the history and practice of the art of photography-第4节

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he　second　and　third　boards；　because　the　same　quantity　of　light　is　diffused　over　a　space　four　and　sixteen　times　greater。　These　same　rays　may　be　collected　and　their　intensity　again　increased。

Rays　of　light　are　reflected　from　one　surface　to　another；　Refracted；　or　bent；　as　they　pass　from　the　surface　of　one　transparent　medium　to　another；　and　Inflected；　or　turned　from　their　course；　by　the　attraction　of　opaque　bodies。　　From　the　first　we　derive　the　principles　on　which　mirrors　are　constructed；　to　the　second　we　are　indebted　for　the　power　of　the　lenses；　and　the　blessings　of　sight；for　the　light　acts　upon　the　retina　of　the　eye　in　the　same　manner　as　on　the　lens　of　a　camera。　The　latter　has　no　important　bearing　upon　our　subject。

When　a　ray　of　light　falls　perpendicularly　upon　an　opaque　body；　it　is　reflected　bark　in　the　same　line　in　which　it　proceeds；　in　this　case　the　reflected　ray　returns　in　the　same　path　the　incident　ray　traversed；　but　when　a　ray　falls　obliquely；　it　is　reflected　obliquely；　that　is；　it　is　thrown　off　in　opposite　direction；　and　as　far　from　the　perpendicular　as　was　the　incident　ray；　as　shown　at　Fig。　　2；　a　representing　the　incident　ray　and　b　the　reflected。　The　point；　or　angle　c　made　by　　＇hipho_2。gif＇　the　incident　ray；　at　the　surface　of　the　reflector　e　f；　with　a　line　c　d；　perpendicular　to　that　surface；　is　called　the　angle　of　incidence；　while　the　angle　formed　by　the　reflected　ray　b　and　the　perpendicular　line　d　is　called　the　angle　of　reflection；　and　these　angles　are　always　equal。

It　is　by　this　reflection　of　light　that　objects　are　made　visible；　but　unless　light　falls　directly　upon　the　eye　they　are　invisible；　and　are　not　sensibly　felt　until　after　a　certain　series　of　operations　upon　the　various　coverings　and　humors　of　the　eye。　Smooth　and　polished　surfaces　reflect　light　most　powerfully；　and　send　to　the　eye　the　images　of　the　objects　from　which　the　light　proceeded　before　reflection。　　Glass；　which　is　transparent　transmitting　lightwould　be　of　no　use　to　us　as　a　mirror；　were　it　not　first　coated　on　one　side　with　a　metalic　amalgam；　which　interrupts　the　rays　in　their　passage　from　the　glass　into　the　air；　and　throws　them　either　directly　in　the　incident　line；　or　in　an　oblique　direction。　　The　reason　why　trees；　rocks　and　animals　are　not　all　mirrors；　reflecting　other　forms　instead　of　their　own；　is；　that　their　surfaces　are　uneven；　and　rays　of　light　reflected　from　an　uneven　surface　are　diffused　in　all　directions。

Parallel　rays　falling　obliquely　upon　a　plane　mirror　are　reflected　parallel；　converging　rays；　with　the　same　degree　of　convergence；　and　diverging　rays　equally　divergent。

Stand　before　a　mirror　and　your　image　is　formed　therein；　and　appears　to　be　as　far　behind　the　glass　as　you　are　before　it；　making　the　angle　of　reflection　equal　to　that　of　incidence；　as　before　stated。　　The　incident　ray　and　the　reflected　ray　form；　together；　what　is　called　the　passage　of　reflection；　and　this　will　therefore　make　the　actual　distance　of　an　image　to　appear　as　far　again　from　the　eye　as　it　really　is。　Any　object　which　reflects　light　is　called　a　radiant。　The　point　behind　a　reflecting　surface；　from　which　they　appear　to　diverge；　is　called　the　virtual　focus。

Rays　of　light　being　reflected　at　the　same　angle　at　which　they　fall　upon　a　mirror；　two　persons　can　stand　in　such　a　position　that　each　can　see　the　image　of　the　other　without　seeing　his　own。　Again；　you　may　see　your　whole　figure　in　a　mirror　half　your　length；　but　if　you　stand　before　one　a　few　inches　shorter　the　whole　cannot　be　reflected；　as　the　incident　ray　which　passes　from　your　feet　into　the　mirror　in　the　former　case；　will　in　the　latter　fall　under　it。　Images　are　always　reversed　in　mirrors。

Convex　mirrors　reflect　light　from　a　rounded　surface　and　disperse　the　rays　in　every　direction；　causing　parallel　rays　to　diverge；　diverging　rays　to　diverge　more；　and　converging　rays　to　converge　less　They　represent　objects　smaller　than　they　really　arebecause　the　angle　formed　by　the　reflected　ray　is　rendered　more　acute　by　a　convex　than　by　a　plane　surface；　and　it　is　the　diminishing　of　the　visual　angle；　by　causing　rays　of　light　to　be　farther　extended　before　they　meet　in　a　point；　which　produces　the　image　of　convex　mirrors。　　The　greater　the　convexity　of　a　mirror；　the　more　will　the　images　of　the　objects　be　diminished；　and　the　nearer　will　they　appear　to　the　surface。　These　mirrors　furnish　science　with　many　curious　and　pleasing　facts。

Concave　mirrors　are　the　reverse　of　convex；　the　latter　being　rounded　outwards；　the　former　hollowed　inwardsthey　render　rays　of　light　more　converging　collect　rays　instead　of　dispersing　them；　and　magnify　objects　while　the　convex　diminishes　them。

Rays　of　light　may　be　collected　in　the　focus　of　a　mirror　to　such　intensity　as　to　melt　metals。　　The　ordinary　burning　glass　is　an　illustration　of　this　fact；　although　the　rays　of　light　are　refracted；　or　passed　through　the　glass　and　concentrated　into　a　focus　beneath。

When　incident　rays　are　parallel；　the　reflected　rays　converge　to　a　focus；　but　when　the　incident　rays　proceed　from　a　focus；　or　are　divergent；　they　are　reflected　parallel。　　It　is　only　when　an　object　is　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