1000000031

fcope, 'i, 'is only meant with regard to the di'Oleter, not

to ,he area nor folidi,y of ,he objeél. But as the inUru:

mt:nt m;:¡gniries the vertical

aiameter,

as much as

clae!

úle

horizontal ,

is eafy to

find how much the whole vi·

fi ble area or furface is magoified : for,

the diameters

be multiplied into one ano,her, the produa \ViII exp«fs

the magnifieation of the \Vhole .ifible area. Thus, . fup–

pofe the focal ditlanee of tbe objea-glaf. be.ten times as

greal as the fo'eal diflanee of the eye-¡Iaf. I then, tbe

objca will be magni6ed te.n times, ' both

length and

bread,h ': ' and

mul,iplied by

10,

produces

100;

",hieh

Ihew., ,hal the area of the oDjeél will 'ppear

100

times

as big when feeo 'hrough fueb a telefeope, as i, does to

tlle

baTe

eye.

Henee i., appears. ,h.. if tbe focal difl.nee of the

eye-gl.fs

\Vere equal ' o ,he focal dillanee of ,he

oojeél-sl.fs

the magnifyiog power of ,he teleleope would be no'hing.

This ,e1efeope may be m ado

magnify in any

gi.en

degree, pro.ided i, be of a fuRieien< leng,h. For, ,he

gre"er ,he focal dillanee of ,he objeél-glafs, ,be

l.ef

may be ,be focal dillaore of

,h~

eye_glafs; ,hough OO!

direétly in proportion. Thus,.n objetl-glafs, of

fee,

focal dillanee, will admi, of an eyo.glafs whofe focal dif–

tanee is

littl~

more tha.n

2-,}

¡nches

which \ViII .magnify

" ear 48 ,imes: bu, an objeél-glafs, of

lOO

fee, fo_eus,

will requi re an

eye-gl.fs

fomewha, more ,hao 6 iDehes ;

aod will thererore

Olf'gnify

almoll

200

times.

A telefeope for viewiog ,erreflri.1 objetls, (hould be

conClruéled, as to Glew them in tbeir

natural

poHllre.

Aod ,his is done by. ooe objeél·glafs

ed,

(No .

2S.)

three eye.gl.ff.s

if,

gh, ik,

fo plaeed, tha, the diflane.

betwc:en any two, which

are

"eareO: to eaeh other, may

he equal ' o ,he fum of ,heir focal diflanees; as in

t~e

li–

gure, where ,he foeus of rhe glaffes

and _.fmeeral

,hofe uf the glaffes

and

mee'" /, and of

and

tn;

the eye being al

iD or near the (ocp, of the

eye-glafs

it,

on ,he o,her fideo Theo, i, is plain, ,ha<

,hefe pencil. of rays, whieh fiow from ,he objea

AB,

and

pafs ,hrough

th~

objeél.glafs

ed,

\ViII mee< and [orm ao

inverted image

CFD

in the foeus of ,h" glafs ; and ,he

¡mage being alfo in the foeus of ,he gl.f.

if,

,he rays of

,he peoeils \ViII beeome parallel, after I',affing through tha!

glaf., and crofs ., /, in ,he foeu. of the glafs

'.fi

from

whenee they paf,

'o the nex, glafs

gh,

and by going

through

they are converged to peines

other

focus,

where they form an ereél image

EmF

of the objeél

AB:

and as tbis im':l:: is a!fo in ,he foeu; of ,he. éye.glafs

iJ.,

and the eye on tbe oppofi,e fide of ,he fame glafs ; the i·

mage is viewed through ,he eye glaf, in this ,elefeop., in

rhe fame manoer " through the eye_glafs in ,he former

one ; only in a contrary porttiao, that ¡s,

lhe

fame

1'Dlition vii,h the objea.

The ,hree glaffe, nex' the eye have' all thei r

f~eal

dillanees rqu.l: and the m.gnifying

po\~er

of this ,eJe·

feope is found ,he fame way as th" of ,he lan above;

viz.

by dividing ,he focal dillanee of the ol-jeél-zlafs

ed,

the focal Giilanee of ,be eye'glafs

ii,

gh,

c.f,

fin ee

all ,h<fe three are equa!.

Wbeo ,he rays of ligh, are

fepa rat~d

I>y

refraélion,

they beeome eoloured·; and if tbey be u"i led.again. they

will be a per reél white. Bu! ,hofe rol" whieh pa:,

through a eoovex

glaf~

near its ed;:es are more or.equally

refraéled ,hao tbole whieh are nearer ,he middle of ,he

glar.. And \Vhen ,he rays of a.y peneil aro uncqualIy

r.efraél, d by ,he

gl.fs,

,hey do notoall mee< again in ono

aod [be Carne paior,

bu~

in reparate painn; which makes

the

ima~e

io"clilliné't", and coloured, about

ilS

edges. The

remedy

!s,

'o have a pl.lte wlth a fmall round hol. in i,.

midcl!e. 6".d in the ,ube at

parallel

,be glalfes.

For, ,he wandering rays abou, the edg'es of the gla{[••

will be flopt,

the pIJte, from

c0ming

to [he eye; ar.d

none ad!uiued

bU I

[hofe which come through the middlc:

of ,he glaf" or "Ie.(t " a good diflaDee froOl ilS ed 6 es,

and pafs tbrough the hole' in the middle .of the pla,e.

BUI Ihis circumf"ibes

t.he

¡magej and Jt:lrens the "fid(t

of vie"" whieh would be muell larger if the piate eoukl

be di fpenfed wi,h.

The great inconvenience attending tbe management of

long [eleCcopes of this

killd,

ha, brougkt them much

¡nlB

difufe ever fince

lhe

refltéting

telt:fcope

was

iovcoted.

Far one of this fort . .fix

feel

in lcngth,

'-magllifies

:¡s much

as ane of the other

hunclrcd.

was invcnted bv Sir {_

jlloc

N~'U1lon,

but has

rec~ived

confiderable

impro;emeflu

Cinee his time; aod is now generally conHruéted in (he fol.

l owing manner, whi eh was Gr(l propc.[ed by Dr.

Cregorf.

A, the boltom

,he

gr~at

'ube

TTTT,

(No. 26.) i.

placed the large eoneave mirrour

DUVF,

whofe prineip;¡\

focus

7/J:

and

in its

middle

is a

round bole P:op.

porile tO

which Ís placed

,the frnlll rnirrour

concave

toward the great one

and fo fixed

a firong wire

lhat

may be

moved

fanher

from'the great

mitrour, 0&

nearer to ir,

meaos"Uf

long Ccrew on the

outfide of

the

lube,

keeping its axis

(hll

in the

fame

lioe

Pllln

with

that of the great one.-Now. fince in viewiog a very

rc..

mote objelt, we cao (caree

fee

a point of it but what is

lean

as bre;;.d

as lhe

great

mirrour,

\Ve

may

confider

the rays 'of eaeh peoei l, which

1Iow

from every poin, of

,he objea, tO be parallel 'o eaeh otber, and 'o eover the

\Vhole reReél;ng furfaee .D UVF_ .B", tb a.• oid eonfufion

in ,he figure, we (hall only draw t\Vo rays of a peneil

Bo{v'ing from each

ext remity

of the objeét ioto the

gren

tubc

and trace

lheir

progreCs, through

?lI their reflt'élions

2nd refraélions, to the eye.f, " the end of ,he ·fwall

tube

tt,

which is j oined to lhe

grelt

one.

Let us theo fuppofe ,he objeél

A!}

'o be o, fueb a cli–

nance,

that

the

rais

may flow from irs Jower extremi-.

, y

aod ,be rays

from its upper e,xtremi,y

Then

the rays

falling parallel upon ,he gre" mirrollr a,

will be! ,benee reBeéled eonvergiog, in tbe direélioo

DC,

and by cronine

the

pTÍucipal

{ocus

lhe

mirrouT

they \ViII form the

upper

extrclllity

of the

¡nverted

j–

magc

IK,

Cornil" 'o the lower extremi,y

of the objeél

/JE:

and paffiog on to the concave mirrour

(whofe fo–

cu' is a'

.n)

they will fall uFon i, . ,

aDd be tbenee re–

fl l!éted convergiog, io

the dircétion

g N,

becaufe

is..

longer ,hon

g"i

and pafiing through tbe hole

io the

large mirrour, they would mect Comewhere

about

r ,

20d

{orm

the lower

cxtremity

of the

cre.tl

image

AB,

,fi·

m:lar ' o ,he lower extreOli,y

of,he objeél

AB.

But by

paOínn ,hrough the plano·eon.." gl.f •.

in ,heir w'y,

thc::y form

that

extremit )'.of

image at

IoJIJ,e

man"

ner, th. ray'

",hio:l come f,OOl tbe top o; the ohjdl

AB,