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

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ed　rays　of　light　to　consist　of　minute　particles　of　matter；　which　are　constantly　emanating　from　luminous　bodies　and　cause　vision；　as　odoriferous　particles；　proceeding　from　certain　bodies；　cause　smelling。

The　effects　of　light　upon　other　bodies；　and　how　light　is　effected　by　them；　involve　some　of　the　most　important　principles；　which　if　properly　understood　by　Daguerreotypists　would　enable　them　to　improve　and　correct　many　of　the　practical　operations　in　their　art。　These　effects　we　shall　exhibit　in　this　and　the　following　chapters。　Before　we　enter　on　this　subject　it　will　be　necessary　to　become　familiar　with　the

DEFINITIONS　of　some　of　the　terms　used　in　the　science　of　optics。

Luminous　bodies　are　of　two　kinds；　those　which　shine　by　their　own　light；　and　those　which　shine　by　reflected　light。

Transparent　bodies　are　such　as　permit　rays　of　light　to　pass　through　them。

Translucent　bodies　permit　light　to　pass　faintly；　but　without　representing　the　figure　of　objects　seen　through　them。

Opaque　bodies　permit　no　light　to　pass　through　them；　but　reflect　light。

A　ray　is　a　line　of　light。

A　beam　is　a　collection　of　parallel　rays。

A　pencil　is　a　collection　of　converging；　or　diverging　rays。

A　medium　is　any　space　through　which　light　passes。

Incident　rays　are　those　which　fall　upon　the　surface　of　a　body。

Reflected　rays　are　those　which　are　thrown　off　from　a　body。

Parallel　rays　are　such　as　proceed　equally　distant　from　each　other　through　their　whole　course。

Converging　rays　are　such　as　approach　and　tend　to　unite　at　any　one　point；　as　at　b。　　fig。　　3。

Diverging　rays　are　those　which　continue　to　recede　from　each　other；　as　at　e。　　Fig。　　3。

A　Focus　is　that　point　at　which　converging　rays　meet。

MOTION　OF　LIGHTRays　of　light　are　thrown　off　from　luminous　bodies　in　every　direction；　but　always　in　straight　lines；　which　cross　each　other　at　every　point；　but　the　particles　of　which　each　ray　consists　are　so　minute　that　the　rays　do　not　appear　to　be　impeded　by　each　other。　A　ray　of　light　passing　through　an　aperture　into　a　dark　room；　proceeds　in　a　straight　line；　a　fact　of　which　any　one　may　be　convinced　by　going　into　a　darkened　room　and　admiting　light　only　through　a　small　aperture。

Light　also　moves　with　great　velocity；　but　becomes　fainter　as　it　recedes　from　the　source　from　which　it　eminates；　in　other　words；　diverging　rays　of　light　diminish　in　intensity　as　the　square　of　the　distance　increases。　For　instance　let　a　fig。　　1；　represent　the　luminous　body　from　　＇hipho_1。gif＇　which　light　proceeds；　and　suppose　three　square　boards；　b。　　c。　　d。　severally　one；　four　and　sixteen　square　inches　in　size　be　placed；　b　one　foot；　c　two　feet；　and　d　four　feet　from　a；　it　will　be　perceived　that　the　smallest　board　b　will　throw　c　into　shadow；　that　is；　obstruct　all　rays　of　light　that　would　otherwise　fall　on　c；　and　if　b　were　removed　c　would　in　like　manner　hide　the　light　from　dNow；　if　b　recieve　as　much　light　as　would　fall　on　c　whose　surface　is　four　times　as　large；　the　light　must　be　four　times　as　powerful　and　sixteen　times　as　powerful　as　that　which　would　fall　on　the　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