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f~m!d;ametcr of tl1e other to the diflance of ,he rocus.
O" divide ¡he doubk produél of ¡he radii by ¡heir f>Jm,
2nd ¡he qUOliem \ViII be ¡he di llanee fought.
Sioee.1I ¡hofe of ¡he fuo whieh p. fs ¡hrough •
conv~x glaf3 are colJeéled togeth!r in its focus, the force
of 311 ¡heir heat is eollcéled io¡o ¡ha¡ part; "nd is io
proportiorl ro the comnlan hca t of the {UD, as the area
of ¡he glafs lO ¡he are. of the foeus . !ience \'le fec ¡he
rearon why ~ con\lex glafs caufes th ! fu o's rays ro bul'"
. f¡er pallir.g ¡hrough il.
AII ¡hefe ra}'s erofs ¡he mirldl. ray in ¡he foeus¡; and
then divcrge {rom it, tO(he conlrary lides, in {he filme
m,,"er FfG, as ¡hey converged io ¡he fpa ee lJfE in eo·
m;ng 10 it.
If 2nother glafs FG, of the Carne cO:lVexi ~y as D E,
be placed iD ¡he rays at ¡he fame diíhnee from ¡he focus,
ir \'IiIJ rcfraél Ibem fa, as thar, alter going out of ¡t,
¡hey \\"ill be all paral!cl, as abe; aod go 00 io ¡he fame
as ¡hcy eJOle to ¡he nrCl gl.f, DÉ, ¡hrough ¡he
fpace ABC ; but on the con:rary lides of thc middle ray
Bfb : for the ray ADfwill go on fromfio the dirtélioo
f (/I1, aod the rayCEfio thedireélioofFe; aod foof¡heren .
T he rays diverge {rom any radiant poinr, as (rom a
priocipal foeus : therefore if 'a e.ndle be placed at¡; in
lhe foeus of lhe con,'ex glafs FG, lhe divergiog roys in
the fpace EfG \ViII b< Ca r. f.. ~:d by ,he gla('., ., ,hat,
after going out of it, they wil! becoOle parallel, as
in the fpace cha.
I f the candle be pl.ced nearer lhe glaC, than i.. focal
¿illance, lhe rays will eooverge afier paffing lhrough ,he
glaf, more or lefs as ,hecandle is more or lefs dillant from the
foeus.
lf lhe candle be plaeed farther from the glaCs th.n its
focal dill.oce, the rayo will converge after palling thro'
the glars, and meet in a point, whích \ViII be more or
lefs dillaot from the glafs as lhe caodle is nearer tO or
farther from ilS focus : aod \Vhere the ray' meet, they will
form an inverted image of lhe Bame of the eaodle; which
may he feeo on a paper plaeed io the me..ing ofthe roy•.
Heoee, if .n]' objeél ABC (No. 10.) be plaeed be·
yood the focus F of tne convex glals de¡;' fome of 'the
"ys which Row from eocry point of lhe obj eét, on lhe
fide nex' lhe glaf" will fall upon it ; and a(ler palling
t hrough it, they will be cooverged inlo/ as maoy point.
on th~ opponte fide of lhe glafs, where the image of eve·
t;y point will . be formed, and eonfequeotly [he image
of ¡be ",hale objeél, whieh will be iovatod. T hus, ¡he
ray' Ad, Ae, A¡; !lowing from the poin[ A , will eo~verge in the fp.ee
da¡; .nd by meeting at a will there
form the image of the poi ot A. Th e rays Bh, Be, Ef,
1Iowiog from the point B, wil! be uoiled at h by the re–
fraélion of ,he glafs. aod will there form ..he image of
the point E . ARd the rays Cd, C" C¡; Rowing from
tbe point C, wil! be united at e, where th ey will f~rm
the image of the point C. A nd fa of all lhe other i ~termediale poiot. between
A .nd· C. The rays whieh
flow from every particular point 01 the objeét, and are
united agai o by the glafs, are called penci/¡ o{ ra)'l.
If tbe objeél A BC be brou!:ht nearer to the glafs, lhe
piélure ahe wil! be removed to a great« dillaoce. For
tbeo more ray' 'flowiog frora every fingle ~OiOl, will . 11
VOL . .HI. N° 87. z
e s. 419
more d1vergiog upon the gI.f, ; . nd lbercfo,.. be
fo Coan collcéled iota ·the eorrcfpondong poiots bc;,i"d il.
Confequeotly, if .he diClaoee cf the objctt ABC (No 1l .)
be equal ¡O ¡he ditlanee tU of the foens of the glafs, lk
rays of eaeh peneil will Q.e [o refraéled by paí!ing [hrollgn
lhe glaf" lha! ,hey \ViII go ou, .<lf it por.llel 10 each 0-
ther; as dI, ,R, fh , (rom : th, poin¡ C; dG, cIt", ID,
from the point D; aod dl(, cE, fL . ¡he roiot ¿J: ",J
lh<"Core, there will be no pitlure fo rOled beh:n; the gI3f•.
Ir lhe focal dillance of the glafs, aod ,he difl.roce oC
lhe cbjfél from the glafs, be kllown, lhe dill'loee of the
pidure from ¡he glafs Olay ~e found by ¡his rule, vi=.
M ultiply lhe dirtanee of the focus by lhe difl.nee of tho
objeét! and divide the prodcél by ,he differcncc; th<
quotitnt ",ill bé the diClaoee of the pitltire.
T he piélure will be as mueh bigger or I,rs ,hao the
objeét, .s it~ din.nee from ¡he gl~fs i. gre.." or leC..
lhan the dillaoee of the objeét. For, as Et (No. 10.) i.
to cE, fa is AC lG e.1. So lhal if ABC be lhe obj eél.
eha wi j be the pitlure; or if da be lhe o"jeél, ABC
will be lhe piélure.
H a.lng defcribed how the rays of light, Howinn from
ohjeéls and p'llliog through conyex glatres. are colleéléd
io<o pOiOlS, and fonl' the images of the objeéls; it will be
eafy 10 uoderfland how lhe rays are affelled by palliog
lhrough the humours of lhe eye, and are thereby collett·
ed ioto innumerable point¡ on [he boaom of lh..: eye, and
thereon form me image, of tbe obj ells whieh <hey BO,"I
fr('lm. Fer, the different humours of the eye, and parti.
cularly lhe ehryllalline humour, are tO be coofidered 3S
a conv~x glaf, ; and the rayo in p.l1ing lhro\lgh lhem to
be :úfeéled in the fame manner as io paGing ,hrough a
convex glafs.-For. defcription of theco"s aad tu mours
of ¡he eye, fee ANATOMY , p. 289.
As everypoiht ofan objeél.ABC, (No. 12 .) fend,o"t
rays in all direaioDS, fome rays, from every paiot on lhe
fide next lhe eye, will fall upon t:" coroea uetwan E
and F; .od by .paffing 00 thrDugb ,he humou" aod pu–
pil of ,he eye, they wilJ be converged 10 .s maoy poinlS
on the retina or bonom of the eye, and .\vill thereoo fol'O\
a diainét inverted piéture eha of the objeél. Thu" tl.e
peoeil of rays 'IN that lIows from the POiOI A of ¡he ob·
jeét, will be conv.erged lO lhe poiot a on lhe relin1 ; lh/ole
from lhe poiot .B \ViII be converged tO the poiot b; ,hof.
from lhe point C will be converged to the point e; .nd
lo of all {he intermcdiate points; by which means Ihe
whole image abe is formed, and the objcét made vilible :
although it mur¡ be owned, lha< t
fenfa!ion is carricd from lhe eye- by the op'ic nerve ¡O lhe
eommon fenfory in tbe brain, .nd there difee rocd, is a–
bove the of our comprehenfion.
But that vifi oo is dreél~d io this m. nner, may be de–
mooflr31ed experimeotally. T ake a bul\ock's eye whi ln
jI is frelh , and haviQg cut oíf lhe lhrce coalS' C ..om lho
back part, quite to tlle VlUeOl1S humour, put a pirce of
\Vhite paper over th.. p. rt, a.d hold ,he eye tow..ds aoy
bright objeél, ood you will fee aD ioverted pidure of lh.
objeél .upoo the papero
Sincc lhe image is ¡nvertedJ many have wondered why'
the objeél appears upright. But \Ve are 10 connder;
J. 'rhat inverl.d is only • relaü.e terOl j and o a.. T lla¡
S N t lhe«