american hand book of the daguerreotype-第16部分

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

＇amdg_5。gif＇

Fig　5　represents　a　double…convex　lens；　Fig。　　6　a　double…concave；　and　Fig。　7　a　concavo…convex　or　meniscus。　　By　these　it　is　seen　that　a　double…convex　lens　tends　to　condense　the　rays　of　light　to　a　focus；　a　double…concave　to　scatter　them；　and　a　concavo…convex　combines　both　powers。

If　parallel　rays　of　light　fall　upon　a　double…convex　lens；　D　D；　Fig。　8；　they　will　be　refracted　（excepting　such　as　pass　directly　through　the　centre）　to　a　point　termed　the　principal　focus。

＇amdg_8a。gif＇

The　lines　A　B　C　represent　parallel　rays　which　pass　through　the　lens；　D　D；　and　meet　at　F；　this　point　being　the　principal　focus；　its　distance　from　the　lens　is　called　the　focal　length。　Those　rays　of　light　which　are　traversing　a　parallel　course；　when　they　enter　the　lens　are　brought　to　a　focus　nearer　the　lens　than　others。　　Hence　the　difficulty　the　operator　sometimes　experiences　by　not　being　able　to　〃obtain　a　focus；〃　when　he　wishes　to　secure　a　picture　of　some　very　distant　objects；　he　does　not　get　his　ground　glass　near　enough　to　the　lenses。　Again；　the　rays　from　an　object　near　by　may　be　termed　diverging　rays。　This　will　be　better　comprehended　by　reference　to　Fig。　9；　where　it　will　be　seen　that　the　dotted　lines；　representing

＇amdg_9。gif＇

parallel　rays；　meet　nearer　the　lenses　than　those　from　the　point　A。　The　closer　the　object　is　to　the　lenses；　the　greater　will　be　the　divergence。　This　rule　is　applicable　to　copying。　　Did　we　wish　to　copy　a　1/6　size　Daguerreotype　on　a　l/l6　size　plate；　we　should　place　it　in　such　a　position　to　the　lenses　at　A　that　the　focus　would　be　at　F；　where　the　image　would　be　represented　at　about　the　proper　size。　Now；　if　we　should　wish　to　copy　the　1/6　size　picture；　and　produce　another　of　exactly　the　same　dimensions；　we　have　only　to　bring　it　nearer　to　the　lenses；　so　that　the　lens　D　E　shall　be　equi…distant　from　the　picture　and　the　focus；　i。　　e。　　from　A　to　B。　The　reason　of　this　is；　that　the　distance　of　the　picture　from　the　lens；　in　the　last　copy；　is　less　than　the　other；　and　the　divergence　has　increased；　throwing；　the　focus　further　from　the　lens。〃

These　remarks　have　been　introduced　here　as　being　important　for　those　who　may　not　understand　the　principles　of　enlarging　or　reducing　pictures　in　copying。

I　would　remark　that　the　points　F　and　A；　in　Fig。　9；　are　termed　〃conjugate　foci。〃

If　we　hold　a　double…convex　lens　opposite　any　object；　we　find　that　an　inverted　image　of　that　object　will　be　formed　on　a　paper　held　behind　it。　To　illustrate　this　more　clearly；　I　will　refer　to　the　following　woodcut：

＇amdg_10。gif＇

〃If　A　B　C　is　an　object　placed　before　a　convex　lens；　L　L；　every　point　of　it　will　send　forth　rays　in　all　directions；　but；　for　the　sake　of　simplicity；　suppose　only　three　points　to　give　out　rays；　one　at　the　top；　one　at　the　middle；　and　one　at　the　bottom；　the　whole　of　the　rays　then　that　proceed　from　the　point　A；　and　fall　on　the　lens　L　L；　will　be　refracted　and　form　an　image　somewhere　on　the　line　A　G　E；　which　is　drawn　direct　through　the　centre　of　the　lens；　consequently　the　focus　E；　produced　by　the　convergence　of　the　rays　proceding　from　A；　must　form　an　image　of　A；　only　in　a　different　relative　position；　the　middle　point　of　C　being　in　a　direct　line　with　the　axis　of　the　lens；　will　have　its　image　formed　on　the　axis　F；　and　the　rays　proceeding　from　the　point　B　will　form　an　image　at　D；　so　that　by　imagining　luminous　objects　to　be　made　up　of　all　infinite　number　of　radiating　points　and　the　rays　from　each　individual　point；　although　falling　on　the　whole　surface　of　the　lens；　to　converge　again　and　form　a　focus　or　representation　of　that　point　from　which　the　rays　first　emerged；　it　will　be　very　easy　to　comprehend　how　images　are　formed；　and　the　cause　of　those　images　being　reversed。

〃It　must　also　be　evident；　that　in　the　two　triangles　A　G　B　and　D　G　E；　that　E　D；　the　length　of　the　image；　must　be　to　A　B；　the　length　of　the　object；　as　G　D；　the　distance　of　the　image；　is　to　G　B；　the　distance　of　the　object　from　the　lens。

It　will　be　observed　that　in　the　last　cut　the　image　produced　by　the　lens　is　curved。　　Now；　it　would　be　impossible　to　produce　a　well…defined　image　from　the　centre　to　the　edge　upon　a　plain　surface；　the　outer　edges　would　be　misty；　indistinct；　or　crayon…like。　The　centre　of　the　image　might　be　represented　clear　and　sharp　on　the　ground　glass；　yet　this　would　be　far　from　the　case　in　regard　to　the　outer　portions。　　This　is　called　spherical　aberration；　and　to　it　is　due　the　want　of　distinctness　which　is　frequently　noticed　around　the　edges　of　pictures　taken　in　the　camera。　To　secure　a　camera　with　a　flat；　sharp；　field；　should　be　the　object　of　every　operator；　and；　in　a　measure；　this　constitutes　the　great　difference　in　cameras　manufactured　in　this　country。

Spherical　aberration　is　overcome　by　proper　care　in　the　formation　of　the　lens：　〃It　can　be　shown　upon　mathematical　data　that　a　lens　similar　to　that　given　in　the　following　diagramone　surface　of　which　is　a　section　of　an　ellipse；　and　the　other　of　a　circle　struck　from　the　furthest　of　the　two　foci　of　that　ellipseproduces　no　aberration。

〃At　the　earliest　period　of　the　employment　of　the　camera　obscura；　a　double…convex　lens　was　used　to　produce　the　image；　but　this　form　was　soon　abandoned；　on　account　of　the　spherical　aberration　so　caused。　Lenses　for　the　photographic　camera　are　now　always　ground　of　a　concavo…convex　form；

＇amdg_11。gif＇

or　meniscus；　which　corresponds　more　nearly　to　the　accompanying　diagram。〃

Chromatic　Aberration　is　another　difficulty　that　opticians　have　to　contend　with　in　the　manufacturing　of　lenses。　　It　will　be　remembered；　that　in　a　former　page　（133）　a　beam　of　light　is　decomposed　by　passing　through　a　glass　prism　giving　seven　distinct　colorsred；　orange；　yellow；　green；　blue；　indigo　and　violet。

Now；　as　has　been　said　before；　the　dissimilar　rays　having　an　unequal　degree　of　refrangibility；　it　will　be　impossible　to　obtain　a　focus　by　the　light　passing　through　a　double…convex　lens　without　its　being　fringed　with　color。　Its　effect　will　be　readily　understood　by　reference　to　the　accompanying　cut。

If　L　L　be　a　double　convex…lens；　and　R　R　R　parallel　rays　of　white　light；　composed　of　the　seven　colored　rays；

＇amdg_8b。gif＇

each　having　a　different　index　of　refraction；　they　cannot　be　refracted　to　one　and　the　same　point；　the　red　rays；　being　the　least　refrangible；　will　be　bent　to　r；　and　the　violet　rays；　being　the　most　refrangible；　to　v：　the　distance　v　r　constitutes　the　chromatic　aberration；　and　the　circle；　of　which　the　diameter　is　a　l；　the　place　or　point　of　mean　refraction；　and　is　called　the　circle　of　least　aberration。　　If　the　rays　of　the　sun　are　refracted　by　means　of　a　lens；　and　the　image　received　on　a　screen　placed　between　C　and　o；　so　as　to　cut　the　cone　L　a　l　L；　a　luminous　circle　will　be　formed　on　the　paper；　only　surrounded　by　a　red　border；　because　it　is　produced　by　a　section　of　the　cone　L　a　l　L；　of　which　the　external　rays　L　a　L　l；　are　red；　if　the　screen　be　moved　to　the　other　side　of　o；　the　luminous　circle　will　be　bordered　with　violet；　because　it　will　be　a　section　of　the　cone　M　a　M　l；　of　which　the　exterior　rays　are　violet。　To　avoid　the　influence　of　spherical　aberration；　and　to　render　the　phenomena　of　coloration　more　evident；　let　an　opaque　disc　be　placed　over　the　central　portion　of　the　lens；　so　as　to　allow　the　rays　only　to　pass　which　are　at　the　edge　of　the　glass；　a　violet　image　of　the　sun　will　then　be　seen　at　v；　red　at　r；　and；　finally；　images　of　all　the　colors　of　the　spectrum　in　the　intermediate　space；　consequently；　the　general　image　will　not　only　be　confused；　but　clothed　with　prismatic　colors。〃

To　overcome　the　difficulty　arising　from　the　chromatic　aberration；　the　optician　has　only　to　employ　a　combination　of　lenses　of　opposite　focal　length；　and　cut　from　glass　possessing　different　refrangible　powers；　so　that　the　rays　of　light　passing　through　the　one　are　strongly　refracted；　and　in　the　other　are　bent　asunder　again；　reproducing　white　light。

To　the　photographer　one　of　the　most　important　features；　requiring　his　particular　attention。　　is；　that　he　be　provided　with　a　good　lens。　By　the　remarks　given　in　the　preceding　pages；　he　will　be　enabled；　in　a　measure；　to　judge　of　some　of　the　difficulties　to　which　he　is　occasionally　subjected。　We　have　in　this　country　but　two　or　three　individuals　who　are　giving　their　attention　to　the　manufacture　of　lenses；　and　their　construction　is　such；　that　they　are　quite　free　from　the　spherical　or　chromatic　aberration。



CHAPTER　V。

To　make　Plates　for　the　DaguerreotypeDetermining　the　Time　of　Exposure　in　the　CameraInstantaneous　Process　for　Producing　Daguerreotype　Galvanizing　the　Daguerreotype　PlateSilvering　Solution　Daguerreotype　without　MercuryManagement　of　Chemicals　Hints　and　CautionsElectrotypingCrayon　Daguerreotypes　Illuminated　DaguerreotypesNatural　Colors　in　Heliography　Multiplying　Daguerreotypes　on　one　PlateDeposit　in　Gilding　Practical　Hints　on　the　Daguerreotype。


TO　MAKE　PLATES　FOR　THE　DAGUERREOTYPE。

I　do　not　give　the　method　employed　by　our　regular　plate　manufacturers；　this　is　not　important；　as　the　operator　could　not　possibly　profit　by　it　from　the　fact　of　the　great　expense　of　manufacturing。　The　following　will　be　found　practical：

Procure　a　well　planished　copper　plate　of　the　required　size；　and　well　polish　it；　first　with　pumice　stone　and　water；　then　with　snake　stone；　jewelers'　rouge。　　Plates　can　be　purchased　in　a　high　state　of　preparation　from　the　engravers。　Having　prepared　the　copper…plate；　well　rub　it　with　salt　and　water；　and　then　with　the　silvering　powder。　　No　kind　answers　better　than　that　used　by　clock…makers　to　silver　their　dial…plates。　It　is　composed　of　one　part　of　well　washed　chloride　of　silver；　five　parts　of　cream　of　tartar；　and　four　parts　of　table　salt。　This　powder　must　be　kept　in　a　dark　vessel；　and　in　a　dry　place。　For　a　plate　six　inches　by　five；　as　much　of　this　composition　as　can　be　taken　up　on　a　shilling　is　sufficient。　It　is　to　be　laid　in　the　centre　of　the　copper；　and　the　figures　being　wetted；　to　be　quickly　rubbed　over　every　part　of　the　plate；　adding　occasionally　a　little　damp　salt。　The　copper　being　covered　with　the　silvering　is　to　be　speedily　well　washed　in　water；　in　which　a　little　soda　is　diss