1000000031

c..ity. the image and ohjea IVill be coiocideot, and equa!

in bulk.

aman pl.ces himlelf

dire~ly

before a large concave

mirrour.

bUl

fanher

fro{II

thao its

centre

ol"

conc:\VilY,

wilJ

fee an ¡overteú image of himfelf in the air, be–

twecn him :lnd tbe mirrour, of a Jefs (ize than

himrclf.

Ana if he holds out his hand IOwanls the mirrour, the

hand of

lhe

image \ViII come

OUt

towards his hand, and

coincide with ir, of an equal bulk, when his }¡,tnd is in

the centre of

conclvity

al\d he \ViII imagine he mayfllake

hands with his image. If he reaches his hasd

hrther,

the haod of the im. ge IViil pafs by his haod, and come

between his hond and his body: and if he moves his

hand towards either Jide, the hand of the image will

move towards th1! other; fo that whatever way [he ob–

jea moves, the

imagé..will

move the

contrary.

AII the ",hile a by:n;nder will fee nothing of the i–

mage, becaufe nooe of tbe reBeaed rays that form it enter

bis eyes.

a fire he made io

large room, and a fmooth ma–

hogony table be placed at a good dinance near the IVall,

before

large concave mirrour; fo pl.ced, tbat the light

of ihe fire may be refleaed from the mirrour to its focus

upon the table; if

perfon f1ands by the table, he will

fee nothing upoo it but a 10ngiO. beam of light: but if

he (lands at a dinance towards the Gre, not direaly be–

tweeo [he fire and rríirrour, he will Cee an image of the

6fe

upon the table, large .od erea. And if another perfon,

who koows nothing of this ma"er before-hand, fhould

chanee to come into ,he room, and fltOuld look from the Gre

to\Vards the table, he would be nartled at the appearaoce;

for the table would feem to be on lire, and, by being near

the waiofcot, 'o endanger ,he who!e houfe. In this e"–

perimeot, ,here fhould be no light in the room buí wha,

proeeeds from th. lire; and the mirrour ought to be at

lean fifteen inches in Eliameter.

the lire be darkened by • fcreen , and a large candle

be placed at ,he back of the fcreen; a perfon f1anding by

the c.ndle will fee tbe 'ppearaoce of a fine large n ar, or

.ather planet, upon the .áble, as bright as Venus or Jupi–

ter. And if a fmal! \Vax taper (whofe llame is much lefs

than .he llame of .he candle) be placed near .he candle, a

latellite to the planet will appear

the table : and if the

taper be moved rouod the candle, the fatellite wiU go

round the planet.

In a

r'írofling ¡'/'!c"P',

the glafs \Vhich is nearen the

?b jetl in viewing i, is called the

objrfl-g/oft,

and that which

nearen the eye i. called

¡he 'Je-g/oft.

The obje{t–

glafs mun be convex, bU! the eye_glafs may be either coo_

vex or concave: and generalIy, in lookiog through a te·

I.fcope, the eye is in the focus of the eye-glafs; though

that is not very material: for [he

dictance

of the eye, as

to difiinét vHion, is jndifFerent, providcd the ray! of [he

peocils fall upoo it parallel: only, the ncarer the eye is

to the end uf the «Iefcope, the larger is th< fcope or area

of the field of view,

L et

(No_

23 )

be a convex glaf. G"ed in a long tube,

2nd bave its focus at

E .

Then, a peneil of rays

gh;,

.8Q

ing from the upper extremity

of the remote objea

AD,

will be fo refraéled

palling through the gl. fs, as

conver~e

and nleet in the poiotí; whilfl the pencil of

r.ly

Um,

1I0wing from the 10\Ver extr<mity

of the

VO l.. lII. N° . 87.

fame objeél

An,

aod paliing through the

gl.fs,

wiU coo–

verge and mece

the point

and the images of the

poiots

and

\Vil! be formed in the pointS í"nd , _ And

as all [he intc.:rmediate points . of lh'e objeEt, belween

and

{cnd out pencils of rays in the fame manner, a fuf·

licient n"mber ofthefe pencils wil! pars through the ob–

jeél:,glars

c.t,

and

converge

as many

intermedlítte points

between

andí; and fo wil! form 'the whole ioverted i–

mage

eEí

of the dinina objea. But

b~caufe

this imag.

is fmall. a concave glafs

is fo placed in the end of the

lube neXl

the

eye, lhal

ilS

virtual

focus

may be al.

And as the pencils of rayo pafs eonverging through the

coneave glafs, but converge lefs after palling through it

than befare, they go on

further,

and

a ,

before

lhey

meet; and the pencils them(cl ves being made

diverge by polling through the

cunc.ve

glafs. they enter

the eye, and form the large piéture

UpOD

the retioa,

whereon it is

magnifiad

uoder the

aDgle

bFao,

BUl

this telcfcope

has

ene inconveniency which. reoders

it unfit for mon purpofes, which is, th.t .he pencil, of

rays being made to diverge by

pailin~

through the con–

c.ve

glafs

no,

very few of them caD enter the pupil of the

eye; and therefore the lield of view i. but very fm.I!,

as is evident by the figure. For DODe of the peocil.

which

Bow

either from the tOP or boltom ofthe objea

can enter the pupil of the eye at

bu' are al! f10pt by

fal!ing upon the iris above aod below the pupil : .nd there–

fore, ooly .he nliddle par< of theobjea can be feeo when

the telefcope lie-, direaly ,oward. it, by means of thofe

rays which proce¿d from the middle of the objea.

that

fee the whole of it, the telefcope muC! be moved

upwards aud dOWOW'fd., unlef. the objea be very re–

mote ; and then it is oever feen dinioélly.

This

inconvenience

remedied

(ubflituting

a con–

vex eye-glafs. as

gh,

(No.

24')

in place of the eoneave

one; and fixing

fo in lhe lube, lhat its foc us may

be coincident with the focus of the objea-glafs

cd,

For theo, the rays of the peneils 1I0wing

from the objea

AH,

and paffing through the objea-glafs

cd,

will meel in its focus, and form the ioverted

i–

mage

mEp:

and as .he image .is forOled in the focus

of the eye-glafs

gh,

the

ra.ys

of each peneil will be pa–

rallel, after palling ,hrough that glafs; but the pencil.

themCe1ves will croCs in ¡ts focus

the other

fide,

as al

e :

aod the pupil of the eye beiog in thi. focus, the_i–

mage \Vill be vie\Ved througb the glafs, under the angle

geh

and being at

it wil! appear magni6ed, fo as tO

lil! the whole fpace

CmepD.

But,

as this

teleCcope

inTerts

the image

Wilh

refpett

lhe objeét,

gives an

unpleafant

view cf terrdlrill

objeas; and is ooly fit for viewillg tbe henenly bo–

dies,

which we

regard

not'their pofitioo, becaufe their

being ¡nverted

does

not

appeu

aCCQunt

their being

round.

But

wh:uever way the objeét íeems

move.

[his

teJeCcope

mull be

moved

che

contrar}"

wt\~-r)

crder

to keep

fight

of it

(o r, fince the ebjeét is ¡overted,

i11

motion will be fo

too .

The m.gnif)'ing power of this telefcope is 's <.he focal

dinance of the objeét-glafs to the focal

diO.nc

< of the

eye-gl.fs.

Therefore. if the former be divid<d -b)' the

l,oltter,

the

quoticnt

will

expre(s che m2gnifying powcr.

V/hen \Ve fpeak of the magnifying of a tel<fcope or micro-

fco!,e,