american hand book of the daguerreotype-第15部分

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　as　used　in　the　battery；　fill　this　also　with　sand；　and　close　the　end　with　plaster　of　Paris。　Place　this　in　a　coating…box；　and　it　will　be　found　to　act　with　great　uniformity　and　be　quite　permanent。

Bromide　of　Lime；　another　accelerator；　can　be　used　in　the　same　manner；　except　it　is；　only　necessary；　when　a　solid　sensitive　is　used；　to　mix　it　with　the　sand　without　placing　it　in　a　bottle。　This　method　is　employed　with　great　success　by　a　few；　who　have　regarded　it　as　a　secret　worth　keeping。

A　Combination；　requiring　the　Use　of　only　One　Coating…box。　It　is　often　wondered　by　beginners；　why　some　solution　requiring　only　one　coating　cannot　be　employed。　　This　can　be　done；　but　the　results　are　not　so　satisfactory　as　when　two　or　more　are　employed。　Such　an　accelerator　may　be　produced　by　adding　alcoholic　solution　of　iodine　to　a　solution　of　chlorate　of　potash；　until　the　latter　will　take　up　no　more　of　the　former；　and　to　each　ounce；　by　measure　of　this　solution；　ten　drops　of　a　saturated　solution　of　bromide　in　water　are　added。　The　solution　of　chlorate　of　potash　is　made　by　diluting；　one　part　of　a　saturated　solution　of　the　salt　with　ten　parts　of　water。　The　use　of　the　chlorate　is　simply　as　a　solvent　of　iodine。

Fats　as　Accelerators。The　use　of　fats；　oils；　or　greasy　substances；　has　been　one　of　the　most　emphatic　prohibitions　about　the　Daguerreotype　plate。　　Yet　it　has　been　proved　that　its　presence　in　a　small　quantity　upon　the　silver　surface　has　the　effect　of　reducing　the　time　of　exposure　in　the　camera　from　two…thirds　to　three…fourths。　An　application　may　be　made　as　follows：　Pour　sweet　oil；　or　rub　beef　or　mutton　fat；　on　a　common　buff；　which　is　free　from　all　polishing　powders。　　With　this；　buff　a　well…cleaned　plate；　and　it　will　leave　a　scum；　which　should　be　mostly　removed　by　using　another　buff；　which　should　be　clean。　Coat　the　plate　in　the　usual　manner；　and　the　result　will　be　a　great　reduction　in　the　time　of　exposure　in　the　camera。　The　impression　produced　upon　a　plate　so　prepared　presents；　when　coming　from　the　vapor　of　mercury；　a　grey；　scummy　appearance；　which；　on　the　application　of　heat　in　gilding；　does　not　improve；　hence　its　use　is　not　generally　adopted。

We　have　instituted　some　investigations　upon　this　subject；　and　in　the　present　volume；　we　shall　not　refer　to　it　further。　Those　wishing　to　learn　more　fully　the　effect　of　light　upon　organic　substances　will　find　Robert　Hunt's　〃Researches　on　Light〃　an　invaluable　work。




LIGHT　AND　OPTICS。

CHAPTER　IV。


LightOpticsSolar　SpectrumDecomposition　of　LightLight；　Heat；　and　ActinismBlue　Paper　and　Color　for　the　Walls　of　the　Operating　Room　Proportions　of　Light；　Heat；　and　Actinism　composing　a　Sunbeam　RefractionReflectionLensesCopying　Spherical　Aberration　Chromatic　Aberration。

It　is　advisable　that　persons　engaging　in　the　Daguerreotype　art　should　have　at　least　a　little　knowledge　of　the　general　principles　of　light　and　optics。　It　is　not　the　author's　design　here　to　give　a　full　treatise　on　these　subjects；　but　he　only　briefly　refers　to　the　matter；　giving　a　few　facts。

It　has　been　well　observed　by　an　able　writer；　that　it　is　impossible　to　trace　the　path　of　a　sunbeam　through　our　atmosphere　without　feeling　a　desire　to　know　its　nature；　by　what　power　it　traverses　the　immensity　of　space；　and　the　various　modifications　it　undergoes　at　the　surfaces　and　interior　of　terrestrial　substances。

Light　is　white　and　colorless；　as　long　as　it　does　not　come　in　contact　with　matter。　　When　in　apposition　with　any　body；　it　suffers　variable　degrees　of　decomposition；　resulting　in　color；　as　by　reflection；　dispersion；　refraction；　and　unequal　absorption。

To　Sir　I。　Newton　the　world　is　indebted　for　proving　the　compound　nature　of　a　ray　of　white　light　emitted　from　the　sun。　The　object　of　this　work　is　not　to　engage　in　an　extended　theory　upon　the　subject　of　light；　but　to　recur　only　to　some　points　of　more　particular　interest　to　the　photographic　operator。

The　decomposition　of　a　beam　of　light　can　be　noticed　by　exposing　it　to　a　prism。　　If；　in　a　dark　room；　a　beam　of　light　be　admitted　through　a　small　hole　in　a　shutter；　it　will　form　a　white　round　spot　upon　the　place　where　it　falls。　　If　a　triangular　prism　of　glass　be　placed　on　the　inside　of　the　dark　room；　so　that　the　beam　of　light　falls　upon　it；　it　no　longer　has　the　same　direction；　nor　does　it　form　a　round　spot；　but　an　oblong　painted　image　of　seven　colorsred；　orange；　yellow；　green；　blue；　indigo；　and　violet。　This　is　called　the　solar　spectrum；　and　will　be　readily　understood　by　reference　to　the　accompanying　diagram；　Fig。　　1。

＇page　133＇

To　those　who　are　unacquainted　with　the　theory　of　light　（and　for　their　benefit　this　chapter　is　given）；　it　may　be　a　matter　of　wonder　how　a　beam　of　light　can　be　divided。

＇amdg_1。gif＇

This　can　be　understood　when　I　say；　that　white　light　is　a　bundle　of　colored　rays　united　together；　and　when　so　incorporated；　they　are　colorless；　but　in　passing　through　the　prism　the　bond　of　union　is　severed；　and　the　colored　rays　come　out　singly　and　separately；　because　each　ray　has　a　certain　amount　of　refracting　（bending）　power；　peculiar　to　itself。　These　rays　always　hold　the　same　relation　to　each　other；　as　may　be　seen　by　comparing　every　spectrum　or　rainbow；　there　is　never　any　confusion　or　misplacement。

There　are　various　other　means　of　decomposing

＇page　134＇

white　light　besides　the　prism；　of　which　one　of　the　principal　and　most　interesting　to　the　Daguerreotypist　is　by　reflection　from　colored　bodies。　　If　a　beam　of　white　light　falls　upon　a　white　surface；　it　is　reflected　without　change；　but　if　it　falls　upon　a　red　surface；　only　the　red　ray　is　reflected：　　so　also　with　yellow　and　other　colors。　The　ray　which　is　reflected　corresponds　with　the　color　of　the　object。　It　is　this　reflected　decomposed　light　which　prevents　the　beautifully…colored　image　we　see　upon　the　ground　glass　in　our　cameras。

＇amdg_2。gif＇

A　sunbeam　may　be　capable　of　three　divisionsLIGHT；　HEAT；　and　ACTINISM；　the　last　causes　all　the　chemical　changes；　and　is　the　acting　power　upon　surfaces　prepared　to　receive　the　photographic　image。　The　accompanying　illustration；　Fig。　　2；　will　readily　bring　to　the　mind　of　the　reader　the　relation　of　these　one　to　another；　and　their　intensities　in　the　different　parts　of　a　decomposed　sunbeam。

The　various　points　of　the　solar　spectrum　are　represented　in　the　order　in　which　they　occur　between　A；　and　B；　this　exhibits　the　limits　of　the　Newtonian　spectrum；　corresponding　with　Fig。　　1。　　Sir　John　Herschel　and　Seebeck　have　shown　that　there　exists；　beyond　the　violet；　a　faint　violet　light；　or　rather　a　lavender　to　b；　to　which　gradually　becomes　colorless；　similarly；　red　light　exists　beyond　the　assigned　limits　of　the　red　ray　to　a。　The　greatest　amount　of　actinic　power　is　shown　at　E　opposite　the　violet；　hence　this　color　〃exerts〃　the　greatest　amount　of　influence　in　the　formation　of　the　photographic　image。

（Blue　paper　and　blue　color　have　been　somewhat　extensively　used　by　our　Daguerreotype　operators　in　their　operating　rooms　and　skylights；　in　order　to　facilitate　the　operation　in　the　camera。　　I　fancy；　however；　that　this　plan　cannot　be　productive　of　as　much　good　as　thought　by　some；　from　the　fact；　that　the　light　falling　upon　the　subject；　and　then　reflected　into　the　camera；　is；　coming　through　colorless　glass；　not　affected　by　such　rays　as　may　be　reflected　from　the　walls　of　the　operating　room；　and　even　if　it　were　so；　I　conceive　that　it　would　be　injurious；　by　destroying　the　harmony　of　shadows　which　might　otherwise　occur。）　The　greatest　amount　of　white　light　is　at　C；　the　yellow　contains　less　of　the　chemical　power　than　any　other　portion　of　the　solar　spectrum。　It　has　been　found　that　the　most　intense　heat　is　at　the　extreme　red；　b。

Artificial　lights　differ　in　their　color；　the　white　light　of　burning　charcoal；　which　is　the　principal　light　from　candles；　oil　and　gas；　contains　three　raysred；　yellow；　and　blue。　The　dazzling　light　emitted　from　lime　intensely　heated；　known　as　the　Drummond　light；　gives　the　colors　of　the　prism　almost　as　bright　as　the　solar　spectrum。

If　we　expose　a　prepared　Daguerreotype　plate　or　sensitive　paper　to　the　solar　spectrum；　it　will　be　observed　that　the　luminous　power　（the　yellow）　occupies　but　a　small　space　compared　with　the　influence　of　heat　and　chemical　power。　　R。　Hunt；　in　his　Researches　on　Light；　has　presented　the　following　remarks　upon　the　accompanying　illustration：

〃If　the　linear　measure；　or　the　diameter　of　a　circle　which　shall　include　the　luminous　rays；

＇amdg_3。gif＇

is　25；　that　of　the　calorific　spectrum　will　be　42。10；　and　of　the　chemical　spectrum　55。10。　Such　a　series　of　circles　may　well　be　used　to　represent　a　beam　from　the　sun；　which　may　be　regarded　as　an　atom　of　Light；　surrounded　with　an　invisible　atmosphere　of　Heat；　and　another　still　more　extended；　which　possesses　the　remarkable　property　of　producing　chemical　and　molecular　change。

A　ray　of　light；　in　passing　obliquely　through　any　medium　of　uniform　density；　does　not　change　its　course；　but　if　it　should　pass　into　a　denser　body；　it　would　turn　from　a　straight　line；　pursue　a　less　oblique　direction；　and　in　a　line　nearer　to　a　perpendicular　to　the　surface　of　that　body。　Water　exerts　a　stronger　refracting　power　than　air；　and　if　a　ray　of　light　fall　upon　a　body　of　this　fluid　its　course　is　changed；　as　may　be　seen　by　reference　to　Fig。　4。

＇amdg_4。gif＇

It　is　observed　that　it　proceeds　in　a　less　oblique　direction　（towards　the　dotted　line）；　and；　on　passing　on　through；　leaves　the　liquid；　proceeding　in　a　line　parallel　to　that　at　which　it　entered。　　It　should　be　observed　that　at　the　surface　of　bodies　the　refractive　power　is　exerted；　and　that　the　light　proceeds　in　a　straight　line　until　leaving　the　body。　The　refraction　is　more　or　less；　and　in　all　cases　in　proportion　as　the　rays　fall　more　or　less　obliquely　on　the　refracting　surface。　It　is　this　law　of　optics　which　has　given　rise　to　the　lenses　in　our　camera　tubes；　by　which　means　we　are　enabled　to　secure　a　well…delineated　representation　of　any　object　we　choose　to　picture。

When　a　ray　of　light　passes　from　one　medium　to　another；　and　through　that　into　the　first　again；　if　the　two　refractions　be　equal；　and　in　opposite　directions；　no　sensible　effect　will　be　produced。

The　reader　may　readily　comprehend　the　phenomena　of　refraction；　by　means　of　light　passing　through　lenses　of　different　curves；　by　reference　to　the　following　diag