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rallc1 10 Ihe eye afler p.lling Ihrough Ihe glaC. ; and Ibeo,
b)' <Iltering Ihe eye al e. •h<y will be converged 10 .s
013ny difFerent pOlOts on the retina, a~d form a large in–
venc:d pillure AD UpOD ¡t, as in the figure.
To 6nd how mueh Ibis magniGes, Ji.ide Ihe le.O:
dinance (whieh is abou, Gx inehes) al whieh an objeél
can b. Ceen diOinélly \Vilh Ihe bare eye, I y me foe.1 dif–
Unce of ,he glaCs; and Ihe '1uolienl will fhe\V how mueh
Ihe glafs magnifies Ihe diame,er of ,he objeél.
The do.hlt or compoul1d miero(cop., (No. 18.) confiO.
of an <d, and an eye-glaf••¡: -The. fmall
objeél "b is placed al a Iiltle crealer diOance from Ihe
glaC. cd Ihan i" principal foeus , fo Ih.. Ihe peneils of ray'
1I0wing from Ihe diiferenl poin.. of Ihe objeél, and paf•.
ing II,"rough Ihe , may be 10 converge and u·
nife in as many points bctu'eeng and h, where the image
of Ihe objeél \ViII be formed : .whieh image is viewed by
,he eye threufh 'the eye-glafs if. For the eye.glafs be·
ing Co plaeed , Ih.. the image gh may be in ils focus. and
'he eye mueh about the Came diOanee on .he olber fide,
the rays of eaeb peocil will be paralle!, afler , oing out
of Ihé eye.glaC., as al • and r. liU ,hey come 10 Ihe eye
.1 1, where, they \ViII begin 10 converge by lhe refraélive
power of lhe bumours; and after having crolred eaeh o –
t~er in the pupil, and palTed through lhe ehry'lalline
a.l d "itreaus humours, they will be colJeéled iota poin .s 00
lhe relÍo3, and (orm ,he large ¡overted image AD lhereoo.
Tbe ,matnifying power of Ihis mierofeope is as follo·Ns.
Suppofe lhe image gh to be fix times lhe dinance of lhe
objeél ab from Ihe -flaC. ed; then \ViII Ihe image
be fix limes the lenglh of the objeél: but finee ,he image
eould nOl be feen dillinll ly by lhe bare eye al a lefs dif–
tance lhan fix inehe. , if it be -"iewed by an eye-glafs if,
of one ioeh foeu., ¡¡ will thereby be brought Gx limes
noarer the ere; and coofcquendy vicwed under an angle
lix times as large ~5 ~efore ; fo thu jt will be agaío mag–
ni6ed Gx limes; that is. fix limes by the objetl-glafs,
and fix times by the eye-glaf.; whieh multiplied in,o one
another, m"kes 36 times; and (o much i5 lhe objeét mag–
nified in diamettr more than what ir appears t6 lhe bare
eye ; and eoofequeDtly 36 time. 36, or 1296 times, in
furface.
But, beeaufe lhe extent or field of ,'i.\V is very fmaU
in this mierofeope, there are generally tWO eye-glalles
placed rgmetimes clore .logether, and fomctimes 3n ¡oeh
.funde<; by wliieh ,means, although lhe ohjcél app.a"
lefs m~gni6ed, yet 'lhe vilible area is nlueh enlarged by
Ihe iDterpof"ion of, feeond eye 'e lac" and conCequently
a mnch plearanter view ¡, obtained.
The fo/ar ",ierofeop., (No. 19. ) invenled l>y Dr L ie–
berkhun, j. eooOruéled in lhe f9Uowing manger, H ..ing
procu rcd a very dark room, let a rouod hole be madc in
the window· OlUuer, aboul three ¡oches diameter, through
\I'hieh th. fun m. y a 'ylinder of AA ioto the
room . In lhis hoJe, pl:tce Ihe cnd of a tube, containing
t\Vo .eonvex glalTe. and an cbjcél, "iz. l . A eonvex glaC.
n.o of ahola t\Va incl1f~5 diameter, and three ¡ochcs focal
dillance, is to be pldced in that fntl of che 11lbc which is
pUl into lhe hol.. 2 . The objeél bb, l>einc pUl be,
t wo glalfe. (which muft be eODeave 10 hold il al ¡il>crty)
e s. 421
is plaeed abou t two inehes and a half from th. &1,Cs aa.
3, A liule more Ihan a quarter of an ineh from Ibe objeét
i. plaeed Ihe fmaU eor.vex glar. ce, whofe focal diOanee
is a quarttr'" of ao ancho
The lube may be fo plaeed, when the fun is low, that
his rayo AA may en'er direélly inlO it: but when he i.
high , his rays nn muO be refieéled inlo tb~ tube by lhe
plane mirroar or looking gl.f. ce.
Tllings being thu. prepared, lhe ra)'s Ihat enter th
tuhe \ViU be eonveyed by the glaC. (la 10IVarcls rhe objeél
¿b, by whieh means it wiU be Orongly iUuminated ; and
the rayo d whieh flo\V from ir lhrough lhe eonvex glaf.
u, wiU make a large inverted piélure of the objeél .1 D D,
whi ch. being recelved on a whitc paper, wiJl repr:fcht
the objeél 0I.gni6ed io len¡¡lh, in proponinn of Ihe dif–
tanee of the piélure from the glaf. u , to lhe diOance of
the obj. él from ,he fa me glafs . T hus, fuppoCe the diC–
tanee of Ihe objeél from the 10 be .;.. por" of an
ineh, and the d;naoee of the dillinél piélqre 10 be 12
ft:et or 144 inches, in which (here are 1440 tenth, of ao
¡neh; and this number dividcd by 3 tenth!, g'ives 480 ;
which is the numbcr o( times the piélure is longer or
broad.. Ihan the objeél; and the length multiplied by
the br.adlh, OleIVs how mueh the \Vhole furface is ml&.–
niGed.
Of TEL<s coPE'.
B E ro R E \\le enter upon the defcriptioD of tclcrcores,.
it \ViU be proper tO fhew bow lhe rayo of light are affoc–
ted by palling lhrough concave glalfes, ¡od al Co by faJlin¡
upon concave mirrnun. ~
When paralleJ ray', as (lbcdifgh. (No. 20,) pafs di·
reélly thr9ugh a gl.C. AB, which il . equaJly oe
both Gde., they will diverge after palling Ihrough Ihe glaCs,
as if they had come from a radiant poinr e, ~ the centre.
of lhe glafs's concavity; which poinr is caJJed the riega".
lave or virtual focus of Ihe glafs. Thu\ the ray n, afte;
p.lling Ihrougb lhe glaCs AB, \ViII &oon in th. direétioD
Al, a. if it had proceeded from he POiOI e, aod no gl.C.
been io Ihe \Vay. Tbe ray b wiU )lo on in Ihe di re~lioo
nlll; the ray e in the direélion gp. &c.-The ray e , th;.t
fall. dir.élly upon the middle o'f the glaCs, (ufft " DO
refraél ion in pafling lhrough il; hUI goes on in th~ fame
rell iline.1 direélion, as i,f no glaC. had been in itS way.
I( lhe glaf. had been eoneave only on one fide, :md lhe
olher fide quite pl. ne, Ihe wnuld have diverted. af–
ler palling lhrough, il, as if lhey had come froOl" radian,
poinl al dQuble lhe diOanee ofe froOl th~ gbfs; Ihat is ,
as if the radiant had bern . t ,he d,lI.nce o( • \Vhole di.:
meter of the glafs', conca\'ity.
lf rays come more eonverging to Cueh a glafs, than P"
rallel ray$ diverge after pat1ing Ihrough il. Ihey \ViII eon–
tinue to converge after p:;l1ing t)l¡,ough it; OUt will nct
mee' Co foon as if no glaCs had beeo in the w.)', aud will
incline tOward..l the fame lide ro which they would h;:!.vc
divereed ir they had come pa ralld 10 Ihe gl.l5. Thus
Ihe rays filnd h, going in a converg:ngflate lowards che
cdge of the n1ars al B, and converging mor~ in th eir w;y
to it than lhe paralld rays diver~e .fter Falling lhrough
it, lhey will go on eonv~rsin& af,er they r afs t1:rouch ir,
thou~'l