american hand book of the daguerreotype-第4部分

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In　regard　to　the　proper　elevation　of　the　camera；　it　may　be　here　stated　that　I　have　found　it　best　in　taking　portraits　where　the　hands　are　introduced；　to　place　the　camera　at　about　equal　height　with　the　eyes　of　the　sitter；　in　order　to　bring　the　face　and　hands　equi…distant　from　the　tube。　It　will　be　found；　if　the　above　be　followed；　that　by　attaching　a　string　to　the　camera　tube；　and　making　a　semi…circle；　that　the　face　and　hands　of　the　sitter　will　occupy　a　corresponding　distance；　and　the　consequence　is　that　the　impression　will　appear　without　the　hands　being　magnified。　It　has　been　found　that　a　person　with　a　freckly　face　can　have　as　fine；　fair；　and　clear　an　impression　as　the　most　perfect　complexion；　this　may　be　done　by　the　subject　rubbing　the　face　until　it　is　very　red。　The　effect　is　to　lessen　the　contrast；　by　giving　the　freckles　and　skin　the　same　color　and　the　photogenic　intensity　of　the　red　and　yellow　being　nearly　the　same；　an　impression　can　be　produced　perfectly　clear。

When　a　child　is　to　be　taken；　and　there　are　doubts　of　its　keeping　still；　the　operation　may　be　accelerated　by　placing　it　nearer　the　window　bringing　the　screen　nearer；　and　placing　a　white　muslin　cloth　over　the　head；　this　will　enable　you　to　work　in　one　third　of　the　usual　time。　Should　the　person　move；　or　the　plate　become　exposed　to　the　light；　it　may　be　restored　to　its　original　sensitiveness　by　placing　it　over　the　quick；　one　or　two　seconds。

Developing　the　Daguerreotype。After　the　plate　has　been　submittedto　the　o　peration　of　the　light；　the　image　is　still　invisible。　It　requires　to　be　exposed　to　the　vapors　of　heated　mercury。　It　is　not　absolutely　necessary　to　apply　artificial　heat　to　the　mercury　to　develop　the　image；　for　fair　proofs　have　been　produced　by　placing　a　plate　over　the　bath　at　the　ordinary　temperature　of　the　atmosphere。　This　plan；　however；　requires　a　long　time　and　cannot　be　adopted　in　practice；　even　if　it　were　advisable。　　The　time　more　usually　required　in　developing　the　image　over　the　mercurial　vapors；　is　about　two　minutes；　and　the　temperature　is　raised　to　a　point　necessary　to　produce　the　desired　effect　in　that　time。　This　point　varies　as　indicated　by　different　scales；　but　for　the　ordinary　scales　it　is　not　far　from　90　deg。　　cen。

The　mercury　bath　is　accompanied　with　a　centigrade　thermometer；　by　which　the　heat　is　regulated。　　Those　furnished　by　the　manufacturers　are　not　always　correct；　and　it　requires　some　experience　to　find　the　proper　degree　on　the　scale。

I　would　here　remark　that　it　is　advisable；　when　placing　the　spirit　lamp　under　the　bath；　to　so　arrange　it　that　the　position　of　applied　heat　should　always　be　on　the　same　point；　viz。；　should　the　heat　be　directly　under　the　bulb　containing　the　thermometer　it　would　raise　the　mercury　in　the　tube　to　the　point　marked；　and　the　temperature　of　that　in　the　bath　would　be　far　below　what　it　should　be；　hence　it　is　（where　time　is　followed　for　developing）　that　many　failures　occur。　　This　is　observed　more　readily　in　the　large　baths　made　of　thick　iron；　particularly　upon　first　heating。　In　practice　I　apply　the　heat　as　nearly　as　possible　between　the　centre　of　the　bottom　of　the　bath　and　the　bulb　containing　the　mercury　tube。　It　is　advisable　to　keep　the　lamp　lighted　under　the　bath　from　the　time　of　commencing　in　the　morning　to　the　close　of　business　at　night。　By　this　means　you　have　a　uniformity　of　action；　that　cannot　be　otherwise　obtained。

It　is　well　known　to　the　experienced　Daguerreotypist；　that　different　atmospheres　have　a　decided　effect　upon　the　mercury　in　developing　the　Daguerreotype。　　It　will　require　a　greater　degree　of　heat　for　one　atmosphere　than　for　another。　Experience　alone　determines　this　little　difference。

In　summer；　on　cloudy　and　stormy　days；　mercurial　vapors　rise　more　readily　and　quickly　than　in　the　temperature　of　autumn　or　winter。　From　60　degrees　upwards　towards　the　boiling　point　（660　deg。）；　the　vapors　of　mercury　rise　in　greater　abundance　and　collect　in　larger　globules　on　cold　surfaces。

For　various　reasons　I　prefer　a　high　temperature　and　short　exposure。　It　accelerates　the　process。　　It　renders　the　lights　of　the　picture　more　strong　and　clear；　while　the　deep　shades　are　more　intense。　It　gives　a　finer　lustre　to　the　drapery。　　The　solarized　portions　also　are　very　seldom　blue；　especially　after　gilding。　If　heated　too　high；　however；　the　light　parts　become　of　a　dead；　chalky　white；　and　the　shadows　are　injured　by　numerous　little　globules　of　mercury　deposited　over　them。　　Just　the　right　quantity　of　mercury　leaves　the　impression　of　a　transparent；　pearly　white　tone；　which　improves　in　the　highest　degree　in　gilding。　To　mercurialize　with　exactness　is　a　nice　point。　　If　there　is　reason　to　suspect　having　timed　rather　short　in　the　camera；　reduce　the　time　over　mercury　in　a　corresponding　proportion。　A　dark　impression　will　be　ruined　by　the　quantity　of　mercury　which　would　only　improve　a　light　one。

If　practicable；　it　is　most　expedient　that　the　plate　be　submitted　to　the　action　of　mercury　immediately　on　coming　from　the　camera。　I　have　frequently；　however；　carried　plates　for　miles　in　the　plate…holders　and　after　exposing　in　the　camera；　brought　them　back　to　expose　to　mercury；　and　obtained　fair　proofs；　but　for　the　reason　before　given；　it　is　advisable　to　carry　along　the　bath；　and　bring　out　the　impression　on　the　spot。

It　is　sometimes　the　practice　of　inexperienced　operators　to　take　the　plate　off　the　bath　and　examine　the　impression　by　solar　light。　This　plan　should　be　abandoned；　as　it　is　almost　sure　to　produce　a　dense　blue　film　over　the　shadows。

This　I　am　led　to　believe　is　occasioned　by　the　action　of　light　on　the　yet　sensitive　portions　of　the　plate；　and　made　to　appear　only　by　subsequent　exposure　to　mercury；　being　equivalent　to　solarization。

There　has　been　little　said　by　our　professors　upon　the　subject　of　the　position　of　the　plates　while　exposed　to　the　mercurial　vapour。　Mr。　Hunt；　in　referring　to　this　subject；　says：　　〃Daguerre　himself　laid　much　stress　upon　the　necessity　of　exposing　the　plate　to　the　mercury　at　an　angle　of　about　45　deg。。　This；　perhaps；　is　the　most　convenient　position　as　it　enables　the　operator　to　view　the　plate　distinctly；　and　watch　the　development　of　the　design；　but　beyond　this；　I　am　satisfied　there　exists　no　real　necessity　for　angular　position。　Both　horizontally　and　vertically；　I　have　often　produced　equally　effective　Daguerreotypes。〃　　I　presume　from　the　last　sentence　of　Mr。　Hunt；　that　he　has　confined　his　experiments　to　the　smaller　sized　plates。　Hence　he　may　not　have　thought　of　the　effect　of　the　vertical　exposure　of　a　large　plate。

In　America　this　is　a　subject　of　no　little　importance。　　When　an　impression　is　to　be　developed　upon　a　plate　fifteen　by　seventeen　inches；　were　we　to　use　an　angle　of　about　45　deg。；　it　would　be　found　to　make　a　perceptible　difference　in　the　appearance　of　the　image。　By　examining　the　wood　tops　of　our　baths　as　formerly　made；　it　will　be　found　that　there　is　a　great　variation　in　the　distance　from　the　mercury　to　the　different　portions　of　the　plate。　By　measuring　one　of　these　tops　for　the　size　plate　above　mentioned；　I　find　the　distance　to　the　nearest　point　between　the　mercury　and　the　plate；　to　be　thirteen；　and　the　middle　point　sixteen；　and　the　furthest　point　twenty…one　and　a　half　inches：　by　this　we　see　that　one　point　of　the　plate　is　eight　and　a　half　inches　further　from　the　mercury　than　the　nearest　point；　even　this　is　not　the　variation　there　would　necessarily　be；　were　we　to　adopt　the　angle　of　45　deg。　　as　urged　by　Daguerre。

Among　our　principal　professors；　the　beveltop　will　not　be　found　in　use　where　the　large　plates　are　used。　　Should　any　one　feel　desirous　to　test　more　minutely　the　effect　produced　by　a　bevel　top　bath；　I　would　suggest　to　them　to　place　a　frame；　so　constructed　as　to　hold　three　sixth　size　plates；　and　fit　it　to　the　top　of　the　bath；　and　so　arrange　it　with　openings　that　the　plates　may　be　placed；　one　at　the　nearest　point　of　the　mercury；　the　second　midway；　and　the　third　to　the　greatest　distance；　and　by　placing　the　plates　over　at　one　and　the　same　time；　the　experimenter　will　be　enabled　to　judge　if　there　exists　a　difference　in　the　developing。　In　speaking　of　the　above；　reference　is　had　to　baths　to　the　ordinary　heights　used　by　operators。

We　will　now　proceed　to　examine　the　effect　produced　by　mercurial　vapor　upon　the　plate　at　different　lengths　of　exposure。　In　some　investigations　which　I　have　made　upon　the　appearance　of　the　Daguerreotype　impressions　when　developed　over　mercury　at　90　deg。　　C。　（194　deg。　　F。）；　the　following　was　the　result。　Plates；　coated　and　exposed　to　light　in　our　usual　manner　of　operating；　produced　on　exposure　of

1/2　minute；　whole　impression；　deep　blue。

1　minute；　ashy　and　flat；　no　shadows；　linen；　deep　blue。

1　1/2　minute；　coarse　and　spongy；　shadows；　muddy；　drapery；　dirty　reddish　brown。

2　minutes；　shallow　or　watery；　shadows；　yellowish；　drapery；　brown。

2　1/4　minutes；　soft；　face；　scarcely　white；　shadows；　neutral；　drapery；　fine　dark　brown　linen　somewhat　blue。

2　1/2　minutes；　clear　and　pearly；　shadows；　clear　and　positive；　of　a　purple　tint；　drapery；　jet　black；　with　the　dark　shades　slightly　frosted　with　mercury。

2　3/4　to　3　minutes；　hard　and　chalky；　shadows；　harsh；　drapery；　roughened；　and　misty　with　excess　of　mercury。

The　foregoing　results　will　be　found　general。

There　are　numerous　opinions　among　our　operators　in　regard　to　the　quantity　of　mercury　necessary　for　a　bath。　　As　regards　this；　I　need　only　say；　similar　results　occur　when　two　pounds　or　two　ounces　are　used；　but　the　quantity　generally　employed　is　about　a　quarter　of　a　pound。　I　am　of　the　opinion　that　one　ounce　will　answer　as　well　as　a　larger　quantity。　I　know　of　no　better　proof　in　favor　of　a　small　quantity　than　that　presented　in　the　following　incident。　　Several　years　since；　an　operator　（Mr。　Senter；　of　Auburn；　N。Y。）　of　my　acquaintance；　was　requested　to　go　several　miles　to　take　a　Daguerreotype　portrait　of　a　deceased　person。　He　packed　up　his　apparatus　and　proceeded　over　a　rough　road　for　some　distance　to　the　house　where　he　was　to　take　the　portrait；　and　arranging　his　apparatus；　with　all　the　expedition　which　the　occasion　required；　after　having　everything　in　usual　order　（as　was　supposed）；　he　proceeded　and　took　some　ten　or　twelve　very　superior　impressions。　They　were　fine；　clear；　and　well　developed。　　After　taking　the　number　ordered；　he　proceeded　to　repack