ORTHEPRODUCTIONOFPICTURESTHROUGHTHEAGENCYOFLIGHT。
CONTAININGALLTHEINSTRUCTIONSNECESSARYFORTHECOMPLETE
PRACTICEOFTHEDAGUERREANANDPHOTOGENICART，BOTHONMETALIC，PLATESANDONPAPER。
TOEDWARDANTHONY，ESQ。，ANESTEEMEDFRIEND。
Whosegentlemanlydeportment，liberalfeelings，andstrictintegrityhavesecuredhimalargecircleoffriends，thisworkisRespectfullyDedicatedBytheAUTHOR。
PREFACE。
TheobjectofthislittleworkistofillavoidmuchcomplainedofbyDaguerreotypists——particularlyyoungbeginers。
Theauthorhaswaitedalongtimeinhopesthatsomemoreablepenwouldbedevotedtothesubject，butthewantsofthenumerous，andconstantlyincreasing，class，justmentioned，induceshimtowaitnolonger。
AlltheEnglishworksonthesubject——particularlyonthepracticalapplication，ofPhotogenicdrawing——aredeficientinmanyminutedetails，whichareessentialtoacompleteunderstandingoftheart。
Manyoftheirmethodsofoperatingareentirelydifferentfrom，andmuchinferiorto，thosepractisedintheUnitedStates：
theirapparatus，also，cannotcomparewithoursforcompleteness，utilityorsimplicity。
Ishall，therefore，confinemyselfprincipally——sofarasPhotogenicdrawinguponmetalicplatesisconcerned——tothemethodspractisedbythemostcelebratedandexperiencedoperators，drawinguponFrenchandEnglishauthorityonlyincaseswhereIfinditessentialtothepurposeforwhichIdesignmywork，namely：furnishingacompletesystemofPhotography;
suchanoneaswillenableanygentleman，orlady，whomaywishtopractisetheart，forprofitoramusement，todosowithoutthetroubleandexpenseofseekinginstructionfromprofessors，whichinmanycaseswithinmyownknowledgehaspreventedpersonsfromembracingtheprofession。
ToEnglishauthorsIamprincipallyindebtedforthatportionofmyworkrelatingtoPhotogenicdrawingonpaper。
Tothemweowenearlyallthemostimportantimprovementsinthatbranchoftheart。Besides，ithasbeenbutseldomattemptedintheUnitedStates，andthenwithoutanydecidedsuccess。
OftheseattemptsIshallspeakfurtherintheHistoricalportionofthisvolume。
Everythingessential，therefore，toacompleteknowledgeofthewholeart，comprisingallthemostrecentdiscoveriesandimprovementsdowntothedayofpublicationwillbefoundhereinlaiddown。
INTRODUCTION
NewYork，January27，1849。
E。ANTHONY，ESQ。
DearSir，——Insubmitingtheaccompanying"HistoryandPracticeofPhotographytoyourperusal，andforyourapprobation，Idosowiththeutmostconfidenceinyourabilityasapracticalman，longengagedinthescienceofwhichittreats，aswellasyourknowledgeofthesciencesgenerally;aswellasyourregardforcandor。
Toyou，therefore，IleavethedecisionwhetherornoIhaveaccomplishedmypurpose，andproducedaworkwhichmaynotonlybeofpracticalbenefittotheDaguerreanartist，butofgeneralinteresttothereadingpublic，andyourdecisionwillinfluencemeinofferingitfor，orwithholdingitfrom，publication。
Ifitmeetsyourapprobation，Iwouldmostrespectfullyaskpermissiontodedicateittoyou，subscribingmyself，Withesteem，Evertrulyyours，HENRYH。SNELLING
NewYork，February1st，1849。
Mr。H。H。SNELLING。
DearSir——YournoteofJanuary27th，requestingpermissiontodedicatetomeyour"HistoryandPracticeofPhotography，"
Iesteemahighcompliment，particularlysinceIhavereadthemanuscriptofyourwork。
Suchatreatisehaslongbeenneeded，andthemannerinwhichyouhavehandledthesubjectwillmakethebookasinterestingtothereadingpublicasitisvaluabletotheDaguerreanartist，ortheamateurdabblerinPhotography。
IhavereadnearlyallofthemanyworksuponthisartthathaveemanatedfromtheLondonandParispresses，andIthinkthereaderwillfindinyoursthepithofthemall，withmuchpracticalandusefulinformationthatI
donotremembertohaveseencommunicatedelsewhere。
ThereismuchinittoarousethereflectiveandinventivefacultiesofourDaguerreotypists。Theyhaveheretoforestumbledalongwithverylittleknowledgeofthetruetheoryoftheirart，andyetthequalityoftheirproductionsisfarinadvanceofthoseoftheFrenchandEnglishartists，mostofwhoseestablishmentsIhavehadthepleasureofvisitingIfeeltherefore，thatwhenasufficientamountoftheoreticknowledgeshallhavebeenaddedtothispracticalskillonthepartofouroperators，andwhentheyshallhavebeenmadefullyacquaintedwithwhathasbeenattainedorattemptedbyothers，astillgreateradvanceintheartwillbemanifested。
AGOODDaguerreotypistisbynomeansameremachinefollowingacertainsetoffixedrules。Successinthisartrequirespersonalskillandartistictastetoamuchgreaterdegreethantheunthinkingpublicgenerallyimagine;infactmorethanisimaginedbynine-tenthsoftheDaguerreotypiststhemselves。
Andweseeasanaturalresult，thatwhilethebusinessnumbersitsthousandsofvotaries，butfewrisetoanydegreeofeminence。
Itisbecausetheylookupontheirbusinessasameremechanicaloperation，andhavingnoaimorpridebeyondtheearningoftheirdailybread，theycalculatewhatwillbeafairpercentageonthecostoftheirplate，case，andchemicals，leavingMIND，whichisasmuchCAPITALasanythingelse（whereitisexercised，）
entirelyoutofthequestion。
TheartoftakingphotographsonPAPER，ofwhichyourworktreatsatconsiderablelength，hasasyetattractedbutlittleattentioninthiscountry，thoughdestined，asIfullybelieve，toattainanimportancefarsuperiortothattowhichtheDaguerreotypehasrisen。
TheAmericanmindneedsawakingupuponthesubject，andI
thinkyourbookwillgiveapowerfulimpulseinthisdirection。
InGermanyahighdegreeofperfectionhasbeenreached，andIhopeyourcountrymenwillnotbeslowtofollow。
YourinterestingaccountoftheexperimentsofMr。Wattleswasentirelynewtome，andisanotheramongthemanyevidencesthatwhentheageisfullyripeforanygreatdiscovery，itisrarethatitdoesnotoccurtomorethanasinglemind。
Trustingthatyourworkwillmeetwiththeencouragementwhichyourtroubleinpreparingitdeserves，andwithgratitudefortheundeservedcomplimentpaidtomeinitsdedication，Iremain，verysincerely，Yourfriendandwellwisher，E。ANTHONY。
PHOTOGRAPHY。
CHAP。I。
ABRIEFHISTORYOFTHEART。
AsinallcasesofgreatandvaluableinventionsinscienceandarttheEnglishlayclaimtothehonorofhavingfirstdiscoveredthatofPhotogenicdrawing。Butweshallseeintheprogressofthishistory，thatlikemanyotherassumptionsoftheirauthors，priorityinthisisnomoreduethem，thentheinventionofsteamboats，orthecottongin。
Thisclaimisfoundeduponthefactthatin1802Mr。WedgwoodrecordedanexperimentintheJournaloftheRoyalInstitutionofthefollowingnature。
"Apieceofpaper，orotherconvenientmaterial，wasplaceduponaframeandspongedoverwithasolutionofnitrateofsilver;
itwasthenplacedbehindapaintingonglassandthelighttraversingthepaintingproducedakindofcopyuponthepreparedpaper，thosepartsinwhichtherayswereleastinterceptedbeingofthedarkesthues。Here，however，terminatedtheexperiment;
foralthoughbothMr。WedgwoodandSirHumphryDaveyexperimentedcarefully，forthepurposeofendeavoringtofixthedrawingsthusobtained，yettheobjectcouldnotbeaccomplished，andthewholeendedinfailure。"
This，bytheirownshowing，wastheearliestattemptoftheEnglishsavans。
ButthismuchoftheprinciplewasknowntotheAlchemistsatanearlydate——
althoughpracticallyproducedinanotherway——asthefollowingexperiment，tobefoundinoldbooks，amplyproves。
"Dissolvechalkinaquafortistotheconsistenceofmilk，andaddtoitastrongsolutionofsilver;keepthisliquorinaglassbottlewellstopped;
thencuttingoutfromapieceofpaperthelettersyouwouldhaveappear，pasteitonthedecanter，andlayitinthesun’sraysinsuchamannerthattheraysmaypassthroughthespacescutoutofthepaperandfallonthesurfaceoftheliquorthepartoftheglassthroughwhichtherayspasswillbeturnedblack，whilethatunderthepaperremainswhite;
butparticularcaremustbeobservedthatthebottlebenotmovedduringtheoperation。"
Hadnotthealchemistsbeensointentuponthedesiretodiscoverthefarfamedphilosopher’sstone，astomakethemunmindfuloftheaccidentaldawningsofmorevaluablediscoveries，thislittleexperimentinchemistrymighthaveinducedthemtoprosecuteamorethoroughsearchintotheprinciple，andPhotogenicartwouldnotnow，asitis，beanewone。
ItisevenassertedthattheJugglersofIndiawereformanyagesinpossessionofasecretbywhichtheywereenabled，inabriefspace，tocopythelikenessofanyindividualbytheactionoflight。
Thisfact，iffactitbe，mayaccountforthecelebratedmagicmirrorssaidtobepossessedbythesejugglers，andprobablecauseoftheirpoweroverthepeople。
However，asearlyas1556thefactwasestablishedthatacombinationofchlorideandsilver。called，fromitsappearance，hornsilver，wasblackenedbythesun’srays;andinthelatterpartofthelastcenturyMrs。Fulhamepublishedanexperimentbywhichachangeofcolorwaseffectedinthechlorideofgoldbytheagencyoflight;
andgaveitasheropinionthatwordsmightbewritteninthisway。
TheseincidentsareconsideredasthefirststepstowardsthediscoveryofthePhotogenicart。
Mr。Wedgwood’sexperimentscanscarcelybesaidtobeanyimprovementonthemsincehefailedtobringthemtopracticalusefulness，andhiscountrymenwillhavetobesatisfiedwithawardingthehonorofitscompleteadaptationtopracticalpurposes，toMM。
NiepceandDaguerreofFrance，andtoProfessorsDraper，andMorseofNew-York。
Thesegentlemen——MM。NiepceandDaguerre——pursuedthesubjectsimultaneously，withouteither，howeverbeingawareoftheexperimentsofhiscolleagueinscience。Forseveralyears，eachpursuedhisresearchesindividuallyuntilchancemadethemacquainted，whentheyenteredintoco-partnership，andconjointlybroughttheartalmosttoperfection。
M。NiepcepresentedhisfirstpaperonthesubjecttotheRoyalSocietyin1827，naminghisdiscoveryHeliography。WhatledhimtothestudyoftheprinciplesoftheartIhavenomeans，atpresent，ofknowing，butitwasprobablyowingtothefactsrecordedbytheAlchemists，Mrs。Fulhameandothers，alreadymentioned。
ButM。Daguerre，whoisacelebrateddioramicpainter，beingdesirousofemployingsomeofthesingularlychangeablesaltsofsilvertoproduceapeculiarclassofeffectsinhispaintings，wasledtopursueaninvestigationwhichresultedinthediscoveryoftheDaguerreotype，orPhotogenicdrawingonplatesofcoppercoatedwithsilver。
Tothisgentleman——tohisliberality——areweAmericansindebtedforthefreeuseofhisinvention;andthelargeandincreasingclassofDaguerreanartistsofthiscountryshouldholdhiminthemostprofoundrespectforit。
Hewasnotwillingthatitshouldbeconfinedtoafewindividualswhomightmonopolisethebenefitstobederivedfromitspractice，andshutoutallchanceofimprovement。
Likeatrue，nobleheartedFrenchgentlemanhedesiredthathisinventionshouldspreadfreelythroughoutthewholeworld。
WiththeseviewsheopenednegociationswiththeFrenchgovernmentwhichwereconcludedmostfavorablytoboththeinventors，andFrancehasthe"gloryofendowingthewholeworldofscienceandartwithoneofthemostsurprisingdiscoveriesthathonortheland。"
Notwithstandingthis，ithasbeenpatentedinEnglandandtheresultiswhatmighthavebeenexpected：EnglishpicturesarefarbelowthestandardofexcellenceofthosetakenbyAmericanartists。
Ihaveseensomemediumportraits，forwhichaguineaeachhadbeenpaid，andtakentoo，byacelebratedartist，thatourpoorestDaguerreotypistswouldbeashamedtoshowtoasecondperson，muchlesssuffertoleavetheirrooms。
CALOTYPE，thenamegiventooneofthemethodsofPhotogenicdrawingonpaper，discovered，andperfectedbyMr。FoxTalbotofEngland，ispreciselyinthesamepredicament，notonlyinthatcountrybutintheUnitedStates，Mr。Talbotbeingpatenteeinboth。
Heisamanofsomewealth，Ibelieve，buthedemandssohighapriceforasinglerightinthiscountry，thatnonecanbefoundwhohavethetemeritytopurchase。
TheexecutionofhispicturesisalsoinferiortothosetakenbytheGermanartists，andIwouldremarkenpassant，thattheMessrs。
MeadexhibitedatthelastfairoftheAmericanInstitute，（of1848，）
fourCalotypes，whichoneofthefirmbroughtfromGermanylastSpring，thatforbeauty，depthoftoneandexcellenceofexecutionsurpassthefineststeelengraving。
WhenMr。Talbot’spatentfortheUnitedStatesexpiresandouringeniousYankeeboyshavetheopportunity，IhavenottheslightestdoubtoftheCalotype，intheirhands，entirelysupercedingtheDaguerreotype。
Letthem，therefore，studytheprinciplesoftheartaslaiddowninthislittlework，experiment，practiceandperfectthemselvesinit，andwhenthattimedoesarrivebepreparedtoproducethatdegreeofexcellenceinCalotypetheyhavealreadyobtainedinDaguerreotype。
ItistoProfessorSamuelF。B。Morse，thedistinguishedinventoroftheMagneticTelegraph，ofNewYork，thatweareindebtedfortheapplicationofPhotography，toportraittaking。
HewasinParis，forthepurposeofpresentingtothescientificworldhisElectro-MagneticTelegraph，atthetime，（1838，）M。Daguerreannouncedhissplendiddiscovery，anditsastoundingresultshavinganimportantbearingontheartsofdesignarrestedhisattention。
Inhislettertomeonthesubject，theProfessorgivesthefollowinginterestingfacts。
"Theprocesswasasecret，andnegociationsweretheninprogress，forthedisclosureofittothepublicbetweentheFrenchgovernmentandthedistinguisheddiscoverer。M。Daguerrehadshownhisresultstotheking，andtoafewonlyofthedistinguishedsavans，andbytheadviceofM。Arago，haddeterminedtowaittheactionoftheFrenchChambers，beforeshowingthemtoanyotherpersons。
Iwasexceedinglydesirousofseeingthem，butknewnothowtoapproachM。Daguerrewhowasastrangertome。
OnmentioningmydesiretoRobertWalsh，Esq。，ourworthyConsul，hesaidtome;’statethatyouareanAmerican，theinventoroftheTelegraph，requesttoseethem，andinvitehiminturntoseetheTelegraph，andIknowenoughoftheurbanityandliberalfeelingsoftheFrench，toinsureyouaninvitation。’
Iwassuccessfullinmyapplication，andwithayoungfriend，sincedeceased，thepromisingsonofEdwardDelevan，Esq。，I
passedamostdelightfulhourwithM。Daguerre，andhisenchantingsun-pictures。Mylettercontaininganaccountofthisvisit，andthesepictures，wasthefirstannouncementinthiscountryofthissplendiddiscovery。"
"Imayhereaddthesingularsequeltothisvisit。OnthesucceedingdayM。DaguerrepaidmeavisittoseetheTelegraphandwitnessitsoperations。
Heseemedmuchgratifiedandremainedwithmeperhapstwohours;
twomelancholyhourstohim，astheyafterwardsproved;orwhilehewaswithme，hisbuildings，includinghisdiorama，hisstudio，hislaboratory，withallthebeautifulpicturesIhadseenthedaybefore，wereconsumedbyfire。Fortunatelyformankind，matteronlywasconsumed，thesoulandmindofthegenius，andtheprocesswerestillinexistence。"
Onhisreturnhome，ProfessorMorsewaitedwithimpatiencefortherevelationofM。Daguerre’sprocess，andnosoonerwasitpublishedthanheprocuredacopyoftheworkcontainingit，andatoncecommencedtakingDaguerreotypepictures。
Atfirsthisobjectwassolelytofurnishhisstudiowithstudiesfromnature;buthisexperimentsledhimintoabeliefofthepracticabilityofprocuringportraitsbytheprocess，andhewasundoubtedlythefirstwhoseattemptswereattendedwithsuccess。Thinking，atthattime，thatitwasnecessarytoplacethesittersinaverystronglight，theywerealltakenwiththeireyesclosed。
Otherswereexperimentingatthesametime，amongthemMr。WolcottandProf。Draper，andMr。Morse，withhisacustomedmodesty，thinksthatitwouldbedifficulttosaytowhomisduethecreditofthefirstDaguerreotypeportrait。
Atallevents，sofarasmyknowledgeservesme，ProfessorMorsedeservesthelaurelwreath，asfromhimoriginatedthefirstofourinumerableclassofDaguerreotypists;andmanyofhispupilshavecarriedthemanipulationtoverygreatperfection。
InconnectionwiththismatterIwillgivetheconcludingparagraphofaprivateletterfromtheProfessortome;Hesays。
"Ifminewerethefirst，otherexperimenterssoonmadebetterresults，andifthereareanywhodisputethatIwasfirst，Ishallhavenoargumentwiththem;forIwasnotsoanxioustobethefirsttoproducetheresult，astoproduceitinanyway。Iesteemitbutthenaturalcarryingoutofthewonderfuldiscovery，andthatthecreditwasafterallduetoDaguerre。
Ilaynoclaimtoanyimprovements。"
SinceIcommencedthecompilationofthiswork，IhavehadthepleasureofmakingtheacquaintanceofanAmericangentleman——JamesM。WattlesEsq。——
whoasearlyas1828——anditwillbeseen，bywhatIhavealreadystated，thatthisisaboutthesamedateofM。Niepce’sdiscovery——hadhisattentionattractedtothesubjectofPhotography，orashetermedit"Solarpicturedrawing，"whiletakinglandscapeviewsbymeansofthecamera-obscura。
Whenwereflectuponallthecircumstancesconnectedwithhisexperiments，thegreatdisadvantagesunderwhichbelabored，andhisextremeyouthfullness，wecannotbutfeelanationalpride——yetwonder——thatamereyankeeboy，surroundedbythedeepestforests，hundredofmilesfromthepopulousportionofourcountry，withoutthenecessarymaterials，orresourcesforprocuringthem，shouldbytheforceofhisnaturalgeniusmakeadiscovery，andputitinpracticaluse，toaccomplishwhich，themostlearnedphilosophersofEurope，witheveryrequisiteapparatus，andaprofoundknowledgeofchemistry——spentyearsoftoiltoaccomplish。
HowmuchmorelatenttalentmaynowbeslumberingfromtheverysamecausewhichkeptMr。Wattlesfrompubliclyrevealinghisdiscoveries，viz;
wantofencouragement——ridicule！
Atthetimewhentheideaoftakingpicturespermanentlyonpaperbymeansofthecamera-obscurafirstoccurredtohim，hewasbutsixteenyearsofage，andundertheinstructionsofMr。CharlesLeSeuer，（atalentedartistfromParis）attheNewHarmonyschool，Indiana。
Drawingandpaintingbeingthenaturalbentofhismind，bewasfrequentlyemployedbytheprofessorstomakelandscapesketchesinthemannermentioned。Thebeautyoftheimageoftheselandscapesproducedonthepaperinthecamera-obscura，causedhimtopauseandadmirethemwithalltheardorofayoungartist，andwishthatbysomemeans，hecouldfixthemthereinalltheirbeauty。
Fromwishinghebroughthimselftothinkthatitwasnotonlypossiblebutactuallycapableofaccomplishmentandfromthinkingitcould，heresolveditshouldbedone。
Hewas，however，whollyignorantofeventhefirstprinciplesofchemistry，andnaturalphilosophy，andalltheknowledgehewasenabledtoobtainfromhisteacherswasofverylittleservicetohim。
Toaddtothis，wheneverhementionedhishopestohisparents，theylaughedathim，andbadehimattendtohisstudiesandletsuchmoonshinethoughtsalone——stillhepersevered，thoughsecretly，andhemetwiththesucceshispeseverancedeserved。
Forthetruthofhisstatement，Mr。Wattlesreferstosomeofourmostrespectablecitizensresidingatthewest，andIaminhopesthatIshallbeenabledtoreceiveintimeforthispublication，aconfirmationfromoneormoreofthesegentlemen。Bethatasitmay，IfeelconfidentintheintegrityofMr。Wattles，andcangivehisstatementtotheworldwithoutadoubtofitstruth。
Thefollowingsketchofhisexperimentsandtheirresultswill，undoubtedly，beinterestingtoeveryAmericanreaderandalthoughsomeoftheprofoundphilosophersofEuropemaysmileathismethodofproceeding，itwillinsomemeasureshowtheinnategeniusofAmericanminds，andprovethatwearenotfarbehindourtrans-atlanticbrethrenintheartsandsciences。
Mr。Wattlessays："Inmyfirsteffortstoeffectthedesiredobject，theywerefeebleindeed，andowingtomylimitedknowledgeofchemistry——
whollyacquiredbyquestioningmyteachers——Imetwithrepeatedfailuresbutfollowingthemupwithadeterminedspirit，Iatlastproduced，whatIthoughtveryfairsamples——buttoproceedtomyexperiments。"
"Ifirstdippedaquartersheetofthinwhitewritingpaperinaweaksolutionofcaustic（asIthencalledit）anddrieditinanemptybox，tokeepitinthedark;whendry，Iplaceditinthecameraandwatcheditwithgreatpatiencefornearlyhalfanhour，withoutproducinganyvisibleresult;evidentlyfromthesolutionbeingtoweak。
Ithensoakedthesamepieceofpaperinasolutionofcommonpotash，andthenagainincausticwateralittlestrongerthanthefirst，andwhendryplaceditinthecamera。Inaboutforty-fiveminutesI
plainlypercievedtheeffect，inthegradualdarkeningofvariouspartsoftheview，whichwastheoldstonefortintherearoftheschoolgarden，withthetrees，fence，&c。Ithenbecameconvincedofthepracticabilityofproducingbeautifulsolarpicturesinthisway;but，alas！mypicturevanishedandwithit，all——nonotall——myhopes。WithreneweddeterminationIbeganagainbystudyingthenatureofthepreparation，andcametotheconclusion，thatifIcoulddestroythepartnotacteduponbythelightwithoutinjuringthatwhichwassoactedupon，Icouldsavemypictures。Ithenmadeastrongsolutionofsal。
sodaIhadinthehouse，andsoakedmypaperinit，andthenwasheditoffinhotwater，whichperfectlyfixedtheviewuponthepaper。
Thispaperwasverypoorwiththickspots，moreabsorbentthanotherparts，andconsequentlymadedarkshadesinthepicturewheretheyshouldnothavebeen;butitwasenoughtoconvincemethatIhadsucceeded，andthatatsomefuturetime，whenIhadthemeansandamoreextensiveknowledgeofchemistry，Icouldapplymyselftoitagain。
Ihavedonesosince，atvarioustimes，withperfectsuccess;
butineveryinstancelaboringunderadversecircumstances。"
Ihaveveryrecentlylearned，that，underthepresentpatentlawsoftheUnitedStates，everyforeignpatenteeisrequiredtoputhisinvention，ordiscovery，intopracticalusewithineighteenmonthsaftertakingouthispapers，orotherwiseforfeithispatent。
WithregardtoMr。Talbot’sCalotypepatent，thistimehasnearly，ifnotquiteexpired，andmycountrymenarenowatperfectlibertytoappropriatetheartiftheyfeeldisposed。
FromthestatementofMr。Wattles，itwillbeperceivedthatthiscanbedonewithoutdishonor，asinthefirstinstanceMr。Talbothadnopositiverighttohispatent。
Photography;orsun-paintingisdivided，accordingtothemethodsadoptedforproducingpictures，intoDAGUERREOTYPE，CHROMATYPE，CALOTYPE，ENERGIATYPE，CHRYSOTYPE，ANTHOTYPEandCYANOTYPE，AMPHITYPE。
CHAP。II。
THETHEORYONLIGHT。——THEPHOTOGRAPHICPRINCIPLE
SomephilosopherscontendthattotheexistenceoflightaloneweowethebeautifuleffectsproducedbythePhotogenicart，whileothersgivesufficientreasonsfordoubtingthecorrectnessoftheassumption。
Thattheresultsareeffectedbyaprincipleassociatedwithlightandnotbytheluminousprincipleitself，isthemostprobableconclusion。
Theimportanceofaknowledgeofthisfactbecomesmostessentialinpractice，aswillpresentlybeseen。TothisprincipleMr。HuntgivesthenameofENERGIA。
THENATUREofLightisnotwhollyknown，butitisgenerallybelievedtobematter，asinitsmotionsitobeysthelawsregulatingmatter。
Socloselyisitconnectedwithheatandelectricitythattherecanbelittledoubtoftheirallbeingbutdifferentmodificationsofthesamesubstance。
Iwillnot，however，enterintoastatementofthevarioustheoriesofPhilosophersonthishead，butcontentmyselfwiththatofSirIsaacNewton;
whosupposedraysoflighttoconsistofminuteparticlesofmatter，whichareconstantlyemanatingfromluminousbodiesandcausevision，asodoriferousparticles，proceedingfromcertainbodies，causesmelling。
Theeffectsoflightuponotherbodies，andhowlightiseffectedbythem，involvesomeofthemostimportantprinciples，whichifproperlyunderstoodbyDaguerreotypistswouldenablethemtoimproveandcorrectmanyofthepracticaloperationsintheirart。
Theseeffectsweshallexhibitinthisandthefollowingchapters。
BeforeweenteronthissubjectitwillbenecessarytobecomefamiliarwiththeDEFINITIONSofsomeofthetermsusedinthescienceofoptics。
Luminousbodiesareoftwokinds;thosewhichshinebytheirownlight，andthosewhichshinebyreflectedlight。
Transparentbodiesaresuchaspermitraysoflighttopassthroughthem。
Translucentbodiespermitlighttopassfaintly，butwithoutrepresentingthefigureofobjectsseenthroughthem。
Opaquebodiespermitnolighttopassthroughthem，butreflectlight。
Arayisalineoflight。
Abeamisacollectionofparallelrays。
Apencilisacollectionofconverging，ordivergingrays。
Amediumisanyspacethroughwhichlightpasses。
Incidentraysarethosewhichfalluponthesurfaceofabody。
Reflectedraysarethosewhicharethrownofffromabody。
Parallelraysaresuchasproceedequallydistantfromeachotherthroughtheirwholecourse。
Convergingraysaresuchasapproachandtendtouniteatanyonepoint，asatb。fig。3。
Divergingraysarethosewhichcontinuetorecedefromeachother，asate。Fig。3。
AFocusisthatpointatwhichconvergingraysmeet。
MOTIONOFLIGHT——Raysoflightarethrownofffromluminousbodiesineverydirection，butalwaysinstraightlines，whichcrosseachotherateverypoint;buttheparticlesofwhicheachrayconsistsaresominutethattheraysdonotappeartobeimpededbyeachother。
Arayoflightpassingthroughanapertureintoadarkroom，proceedsinastraightline;afactofwhichanyonemaybeconvincedbygoingintoadarkenedroomandadmitinglightonlythroughasmallaperture。
Lightalsomoveswithgreatvelocity，butbecomesfainterasitrecedesfromthesourcefromwhichiteminates;inotherwords，divergingraysoflightdiminishinintensityasthesquareofthedistanceincreases。
Forinstanceletafig。1，representtheluminousbodyfrom[hipho_1。gif]
whichlightproceeds，andsupposethreesquareboards，b。c。d。
severallyone，fourandsixteensquareinchesinsizebeplaced;
bonefoot，ctwofeet，anddfourfeetfroma，itwillbeperceivedthatthesmallestboardbwillthrowcintoshadow;
thatis，obstructallraysoflightthatwouldotherwisefallonc，andifbwereremovedcwouldinlikemannerhidethelightfromd——Now，ifbrecieveasmuchlightaswouldfalloncwhosesurfaceisfourtimesaslarge，thelightmustbefourtimesaspowerfulandsixteentimesaspowerfulasthatwhichwouldfallonthesecondandthirdboards，becausethesamequantityoflightisdiffusedoveraspacefourandsixteentimesgreater。
Thesesameraysmaybecollectedandtheirintensityagainincreased。
Raysoflightarereflectedfromonesurfacetoanother;Refracted，orbent，astheypassfromthesurfaceofonetransparentmediumtoanother;andInflected，orturnedfromtheircourse，bytheattractionofopaquebodies。Fromthefirstwederivetheprinciplesonwhichmirrorsareconstructed;
tothesecondweareindebtedforthepowerofthelenses，andtheblessingsofsight，——forthelightactsupontheretinaoftheeyeinthesamemannerasonthelensofacamera。
Thelatterhasnoimportantbearinguponoursubject。
Whenarayoflightfallsperpendicularlyuponanopaquebody，itisreflectedbarkinthesamelineinwhichitproceeds;
inthiscasethereflectedrayreturnsinthesamepaththeincidentraytraversed;butwhenarayfallsobliquely，itisreflectedobliquely，thatis，itisthrownoffinoppositedirection，andasfarfromtheperpendicularaswastheincidentray，asshownatFig。2;arepresentingtheincidentrayandbthereflected。
Thepoint，oranglecmadeby[hipho_2。gif]
theincidentray，atthesurfaceofthereflectoref，withalinecd，perpendiculartothatsurface，iscalledtheangleofincidence，whiletheangleformedbythereflectedraybandtheperpendicularlinediscalledtheangleofreflection，andtheseanglesarealwaysequal。
Itisbythisreflectionoflightthatobjectsaremadevisible;
butunlesslightfallsdirectlyupontheeyetheyareinvisible，andarenotsensiblyfeltuntilafteracertainseriesofoperationsuponthevariouscoveringsandhumorsoftheeye。
Smoothandpolishedsurfacesreflectlightmostpowerfully，andsendtotheeyetheimagesoftheobjectsfromwhichthelightproceededbeforereflection。Glass，whichistransparent——
transmittinglight——wouldbeofnousetousasamirror，wereitnotfirstcoatedononesidewithametalicamalgam，whichinterruptstheraysintheirpassagefromtheglassintotheair，andthrowsthemeitherdirectlyintheincidentline，orinanobliquedirection。Thereasonwhytrees，rocksandanimalsarenotallmirrors，reflectingotherformsinsteadoftheirown，is，thattheirsurfacesareuneven，andraysoflightreflectedfromanunevensurfacearediffusedinalldirections。
Parallelraysfallingobliquelyuponaplanemirrorarereflectedparallel;
convergingrays，withthesamedegreeofconvergence;anddivergingraysequallydivergent。
Standbeforeamirrorandyourimageisformedtherein，andappearstobeasfarbehindtheglassasyouarebeforeit，makingtheangleofreflectionequaltothatofincidence，asbeforestated。Theincidentrayandthereflectedrayform，together，whatiscalledthepassageofreflection，andthiswillthereforemaketheactualdistanceofanimagetoappearasfaragainfromtheeyeasitreallyis。
Anyobjectwhichreflectslightiscalledaradiant。
Thepointbehindareflectingsurface，fromwhichtheyappeartodiverge，iscalledthevirtualfocus。
Raysoflightbeingreflectedatthesameangleatwhichtheyfalluponamirror，twopersonscanstandinsuchapositionthateachcanseetheimageoftheotherwithoutseeinghisown。
Again;youmayseeyourwholefigureinamirrorhalfyourlength，butifyoustandbeforeoneafewinchesshorterthewholecannotbereflected，astheincidentraywhichpassesfromyourfeetintothemirrorintheformercase，willinthelatterfallunderit。
Imagesarealwaysreversedinmirrors。
Convexmirrorsreflectlightfromaroundedsurfaceanddispersetheraysineverydirection，causingparallelraystodiverge，divergingraystodivergemore，andconvergingraystoconvergeless——
Theyrepresentobjectssmallerthantheyreallyare——becausetheangleformedbythereflectedrayisrenderedmoreacutebyaconvexthanbyaplanesurface，anditisthediminishingofthevisualangle，bycausingraysoflighttobefartherextendedbeforetheymeetinapoint，whichproducestheimageofconvexmirrors。Thegreatertheconvexityofamirror，themorewilltheimagesoftheobjectsbediminished，andthenearerwilltheyappeartothesurface。
Thesemirrorsfurnishsciencewithmanycuriousandpleasingfacts。
Concavemirrorsarethereverseofconvex;thelatterbeingroundedoutwards，theformerhollowedinwards——theyrenderraysoflightmoreconverging——
collectraysinsteadofdispersingthem，andmagnifyobjectswhiletheconvexdiminishesthem。
Raysoflightmaybecollectedinthefocusofamirrortosuchintensityastomeltmetals。Theordinaryburningglassisanillustrationofthisfact;
althoughtheraysoflightarerefracted，orpassedthroughtheglassandconcentratedintoafocusbeneath。
Whenincidentraysareparallel，thereflectedraysconvergetoafocus，butwhentheincidentraysproceedfromafocus，oraredivergent，theyarereflectedparallel。Itisonlywhenanobjectisnearertoaconcavemirrorthanitscentreofconcavity，thatitsimageismagnified;
forwhentheobjectisfartherfromthemirror，thiscentrewillappearlessthantheobject，andinaninvertedposition。
Thecentreofconcavityinaconcavemirror，isanimaginarypointplacedinthecentreofacircleformedbycontinuingtheboundaryoftheconcavityofthemirrorfromanyonepointoftheedgetoanotherparalleltoandbeneathit。
REFRACTIONOFLIGHT：——Inowpasstotheconsiderationofthepassageoflightthroughbodies。
Arayoflightfailingperpendicularlythroughtheairuponasurfaceofglassorwaterpassesoninastraightlinethroughthebody;
butifit，inpassingfromonemediumtoanotherofdifferentdensity，fallobliquely，itisbentfromitsdirectcourseandrecedesfromit，eithertowardstherightorleft，andthisbendingiscalledrefraction;
（seefig。3，b。）Ifarayoflightpassesfromararerintoadensermediumitisrefractedtowardsaperpendicularinthatmedium;butifitpassesfromadenserintorareritisbentfurtherfromaperpendicularinthatmedium。
Owingtothisbendingoftheraysoflighttheanglesofrefractionandincidenceareneverequal。
Transparentbodiesdifferintheirpowerofbendinglight——
asageneralrule，therefractivepowerisproportionedtothedensity——butthechemicalconstitutionofbodiesaswellastheirdensity，isfoundtoeffecttheirrefractingpower。
Inflamablebodiespossessthispowertoagreatdegree。
Thesinesoftheangleofincidenceandrefraction（thatis，theperpendiculardrawnfromtheextremityofanarctothediameterofacircle，）arealwaysinthesameratio;viz：fromairintowater，thesineoftheangleofrefractionisnearlyasfourtothree，whateverbethepositionoftheraywithrespecttotherefractingsurface。
Fromairintosulphur，thesineoftheangleofrefractionisastwotoone——
thereforetheraysoflightcannotberefractedwheneverthesineoftheangleofrefractionbecomesequaltotheradius*ofacircle，andlightfallingveryobliquelyuponatransparentmediumceasestoberefracted;thisistermedtotalreflection。
*TheRADIUSofacircleisastraightlinepassingfromthecentretothecircumference。
Sincethebrightnessofareflectedimagedependsuponthequantityoflight，itisquiteevidentthatthoseimageswhicharisefromtotalreflectionarebyfarthemostvivid，asinordinarycasesofreflectionaportionoflightisabsorbed。
Ishouldbepleasedtoentermorefullyintothisbranchofthescienceofoptics，buttheboundstowhichIamnecessarilylimitedinaworkofthiskindwillnotadmitofit。
Inthenextchapter，however，IshallgiveasynopsisofMr。Hunt’streatiseonthe"InfluenceoftheSolarRaysonCompoundBodies，withespecialreferencetotheirPhotographicapplication"——
AworkwhichshouldbeinthehandsofeveryDaguerreotypist，andwhichIhopesoontoseerepublishedinthiscountry。
IwillconcludethischapterwithabriefstatementoftheprinciplesuponwhichthePhotographicartisfounded。
SOLARandStellerlightcontainsthreekindsofrays，viz：
1。Colorific，orraysofcolor。
2。Calorific，orraysofheat。
3。Chemicalrays，orthosewhichproducechemicaleffects。
OnthefirstandthirdthePhotographicprincipledepends。
Inexplainingthisprincipletheaccompanyingwoodcuts，（figs。3and4）willrenderitmoreintelligible。
Ifapencilofthesun’sraysfalluponaprism，itisbentinpassingthroughthetransparentmedium;andsomeraysbeingmorerefractedthanothers，weprocureanelongatedimageoftheluminousbeam，exhibitingthreedistinctcolors，red，yellowandblue，whicharetoberegardedasprimitives——andfromtheirinterblending，seven，asrecordedbyNewton，andshownintheaccompanyingwoodcut。
Theseraysbeingabsorbed，orreflecteddifferentlybyvariousbodies，givetonaturethecharmofcolor。Thustotheeveisgiventhepleasurewederiveinlookinguponthegreenfieldsandforests，theenumerablevarietiesofflowers，theglowingruby，jasper，topaz，amethist，andemerald，thebrilliantdiamond，andalltherichandvariedhuesofnature，bothanimateandinanimate。
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Now，ifweallowthisprismaticspectrum（b。fig。3。）tofalluponanysurface（asatc。）preparedwithasensitivephotographiccompound，weshallfindthatthechemicaleffectproducedbearsnorelationtotheintensityofthelightofanyparticularcoloredray，butthat，onthecontrary，itisdispersedoverthelargestportionofthespectrum，beingmostenergeticintheleastluminousrays，andeveractiveoveranextensivespace，wherenotracesoflightcanbedetected。Fig。4，willgivethestudentabetterideaofthisprinciple。Itisacopyofthekindofimpressionwhichthespectrum，spokenof，wouldmakeonapieceofpapercoveredwithaverysensitivephotographicpreparation。
Thewhitespacea。correspondswiththemostluminous，oryellowray，（5，fig。3）overlimitsofwhichallchemicalchangeisprevented。
Asimilaractionisalsoproducedbythelowerendoftheredrayc;
butintheupperportion，howeverwefindadecidedchange（asatd）。Themostactivechemicalchange，youwillpercieve，isproducedbytheraysabovetheyellowa;viz。4，3，2and1
（asatb）thegreen（4）beingtheleastactive，andtheblue（3）andviolet（1）raysthemostso，theactionstillcontinuingfarbeyondthepointbwhichistheendoftheluminousimage。
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SupposewewishtocopybytheDaguerreotype，orCalotypeprocess，anyobjectshighlycolored——blue，redandyellow，forinstancepredominating——
thelastofcoursereflectsthemostlight，thebluetheleast;
buttheraysfromthebluesurfacewillmakethemostintenseimpression，whilsttheredradiationsareworkingveryslowly，andtheyellowremainsentirelyinactive。Thisaccountsforthedifficultyexperiencedincopyingbrightgreenfoliage，orwarmlycoloredportraits;alargeportionoftheyellowandredraysenteringintothecompositionofboth——
andtheimperfectionsofaDaguerreotypeportraitofapersonwithafreckledfacedependsuponthesamecause。
Ayellow，hazyatmosphere，evenwhenthelightisverybright，willeffectuallypreventanygoodphotographicresult——
andintheheightofsummer，withthemostsensativeprocess，itnotunfrequentlyhappensthatthemostanoyingfailuresarisefromthisagencyofayellowmedium。Abuildingpaintedofayellowcolor，whichmayreflectthesun’sraysdirectlyintotheoperator’sroomwillhavethesameeffect。
Daguerreotypists，beingignorantofthesefacts，areveryapttochargetheirwantofsuccesstotheplates，orchemicals，oranythingbuttherealcause;anditwouldbewelltobearthesefactsconstantlyinmindandasfaraspossibleavoidthem。
This，maybeaccomplished，inameasure，byachoiceoflocationorbyhavingtheglassofyourwindowstingedwithblue;orascreenofthinbluepapermaybeinterposedbetweenthelightandsitter。
Inselectingsubjects，allstrikingcontrastsincolorshouldbeavoided，andsittersforportraitsshouldbecautionednottowearanythingthatmayproducetheeffectspokenof——
darkdressesalwaysbeingthebest。
Theactionoflightbothcombinesanddecomposesbodies。
Forinstance，chlorineandhydrogenwillremaininaglassvesselwithoutalterationifkeptinthedark;butifexposedtotheraysofthesun，theyimmediatelyenterintocombination，andproducehydrochloricacid。
Ontheotherhand，ifcolorlessnitricacidbeexposedtothesun，itbecomesyellow，thenchangestored，andoxygenisliberatedbythepartialdecompositioneffectedbythesolarrays。
Oftheorganicsubstancesnonearemorereadilyacteduponbylightthanthevariouscombinationsofsilver。
Ofthesesomearemore，andotherslesssensitive。IfChlorideofsilver，whichisawhiteprecipitateformedbyaddingchlorideofsodium（commonsalt）
toasolutionofnitrateofsilver，beexposedtodiffusedlight，itspeedilyassumesaviolettint，andultimatelybecomesnearlyblack。
Withiodideofsilver，bromideofsilver，ammonio-nitrateofsilver，andothersaltsofthismetal，theresultwillbemuchthesame。
Somebodies，whichundertheinfluenceoflight，undergochemicalchanges，havethepowerofrestoringthemselvestotheiroriginalconditioninthedark。Thisismoreremarkablydisplayedintheiodideofplatinum，whichreadilyrecievesaphotogenicimagebydarkeningovertheexposedsurfaces，butspeedilylosesitbybleachinginthedark。
TheioduretofDaguerre’splate，andsomeotheriodides，exhibitthesamepeculiarity——Thisleadsustothestrikingfact，thatbodieswhichhaveundergoneachangeofestateundertheinfluenceofday-lighthavesomelatentpowerbywhichtheycanrenovatethemselves。
Possiblythehoursofnightareasnecessarytoinanimatenatureastheyaretotheanimate。Duringtheday，anexcitementwhichwedonotheed，unlessinastateofdisease，ismaintainedbytheinfluenceoflightandthehoursofrepose，duringwhichtheequilibriumisrestored，areabsolutelynecessarytothecontinuanceofhealth。
Insteadofafewchemicalcompoundsofgoldandsilver，whichatfirstwerealonesupposedtobephotographic，wearenowawarethatcopper，platinum，lead，nikel，andindeed，probablyalltheelements，areequallyliablytochangeunderthesun’sinfluence。Thisfactmaybeofbenefittoengravers，forifsteelcanbemadetotakephotographicimpressions，themorelaboriousprocessofetchingmaybedispensedwith。
Infact，inthelatterpartofthiswork，aprocessisdescribedforetchingandtakingprintedimpressionsfromDaguerreotypeplates。
Asyetthisprocesshasproducednodecidedbeneficialresults——
butfutureexperimentsmayaccomplishsomepracticaldiscoveryofintrinsicvaluetotheartofengraving。
Averysimpleexperimentwillprovehowessentiallightistothecoloringofthevariousspeciescomprisingthevegetableandanimalkingdoms。
Ifwetransplantanyshrubfromthelightofdayintoadarkcellar，wewillsoonseeitloseitsbrightgreencolor，andbecomeperfectlywhite。
Anothereffectoflightisthatitappearstoimparttobodiessomepowerbywhichtheymorereadilyenterintochemicalcombinationwithothers。
Wehavealreadysaidthatchlorineandhydrogen，ifkeptinthedark，willremainunaltered;butifthechlorinealonebepreviouslyexposedtothesun，thechlorinethussolarisedwillunitewiththehydrogeninthedark。
Sulphateofironwillthrowdowngoldorsilverfromtheirsolutionsslowlyinthedark;butifeithersolutionbefirstexposedtosunshine，andthemixturebethenmade，inthedark，theprecipitationtakesplaceinstantly。
Hereisagain，evidenceofeitheranabsorptionofsomematerialagentfromthesunbeam，oranalterationinthechemicalconstitutionofthebody。
ItwasfromunderstandingtheseprinciplesandapplyingthemthatphilosopherswereenabledtoproducetheCalotype，Daguerreotype，&c。Fortheeffectsandactionoflightonthecamera，seeChapterV。
SomeadvanceshavebeenmadetowardsproducingPhotographicimpressionsincolor——theimpossibilityofwhichsomeofourbestandoldestartistshavemostpertinaciouslymaintained。
Thecoloredimageofthespectrumhasbeenmostfaithfullycopied，rayforray，onpaperspreadwiththejuiceoftheCochorusJaponica，（aspeciesofplant）andthefluorideofsilver;
andonsilverplatecoveredwithathinfilmofchloride。
Thedaymaybestillremotewhenthismuchtobedesireddecideratumshallbeaccomplishedinportraittaking;
butIamledtohopethatfutureexperimentsmaymasterthesecretwhichnowcausesittobelookedupon，bymany，asanimpossibility。
Thatgreatadvantageshaveresulted，andthatgreaterstillwillresultfromthediscoveryofthePhotographicart，fewwilldeny。
Thefaithfulmannerinwhichitcopiesnature，eventothemostminutedetails，rendersitofmuchvaluetothepainter;butafewminutessufficingtotakeaviewthatformerlywouldhaveoccupiedseveraldays。
Itssuperiorityinportraits，overminiatureoroilpaintinghasbeentacitlyacknowledgedbythethousandswhoemployittosecuretheirown，orafriendslikeness，andbythesteadyincreaseinthenumberofartistswhoareweekly，ayedailyspringingupineverytownandvillageintheland。
CHAP。III。
SYNOPSISOFMR。HUNT’STREATISEON"THEINFLUENCEOFTHE
SOLARRAYSONCOMPOUNDBODIES，WITHESPECIALREFERENCETO
THEIRPHOTOGRAPHICAPPLICATION。"
OXIDEOFSILVERexposedforafewhourstogoodsunshine，passesintoamoredecidedolivecolor，thancharacterisesitwhenfirstpreparedbyprecipitationfromnitrateofsilver。
Longerexposurerendersthiscolorverymuchlighter，andthecoveredparts，arefoundmuchdarker，thanthoseonwhichthelighthasacteddirectly。Insomeinstanceswheretheoxideofsilverhasbeenspreadonthepaperadecidedwhiteningprocessinsomeparts，afterafewdaysexposure，isnoticed。
Oxideofsilverdisolvedinammoniaisavaluablephotographicfluid;
oneapplicationofastrongsolutionforminganexceedinglysensitivesurface。Thepicturesonthispaperareeasilyfixedbysaltorweakammonia。
NITRATEOFSILVER。——Thissaltinastateofpurity，doesnotappeartobesensiblyaffectedbylight，butthepresenceofthesmallestportionoforganicmatterrendersitexceedinglyliabletochangeunderluminousinfluence。
Ifapieceofnitratedpaperisplaceduponhotiron，orheldnearthefire，itwillbefoundthatataheatjustbelowthatatwhichthepaperchars，thesaltisdecomposed。
Wheretheheatisgreatest，thesilverisrevived，andimmediatelyaroundit，thepaperbecomesadeepblue;
beyondthisaprettydecidedgreencolorresults，andbeyondthegreen，ayelloworyellowbrownstainismade。
Thisexhibitsaremarkableanalogybetweenheatandlight，——
beforespokenofinchap。II——andisofsomepracticalimportanceinthepreparationofthepaper。
PRISMATICANALYSIS。——Themethodofaccomplishingtheprismaticdecompositionofraysoflightbythespectrumhasalreadybeendescribedonpages22
and23。Thecoloroftheimpressedspectrum，onpaperwashedwithnitrateofsilver，isatfirst，apalebrown，whichpassesslowlyintoadeepershade;
thatportioncorrespondingwiththeblueraysbecomingabluebrown;
andunderthevioletofapeculiarpinkeyshade，averydecidedgreentint，onthepointwhichcorrespondswiththeleastrefrangiblebluerays，maybeobserved，itslimitsofactionbeingnearthecentreoftheyellowray，anditsmaximumaboutthecentreoftheblue，althoughtheactionuptotheedgeofthevioletrayiscontinuedwithverylittlediminutionofeffect;
beyondthispointtheactionisveryfeeble。
Whenthespectrumismadetoactonpaperwhichhasbeenpreviouslydarkened，byexposuretosunshineundercupro-sulphateofammonia，thephenomenaaremateriallydifferent。
Thephotographicspectrumislengthenedoutontheredornegativesidebyafaintbutveryvisibleredportion，whichextendsfullyuptotheendoftheredrays，asseenbythenakedeye。
Thetintofthegeneralspectrum，too，insteadofbrownisdarkgrey，passing，however，atitsmostrefractedorpositiveendintoaruddybrown。
InitsPhotographicapplication，thenitrateofsilveristhemostvaluableofthesaltsofthatmetal，asfromitmostoftheotherargentinecompoundscanbeprepared，althoughitisnotofitselfsufficientlysensibletolighttorenderitofmuchuse。
CHLORIDEOFSILVER。——Thissaltofsilver，whetherinitsprecipitatedstate，orwhenfused，changesitscolortoafinebluishgreybyaveryshortexposuretothesun’srays。
Ifcombinedwithasmallquantityofnitrate，thechangeismorerapid，itattainsadeepbrown，thenslowlypassesintoafineolive，andeventually，afterafewweeks，themetalicsilverisseentoberevivedonthesurfaceofthesalt。
Greatdifferencesofcolorareproducedonchloridesofsilverprecipitatedbydifferentmuriates。Nearlyeveryvarietyincombinationwiththenitrate，becomesatlastofthesameolivecolor，thefollowingexamples，therefore，havereferencetoafewminutesexposure，only，togoodsunshine;itmustalsoberecollectedthatthechlorideofsilverinthesecasesiscontaminatedwiththeprecipitant。
Muriateofammoniaprecipitateschloridetodarkentoafinechocolatebrown，whilstmuriateoflimeproducesabrick-redcolor。
Muriatesofpotashandsodaaffordaprecipitate，whichdarkensspeedlytoapuredarkbrown，andmuriaticacid，oraqueouschlorine，donotappeartoincreasethedarkeningpowerbeyondthelilactowhichthepurechlorideofsilverchangesbyexposure。
Thisdifferenceofcolorappearstobeowingtotheadmixtureoftheearthoralkaliusedwiththesilversalt。
Theprismaticimpressiononpaperspreadwiththechlorideofsilverisoftenverybeautifullytinted，theintensityofcolorvaryingwiththekindofmuriateused。Spreadpaperwithmuriateofammoniaorbarytaandyouobtainarangeofcolorsnearlycorrespondingwiththenaturalhuesoftheprismaticspectrum。
Underfavorablecircumstancesthemeanredray，leavesaredimpression，whichpassesintoagreenoverthespaceoccupiedbytheyellowrays。
Abovethisaleadenhueisobserved，andaboutthemeanblueray，wheretheactionisgreatest，itrapidlypassesthroughbrownintoblack，andthroughthemostrefrangibleraysitgraduallydeclinesintoabluishbrown，whichtintiscontinuedthroughouttheinvisiblerays。Attheleastrefrangibleendofthespectrum，theveryremarkablephenomenonhasbeenobserved，oftheextremeredraysexertingaprotectinginfluence，andpreservingthepaperfromthatchange，whichitwouldotherwiseundergo，undertheinfluenceofthedispersedlightwhichalwayssurroundsthespectrum。
Notonlytheextremeredrayexertsthisverypeculiarproperty，buttheordinaryredraythroughnearlyitswholelength。
Inphotographicdrawingthissaltisoftheutmostimportance。
Mr。Talbot’sapplicationofitwillbegivenhereafterinanotherportionofthiswork。
IODIDEOFSILVER——Perfectlypure，undergoesverylittlechangeundertheinfluenceoflightorheat;butifaveryslightexcessofthenitrateofsilverbeaddeditbecomesinfinitelymoresenitivethanthechlorideThespectrumimpresseduponpaperpreparedwithaweaksolutionofthehydriodateofpotashpresentssomeveryremarkablepeculiarities。
Themaximumofintensityisfoundattheedgeofthemostrefrangiblevioletrays，oralittlebeyondit，varyingslightlyaccordingtothekindofpaperused，andthequantityoffreenitrateofsilverpresent。
Theactioncommencesatapointnearlycoincidentwiththemeanredoftheluminousspectrum，whereitgivesadullashorleadcolor，whilethemostrefrangibleraysimpressaruddysnuff-brown，thechangeoftintcomingonrathersuddenlyabouttheendoftheblueorbeginningofthevioletraysoftheluminousspectrum。
Beyondtheextremevioletrays，theactionrapidlydiminishes，butthedarkeningproducedbytheseinvisiblerays，extendsaverysmallspacebeyondthepointatwhichtheyceasetoactonthechlorideofsilver。
Initsphotographicapplication，itis，alone，ofverylittleuse;
butincombinationwithotherreagentsitbecomesexquisitelysensitive。
Withgallicacidandtheferrocyanateofpotashitformstwoofthemostsensitivephotographicsolutionswithwhichweareacquainted。
Theseareusedinthecalotypeprocess。
IODURETOFSILVER。——Ifuponaplateofpolishedsilverweplaceasmallpieceofiodine，andapplytheheatofalampbeneaththeplateforamoment，asystemofringsisspeedilyformed。
Thefirstring，whichspreadingconstantlyformstheexteriorofthecircle，isofabrightyellowcolor;withinthis，therearises，sucessively，ringsofgreen，redandbluecolors，andthenagainafineyellowcircle，centredbyagreyishspotontheplaceoccupiedbytheiodine。Onexposingthesetothelight，theouteryellowcirclealmostinstantlychangescolor，theothersslowly，intheorderoftheirposition，theinterioryellowcircleresistingforalongtimethesolarinfluence。
Theseringsmustberegardedasfilmsoftheioduretofsilver，varying，notonlyinthickness，butinthemoreorlessperfectstatesofcombinationinwhichtheiodineandmetalare。
Theexteriorcircleisanioduretinaveryloosestateofchemicalagregation;theattractiveforcesincreaseasweproceedtowardsthecentre，whereawellformedioduret，orprobablyatrueiodideofsilver，isformed，whichisacteduponbysunlightwithdifficulty。TheexteriorandmostsensitivefilmconstitutesthesurfaceofDaguerreotypeplates。
Thechangeswhichthesecoloredringsundergoareremarkable;
byafewminutesexposuretosunlight，aninversionofnearlyallthecolorstakesplace，thetwofirstringsbecomingadeepolivegreen;andadeepblueincliningtoblack。