression；　which　passes　into　a　green　over　the　space　occupied　by　the　yellow　rays。　Above　this　a　leaden　hue　is　observed；　and　about　the　mean　blue　ray；　where　the　action　is　greatest；　it　rapidly　passes　through　brown　into　black；　and　through　the　most　refrangible　rays　it　gradually　declines　into　a　bluish　brown；　which　tint　is　continued　throughout　the　invisible　rays。　　At　the　least　refrangible　end　of　the　spectrum；　the　very　remarkable　phenomenon　has　been　observed；　of　the　extreme　red　rays　exerting　a　protecting　influence；　and　preserving　the　paper　from　that　change；　which　it　would　otherwise　undergo；　under　the　influence　of　the　dispersed　light　which　always　surrounds　the　spectrum。　Not　only　the　extreme　red　ray　exerts　this　very　peculiar　property；　but　the　ordinary　red　ray　through　nearly　its　whole　length。

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