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

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ch；　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　perceptible　at　the　least　refracted　end。　　Beyond　the　red　ray；　an　extended　space　is　protected　from　the　agency　of　the　dispersed　light；　and　its　whiteness　maintained；　thus　confirming　the　evidence　of　some　chemical　power　in　action；　over　a　space　beyond　the　luminous　spectrum；　which　corresponds　with　the　rays　of　the　least　refrangibility。

This　salt　is　extensively　used　in　photographic　drawing。

PREPARATIONS　OF　GOLD。Chloride　of　Gold；　freed　from　an　excess　of　acid　is　slowly　changed　under　the　action　of　light；　a　regularly　increasing　darkness　taking　place　until　it　becomes　purple；　the　first　action　of　the　light　being　to　whiten　the　paper；　which；　if　removed　from　the　light　at　this　stage；　will　gradually　darken　and　eventually　develope　the　picture。　This　process　may　be　quickened　by　placing　the　paper　in　cold　water。

Chloride　of　gold　with　nitrate　of　silver　gives　a　precipitate　of　a　yellow　brown　color。　　Paper　impregnated　with　the　acetate　of　lead；　when　washed　with　perfectly　neutral　chloride　of　gold；　acquires　a　brownish…yellow　hue。　The　first　impression　of　light　seems　rather　to　whiten　than　darken　the　paper；　by　discharging　the　original　color；　and　substituting　for　it　a　pale　greyish　tint；　which　by　slow　degrees　increases　to　a　dark　slate　color；　but　if　arrested；　while　yet；　not　more　than　a　moderate　ash　grey；　and　held　in　a　current　of　steam；　the　color　of　the　parts　acted　upon　by　light　and　of　that　onlydarkens　immediately　to　a　deep　purple。

Here　I　must　leave　the　subject　of　the　action　of　light　upon　metalic　compounds　referring　to　Mr。　Hunts　work　for　any　further　information　the　student　may　desire　on　the　other　metalsas　I　find　myself　going　beyond　my　limits。　I　cannot；　however；　entirely　dismiss　the　subject　without　giving　a　few　examples　of　the　action　of　light　on　the　juices　of　plants；　some　of　which　produce　very　good　photographic　effect。

CORCHORUS　JAPONICAThe　juice　of　the　flowers　of　this　plant　impart　a　fine　yellow　color　to　paper；　and；　so　far　as　ascertained；　is　the　most　sensitive　of　any　vegetable　preparation；　but　owing　to　its　continuing　to　change　color　even　in　the　dark；　photographic　images　taken　on　paper　prepared　with　it　soon　fade　out。

WALL　FLOWER。This　flower　yields　a　juice；　when　expressed　with　alcohol；　from　which　subsides；　on　standing；　a　bright　yellow　finely　divided　faecula；　leaving　a　greenish…yellow　transparent　liquid；　only　slightly　colored　supernatant。　The　faecula　spreads　well　on　paper；　and　is　very　sensitive　to　light；　but　appears　at　the　same　time　to　undergo　a　sort　of　chromatic　analysis；　and　to　comport　itself　as　if　composed　of　two　very　distinct　coloring　principles；　very　differently　affected。　　The　one　on　which　the　intensity　and　sub…orange　tint　of　the　color　depends；　is　speedily　destroyed；　but　the　paper　is　not　thereby　fully　whitened。　A　paler　yellow　remains　as　a　residual　tint；　and　this　on　continued　exposure　to　the　light；　slowly　darkens　to　brown。　Exposed　to　the　spectrum；　the　paper　is　first　reduced　nearly　to　whiteness　in　the　region　of　the　blue　and　violet　rays。　More　slowly；　an　insulated　solar　image　is　whitened　in　the　less　refrangible　portion　of　the　red。　　Continue　the　exposure；　and　a　brown　impression　begins　to　be　percieved　in　the　midst　of　the　white　streak；　which　darkens　slowly　over　the　region　between　the　lower　blue　and　extreme　violet　rays。

THE　RED　POPPY　yields　a　very　beautiful　red　color；　which　is　entirely　destroyed　by　light。　　When　perfectly　dried　on　paper　the　color　becomes　blue。　This　blue　color　is　speedily　discharged　by　exposure　to　the　sun's　rays；　and　papers　prepared　with　it　afford　very　interesting　photographs。　Future　experiments　will　undoubtedly　more　fully　develope　the　photogenic　properties　of　flowers；　and　practically　apply　them。

Certain　precautions　are　necessary　in　extracting　the　coloring　matter　of　flowers。　　The　petals　of　fresh　flowers；　carefully　selected；　are　crushed　to　a　pulp　in　a　mortar；　either　alone　or　with　the　addition　of　a　litte　alcohol；　and　the　juice　expressed　by　squeezing　the　pulp　in　a　clean　linen　or　cotton　cloth。　　It　is　then　to　be　spread　upon　paper　with　a　flat　brush；　and　dried　in　the　air。　If　alcohol　be　not　added；　it　must　be　applied　immediately；　as　the　air　changes　or　destroys　the　color　instantly。

Most　flowers　give　out　their　coloring　matter　to　alcohol　or　water　but　the　former　is　found　to　weaken；　and　in　some　cases　to　discharge　altogether　these　colors；　but　they　are　in　most　cases　restored　in　drying。　Paper　tinged　with　vegetable　colors　must　be　kept　perfectly　dry　and　in　darkness。

To　secure　an　eveness　of　tint　on　paper　it　should　be　first　moistened　on　the　back　by　sponging；　and　blotting　off　with　bibulous　paper。　　It　should　then　be　pinned　on　a　board；　the　moist　side　downwards；　so　that　two　of　its　edges　the　right　and　lower　onesproject　a　little　over　those　of　the　board。　Incline　the　board　twenty　or　thirty　degrees　to　the　horizon；　and　apply　the　tincture　with　a　brush　in　strokes　from　right　to　left；　taking　care　not　to　go　over　the　edges　which　rests　on　the　board；　but　to　pass　clearly　over　those　that　project；　and　also　observing　to　carry　the　tint　from　below　upwards　by　quick　sweeping　strokes；　leaving　no　dry　spaces　between　them。　　Cross　these　with　other　strokes　from　above　downwards；　leaving　no　floating　liquid　on　the　paper。　Dry　as　quickly　as　possible；　avoiding；　however；　such　heat　as　may　injure　the　tint



CHAP。　IV。

A　FEW　HINTS　AND　SUGGESTIONS　TO　DAGUERREOTYPISTS。


There　are　very　few　who　may　not　be　capable　of　practising　the　Photographic　art；　either　on　paper；　or　metalic　platesbut；　like　all　other　professions；　some　are　more　clever　in　its　various　processes　than　others。

Impatience　is　a　great　drawback　to　perfect　success；　and　combined　with　laziness　is　a　decided　enemy。　　Besides　this；　no　one　can　excel　in　Photography　who　does　not　possess　a　natural　taste　for　the　fine　arts；　who　is　not　quick　in　discerning　grace　and　beautyis　regardless　of　the　principles　of　perspective；　foreshorting　and　other　rules　of　drawing；　and　who　sets　about　it　merely　for　the　sake　of　gainwithout　the　least　ambition　to　rise　to　the　first　rank；　both　in　its　practice　and　theory。　There　is　no　profession　or　trade　in　which　a　slovenly　manner　will　not　show　itself；　and　none　where　its　effects　will　be　more　apparent　than　this。

In　order　to　be　great　in　any　pursuit；　we　must　be　ourselves；　and　keep　all　things；　in　order。　　In　your　show　and　reception　rooms；　let　neatness　prevail；　have　your　specimens　so　placed　leaning　slightly　forwardas　to　obtain　the　strongest　light　upon　them；　and　at　the　same　time　prevent　that　glassiness　of　apearance　which　detracts　so　materially　from　the　effect　they　are　intended　to　produce。　　If　possible；　let　the　light　be　of　a　north…western　aspect；　mellowed　by　curtains　of　a　semitransparent　hue。　Your　show…cases；　at　the　door；　should　be　kept　well　cleaned。　I　have　often　been　disgusted　while　attempting　to　examine　portraits　in　the　cases　of　our　artists；　at　the　greasy　coating　and　marks　of　dirty　fingers　upon　the　glass　and　frame　enclosing　them。　Believe　it；　many　a　good　customer　is　lost　for　no　other　reason。

In　your　operating　room；　dust　should　be　carefully　excluded。　It　should　be　furnished　with　nothing　apt　to　collect　and　retain　dust；　a　carpet　is　therefore　not　only　a　useless　article；　but　very　improper。　A　bare　floor　is　to　be　prefered；　but　if　you