the history and practice of the art of photography-第5节

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forests；　the　enumerable　varieties　of　flowers；　the　glowing　ruby；　jasper；　topaz；　amethist；　and　emerald；　the　brilliant　diamond；　and　all　the　rich　and　varied　hues　of　nature；　both　animate　and　inanimate。　　＇hipho_3。gif＇

Now；　if　we　allow　this　prismatic　spectrum　（b。　fig。　　3。）　to　fall　upon　any　surface　（as　at　c。）　prepared　with　a　sensitive　photographic　compound；　we　shall　find　that　the　chemical　effect　produced　bears　no　relation　to　the　intensity　of　the　light　of　any　particular　colored　ray；　but　that；　on　the　contrary；　it　is　dispersed　over　the　largest　portion　of　the　spectrum；　being　most　energetic　in　the　least　luminous　rays；　and　ever　active　over　an　extensive　space；　where　no　traces　of　light　can　be　detected。　　Fig。　　4；　will　give　the　student　a　better　idea　of　this　principle。　　It　is　a　copy　of　the　kind　of　impression　which　the　spectrum；　spoken　of；　would　make　on　a　piece　of　paper　covered　with　a　very　sensitive　photographic　preparation。　The　white　space　a。　　corresponds　with　the　most　luminous；　or　yellow　ray；　（5；　fig。　　3）　over　limits　of　which　all　chemical　change　is　prevented。　A　similar　action　is　also　produced　by　the　lower　end　of　the　red　ray　c；　but　in　the　upper　portion；　however　we　find　a　decided　change　（as　at　d）。　The　most　active　chemical　change；　you　will　percieve；　is　produced　by　the　rays　above　the　yellow　a；　viz。　　4；　3；　2　and　1　（as　at　b）　the　green　（4）　being　the　least　active；　and　the　blue　（3）　and　violet　（1）　rays　the　most　so；　the　action　still　continuing　far　beyond　the　point　b　which　is　the　end　of　the　luminous　image。　　＇hipho_4。gif＇

Suppose　we　wish　to　copy　by　the　Daguerreotype；　or　Calotype　process；　any　objects　highly　coloredblue；　red　and　yellow；　for　instance　predominating　the　last　of　course　reflects　the　most　light；　the　blue　the　least；　but　the　rays　from　the　blue　surface　will　make　the　most　intense　impression；　whilst　the　red　radiations　are　working　very　slowly；　and　the　yellow　remains　entirely　inactive。　　This　accounts　for　the　difficulty　experienced　in　copying　bright　green　foliage；　or　warmly　colored　portraits；　a　large　portion　of　the　yellow　and　red　rays　entering　into　the　composition　of　both　and　the　imperfections　of　a　Daguerreotype　portrait　of　a　person　with　a　freckled　face　depends　upon　the　same　cause。

A　yellow；　hazy　atmosphere；　even　when　the　light　is　very　bright；　will　effectually　prevent　any　good　photographic　result　and　in　the　height　of　summer；　with　the　most　sensative　process；　it　not　unfrequently　happens　that　the　most　anoying　failures　arise　from　this　agency　of　a　yellow　medium。　　A　building　painted　of　a　yellow　color；　which　may　reflect　the　sun's　rays　directly　into　the　operator's　room　will　have　the　same　effect。　Daguerreotypists；　being　ignorant　of　these　facts；　are　very　apt　to　charge　their　want　of　success　to　the　plates；　or　chemicals；　or　any　thing　but　the　real　cause；　and　it　would　be　well　to　bear　these　facts　constantly　in　mind　and　as　far　as　possible　avoid　them。　This；　may　be　accomplished；　in　a　measure；　by　a　choice　of　location　or　by　having　the　glass　of　your　windows　tinged　with　blue；　or　a　screen　of　thin　blue　paper　may　be　interposed　between　the　light　and　sitter。　In　selecting　subjects；　all　striking　contrasts　in　color　should　be　avoided；　and　sitters　for　portraits　should　be　cautioned　not　to　wear　anything　that　may　produce　the　effect　spoken　of　dark　dresses　always　being　the　best。

The　action　of　light　both　combines　and　decomposes　bodies。　For　instance；　chlorine　and　hydrogen　will　remain　in　a　glass　vessel　without　alteration　if　kept　in　the　dark；　but　if　exposed　to　the　rays　of　the　sun；　they　immediately　enter　into　combination；　and　produce　hydrochloric　acid。　On　the　other　hand；　if　colorless　nitric　acid　be　exposed　to　the　sun；　it　becomes　yellow；　then　changes　to　red；　and　oxygen　is　liberated　by　the　partial　decomposition　effected　by　the　solar　rays。

Of　the　organic　substances　none　are　more　readily　acted　upon　by　light　than　the　various　combinations　of　silver。

Of　these　some　are　more；　and　others　less　sensitive。　　If　Chloride　of　silver；　which　is　a　white　precipitate　formed　by　adding　chloride　of　sodium　（common　salt）　to　a　solution　of　nitrate　of　silver；　be　exposed　to　diffused　light；　it　speedily　assumes　a　violet　tint；　and　ultimately　becomes　nearly　black。　With　iodide　of　silver；　bromide　of　silver；　ammonio…nitrate　of　silver；　and　other　salts　of　this　metal；　the　result　will　be　much　the　same。

Some　bodies；　which　under　the　influence　of　light；　undergo　chemical　changes；　have　the　power　of　restoring　themselves　to　their　original　condition　in　the　dark。　　This　is　more　remarkably　displayed　in　the　iodide　of　platinum；　which　readily　recieves　a　photogenic　image　by　darkening　over　the　exposed　surfaces；　but　speedily　loses　it　by　bleaching　in　the　dark。　The　ioduret　of　Daguerre's　plate；　and　some　other　iodides；　exhibit　the　same　peculiarityThis　leads　us　to　the　striking　fact；　that　bodies　which　have　undergone　a　change　of　estate　under　the　influence　of　day…light　have　some　latent　power　by　which　they　can　renovate　themselves。　Possibly　the　hours　of　night　are　as　necessary　to　inanimate　nature　as　they　are　to　the　animate。　　During　the　day；　an　excitement　which　we　do　not　heed；　unless　in　a　state　of　disease；　is　maintained　by　the　influence　of　light　and　the　hours　of　repose；　during　which　the　equilibrium　is　restored；　are　absolutely　necessary　to　the　continuance　of　health。

Instead　of　a　few　chemical　compounds　of　gold　and　silver；　which　at　first　were　alone　supposed　to　be　photographic；　we　are　now　aware　that　copper；　platinum；　lead；　nikel；　and　indeed；　probably　all　the　elements；　are　equally　liably　to　change　under　the　sun's　influence。　　This　fact　may　be　of　benefit　to　engravers；　for　if　steel　can　be　made　to　take　photographic　impressions；　the　more　laborious　process　of　etching　may　be　dispensed　with。　In　fact；　in　the　latter　part　of　this　work；　a　process　is　described　for　etching　and　taking　printed　impressions　from　Daguerreotype　plates。　As　yet　this　process　has　produced　no　decided　beneficial　results　but　future　experiments　may　accomplish　some　practical　discovery　of　intrinsic　value　to　the　art　of　engraving。

A　very　simple　experiment　will　prove　how　essential　light　is　to　the　coloring　of　the　various　species　comprising　the　vegetable　and　animal　kingdoms。　If　we　transplant　any　shrub　from　the　light　of　day　into　a　dark　cellar；　we　will　soon　see　it　lose　its　bright　green　color；　and　become　perfectly　white。

Another　effect　of　light　is　that　it　appears　to　impart　to　bodies　some　power　by　which　they　more　readily　enter　into　chemical　combination　with　others。　We　have　already　said　that　chlorine　and　hydrogen；　if　kept　in　the　dark；　will　remain　unaltered；　but　if　the　chlorine　alone　be　previously　exposed　to　the　sun；　the　chlorine　thus　solarised　will　unite　with　the　hydrogen　in　the　dark。　Sulphate　of　iron　will　throw　down　gold　or　silver　from　their　solutions　slowly　in　the　dark；　but　if　either　solution　be　first　exposed　to　sunshine；　and　the　mixture　be　then　made；　in　the　dark；　the　precipitation　takes　place　instantly。　Here　is　again；　evidence　of　either　an　absorption　of　some　material　agent　from　the　sunbeam；　or　an　alteration　in　the　chemical　constitution　of　the　body。　It　was　from　understanding　these　principles　and　applying　them　that　philosophers　were　enabled　to　produce　the　Calotype；　Daguerreotype；　&c。　For　the　effects　and　action　of　light　on　the　camera；　see　Chapter　V。

Some　advances　have　been　made　towards　producing　Photographic　impressions　in　colorthe　impossibility　of　which　some　of　our　best　and　oldest　artists　have　most　pertinaciously　maintained。　The　colored　image　of　the　spectrum　has　been　most　faithfully　copied；　ray　for　ray；　on　paper　spread　with　the　juice　of　the　Cochorus　Japonica；　（a　species　of　plant）　and　the　fluoride　of　silver；　and　on　silver　plate　covered　with　a　thin　film　of　chloride。　The　day　may　be　still　remote　when　this　much　to　be　desired　decideratum　shall　be　accomplished　in　portrait　taking；　but　I　am　led　to　hope　that　future　experiments　may　master　the　secret　which　now　causes　it　to　be　looked　upon；　by　many；　as　an　impossibility。

That　great　advantages　have　resulted；　and　that　greater　still　will　result　from　the　discovery　of　the　Photographic　art；　few　will　deny。　The　faithful　manner　in　which　it　copies　nature；　even　to　the　most　minute　details；　renders　it　of　much　value　to　the　painter；　but　a　few　minutes　sufficing　to　take　a　view　that　formerly　would　have　occupied　several　days。　Its　superiority　in　portraits；　over　miniature　or　oil　painting　has　been　tacitly　acknowledged　by　the　thousands　who　employ　it　to　secure　their　own；　or　a　friends　likeness；　and　by　the　steady　increase　in　the　number　of　artists　who　are　weekly；　aye　daily　springing　up　in　every　town　and　village　in　the　land。



CHAP。　III。

SYNOPSIS　OF　MR。　HUNT'S　TREATISE　ON　〃THE　INFLUENCE　OF　THE　SOLAR　RAYS　ON　COMPOUND　BODIES；　WITH　ESPECIAL　REFERENCE　TO　THEIR　PHOTOGRAPHIC　APPLICATION。〃


OXIDE　OF　SILVER　exposed　for　a　few　hours　to　good　sunshine；　passes　into　a　more　decided　olive　color；　than　characterises　it　when　first　prepared　by　precipitation　from　nitrate　of　silver。　Longer　exposure　renders　this　color　very　much　lighter；　and　the　covered　parts；　are　found　much　darker；　than　those　on　which　the　light　has　acted　directly。　　In　some　instances　where　the　oxide　of　silver　has　been　spread　on　the　paper　a　decided　whitening　process　in　some　parts；　after　a　few　days　exposure；　is　noticed。　Oxide　of　silver　disolved　in　ammonia　is　a　valuable　photographic　fluid；　one　application　of　a　strong　solution　forming　an　exceedi