the history and practice of the art of photography-第5章

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　nearer　to　a　concave　mirror　than　its　centre　of　concavity；　that　its　image　is　magnified；　for　when　the　object　is　farther　from　the　mirror；　this　centre　will　appear　less　than　the　object；　and　in　an　inverted　position。

The　centre　of　concavity　in　a　concave　mirror；　is　an　imaginary　point　placed　in　the　centre　of　a　circle　formed　by　continuing　the　boundary　of　the　concavity　of　the　mirror　from　any　one　point　of　the　edge　to　another　parallel　to　and　beneath　it。

REFRACTION　OF　LIGHT：I　now　pass　to　the　consideration　of　the　passage　of　light　through　bodies。

A　ray　of　light　failing　perpendicularly　through　the　air　upon　a　surface　of　glass　or　water　passes　on　in　a　straight　line　through　the　body；　but　if　it；　in　passing　from　one　medium　to　another　of　different　density；　fall　obliquely；　it　is　bent　from　its　direct　course　and　recedes　from　it；　either　towards　the　right　or　left；　and　this　bending　is　called　refraction；　（see　fig。　　3；　b。）　If　a　ray　of　light　passes　from　a　rarer　into　a　denser　medium　it　is　refracted　towards　a　perpendicular　in　that　medium；　but　if　it　passes　from　a　denser　into　rarer　it　is　bent　further　from　a　perpendicular　in　that　medium。　Owing　to　this　bending　of　the　rays　of　light　the　angles　of　refraction　and　incidence　are　never　equal。

Transparent　bodies　differ　in　their　power　of　bending　light　as　a　general　rule；　the　refractive　power　is　proportioned　to　the　densitybut　the　chemical　constitution　of　bodies　as　well　as　their　density；　is　found　to　effect　their　refracting　power。　Inflamable　bodies　possess　this　power　to　a　great　degree。

The　sines　of　the　angle　of　incidence　and　refraction　（that　is；　the　perpendicular　drawn　from　the　extremity　of　an　arc　to　the　diameter　of　a　circle；）　are　always　in　the　same　ratio；　viz：　　from　air　into　water；　the　sine　of　the　angle　of　refraction　is　nearly　as　four　to　three；　whatever　be　the　position　of　the　ray　with　respect　to　the　refracting　surface。　From　air　into　sulphur；　the　sine　of　the　angle　of　refraction　is　as　two　to　one　therefore　the　rays　of　light　cannot　be　refracted　whenever　the　sine　of　the　angle　of　refraction　becomes　equal　to　the　radius*　of　a　circle；　and　light　falling　very　obliquely　upon　a　transparent　medium　ceases　to　be　refracted；　this　is　termed　total　reflection。

*　The　RADIUS　of　a　circle　is　a　straight　line　passing　from　the　centre　to　the　circumference。

Since　the　brightness　of　a　reflected　image　depends　upon　the　quantity　of　light；　it　is　quite　evident　that　those　images　which　arise　from　total　reflection　are　by　far　the　most　vivid；　as　in　ordinary　cases　of　reflection　a　portion　of　light　is　absorbed。

I　should　be　pleased　to　enter　more　fully　into　this　branch　of　the　science　of　optics；　but　the　bounds　to　which　I　am　necessarily　limited　in　a　work　of　this　kind　will　not　admit　of　it。　In　the　next　chapter；　however；　I　shall　give　a　synopsis　of　Mr。　Hunt's　treatise　on　the　〃Influence　of　the　Solar　Rays　on　Compound　Bodies；　with　especial　reference　to　their　Photographic　application〃　A　work　which　should　be　in　the　hands　of　every　Daguerreotypist；　and　which　I　hope　soon　to　see　republished　in　this　country。　I　will　conclude　this　chapter　with　a　brief　statement　of　the　principles　upon　which　the　Photographic　art　is　founded。

SOLAR　and　Steller　light　contains　three　kinds　of　rays；　viz：

1。　　Colorific；　or　rays　of　color。

2。　　Calorific；　or　rays　of　heat。

3。　　Chemical　rays；　or　those　which　produce　chemical　effects。

On　the　first　and　third　the　Photographic　principle　depends。　In　explaining　this　principle　the　accompanying　wood　cuts；　（figs。　3　and　4）　will　render　it　more　intelligible。

If　a　pencil　of　the　sun's　rays　fall　upon　a　prism；　it　is　bent　in　passing　through　the　transparent　medium；　and　some　rays　being　more　refracted　than　others；　we　procure　an　elongated　image　of　the　luminous　beam；　exhibiting　three　distinct　colors；　red；　yellow　and　blue；　which　are　to　be　regarded　as　primitivesand　from　their　interblending；　seven；　as　recorded　by　Newton；　and　shown　in　the　accompanying　wood　cut。　These　rays　being　absorbed；　or　reflected　differently　by　various　bodies；　give　to　nature　the　charm　of　color。　　Thus　to　the　eve　is　given　the　pleasure　we　derive　in　looking　upon　the　green　fields　and　forests；　the　enumerable　varieties　of　flowers；　the　glowing　ruby；　jasper；　topaz；　amethist；　and　emerald；　the　brilliant　diamond；　and　all　the　rich　and　varied　hues　of　nature；　both　animate　and　inanimate。　　＇hipho_3。gif＇

Now；　if　we　allow　this　prismatic　spectrum　（b。　fig。　　3。）　to　fall　upon　any　surface　（as　at　c。）　prepared　with　a　sensitive　photographic　compound；　we　shall　find　that　the　chemical　effect　produced　bears　no　relation　to　the　intensity　of　the　light　of　any　particular　colored　ray；　but　that；　on　the　contrary；　it　is　dispersed　over　the　largest　portion　of　the　spectrum；　being　most　energetic　in　the　least　luminous　rays；　and　ever　active　over　an　extensive　space；　where　no　traces　of　light　can　be　detected。　　Fig。　　4；　will　give　the　student　a　better　idea　of　this　principle。　　It　is　a　copy　of　the　kind　of　impression　which　the　spectrum；　spoken　of；　would　make　on　a　piece　of　paper　covered　with　a　very　sensitive　photographic　preparation。　The　white　space　a。　　corresponds　with　the　most　luminous；　or　yellow　ray；　（5；　fig。　　3）　over　limits　of　which　all　chemical　change　is　prevented。　A　similar　action　is　also　produced　by　the　lower　end　of　the　red　ray　c；　but　in　the　upper　portion；　however　we　find　a　decided　change　（as　at　d）。　The　most　active　chemical　change；　you　will　percieve；　is　produced　by　the　rays　above　the　yellow　a；　viz。　　4；　3；　2　and　1　（as　at　b）　the　green　（4）　being　the　least　active；　and　the　blue　（3）　and　violet　（1）　rays　the　most　so；　the　action　still　continuing　far　beyond　the　point　b　which　is　the　end　of　the　luminous　image。　　＇hipho_4。gif＇

Suppose　we　wish　to　copy　by　the　Daguerreotype；　or　Calotype　process；　any　objects　highly　coloredblue；　red　and　yellow；　for　instance　predominating　the　last　of　course　reflects　the　most　light；　the　blue　the　least；　but　the　rays　from　the　blue　surface　will　make　the　most　intense　impression；　whilst　the　red　radiations　are　working　very　slowly；　and　the　yellow　remains　entirely　inactive。　　This　accounts　for　the　difficulty　experienced　in　copying　bright　green　foliage；　or　warmly　colored　portraits；　a　large　portion　of　the　yellow　and　red　rays　entering　into　the　composition　of　both　and　the　imperfections　of　a　Daguerreotype　portrait　of　a　person　with　a　freckled　face　depends　upon　the　same　cause。

A　yellow；　hazy　atmosphere；　even　when　the　light　is　very　bright；　will　effectually　prevent　any　good　photographic　result　and　in　the　height　of　summer；　with　the　most　sensative　process；　it　not　unfrequently　happens　that　the　most　anoying　failures　arise　from　this　agency　of　a　yellow　medium。　　A　building　painted　of　a　yellow　color；　which　may　reflect　the　sun's　rays　directly　into　the　operator's　room　will　have　the　same　effect。　Daguerreotypists；　being　ignorant　of　these　facts；　are　very　apt　to　charge　their　want　of　success　to　the　plates；　or　chemicals；　or　any　thing　but　the　real　cause；　and　it　would　be　well　to　bear　these　facts　constantly　in　mind　and　as　far　as　possible　avoid　them。　This；　may　be　accomplished；　in　a　measure；　by　a　choice　of　location　or　by　having　the　glass　of　your　windows　tinged　with　blue；　or　a　screen　of　thin　blue　paper　may　be　interposed　between　the　light　and　sitter。　In　selecting　subjects；　all　striking　contrasts　in　color　should　be　avoided；　and　sitters　for　portraits　should　be　cautioned　not　to　wear　anything　that　may　produce　the　effect　spoken　of　dark　dresses　always　being　the　best。

The　action　of　light　both　combines　and　decomposes　bodies。　For　instance；　chlorine　and　hydrogen　will　remain　in　a　glass　vessel　without　alteration　if　kept　in　the　dark；　but　if　exposed　to　the　rays　of　the　sun；　they　immediately　enter　into　combination；　and　produce　hydrochloric　acid。　On　the　other　hand；　if　colorless　nitric　acid　be　exposed　to　the　sun；　it　becomes　yellow；　then　changes　to　red；　and　oxygen　is　liberated　by　the　partial　decomposition　effected　by　the　solar　rays。

Of　the　organic　substances　none　are　more　readily　acted　upon　by　light　than　the　various　combinations　of　silver。

Of　these　some　are　more；　and　others　less　sensitive。　　If　Chloride　of　silver；　which　is　a　white　precipitate　formed　by　adding　chloride　of　sodium　（common　salt）　to　a　solution　of　nitrate　of　silver；　be　exposed　to　diffused　light；　it　speedily　assumes　a　violet　tint；　and　ultimately　becomes　nearly　black。　With　iodide　of　silver；　bromide　of　silver；　a