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

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umes　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　exceedingly　sensitive　surface。　　The　pictures　on　this　paper　are　easily　fixed　by　salt　or　weak　ammonia。

NITRATE　OF　SILVER。This　salt　in　a　state　of　purity；　does　not　appear　to　be　sensibly　affected　by　light；　but　the　presence　of　the　smallest　portion　of　organic　matter　renders　it　exceedingly　liable　to　change　under　luminous　influence。

If　a　piece　of　nitrated　paper　is　placed　upon　hot　iron；　or　held　near　the　fire；　it　will　be　found　that　at　a　heat　just　below　that　at　which　the　paper　chars；　the　salt　is　decomposed。　Where　the　heat　is　greatest；　the　silver　is　revived；　and　immediately　around　it；　the　paper　becomes　a　deep　blue；　beyond　this　a　pretty　decided　green　color　results；　and　beyond　the　green；　a　yellow　or　yellow　brown　stain　is　made。　This　exhibits　a　remarkable　analogy　between　heat　and　light；　before　spoken　of　in　chap。　IIand　is　of　some　practical　importance　in　the　preparation　of　the　paper。

PRISMATIC　ANALYSIS。The　method　of　accomplishing　the　prismatic　decomposition　of　rays　of　light　by　the　spectrum　has　already　been　described　on　pages　22　and　23。　　The　color　of　the　impressed　spectrum；　on　paper　washed　with　nitrate　of　silver；　is　at　first；　a　pale　brown；　which　passes　slowly　into　a　deeper　shade；　that　portion　corresponding　with　the　blue　rays　becoming　a　blue　brown；　and　under　the　violet　of　a　peculiar　pinkey　shade；　a　very　decided　green　tint；　on　the　point　which　corresponds　with　the　least　refrangible　blue　rays；　may　be　observed；　its　limits　of　action　being　near　the　centre　of　the　yellow　ray；　and　its　maximum　about　the　centre　of　the　blue；　although　the　action　up　to　the　edge　of　the　violet　ray　is　continued　with　very　little　diminution　of　effect；　beyond　this　point　the　action　is　very　feeble。

When　the　spectrum　is　made　to　act　on　paper　which　has　been　previously　darkened；　by　exposure　to　sunshine　under　cupro…sulphate　of　ammonia；　the　phenomena　are　materially　different。　The　photographic　spectrum　is　lengthened　out　on　the　red　or　negative　side　by　a　faint　but　very　visible　red　portion；　which　extends　fully　up　to　the　end　of　the　red　rays；　as　seen　by　the　naked　eye。　The　tint　of　the　general　spectrum；　too；　instead　of　brown　is　dark　grey；　passing；　however；　at　its　most　refracted　or　positive　end　into　a　ruddy　brown。

In　its　Photographic　application；　the　nitrate　of　silver　is　the　most　valuable　of　the　salts　of　that　metal；　as　from　it　most　of　the　other　argentine　compounds　can　be　prepared；　although　it　is　not　of　itself　sufficiently　sensible　to　light　to　render　it　of　much　use。

CHLORIDE　OF　SILVER。This　salt　of　silver；　whether　in　its　precipitated　state；　or　when　fused；　changes　its　color　to　a　fine　bluish　grey　by　a　very　short　exposure　to　the　sun's　rays。　If　combined　with　a　small　quantity　of　nitrate；　the　change　is　more　rapid；　it　attains　a　deep　brown；　then　slowly　passes　into　a　fine　olive；　and　eventually；　after　a　few　weeks；　the　metalic　silver　is　seen　to　be　revived　on　the　surface　of　the　salt。　Great　differences　of　color　are　produced　on　chlorides　of　silver　precipitated　by　different　muriates。　　Nearly　every　variety　in　combination　with　the　nitrate；　becomes　at　last　of　the　same　olive　color；　the　following　exam