1000000031

there tS 3 v:ry great diffaence h:tween the Tell o'.Jj.d't

2.nd

the means or image

which \Ve perceive lt. W i1t::n

all lhe parts of a

d.n.nl

proCpett are painled upon lhe

retina, theyare all right

\Yi~h

refpeét to one another,

élS

wcll as the

PUtS

of the

profpca

¡trelr

and we can

only

judge of ao objetl.'s being ¡nverted, when ¡t/is.. turo·

cd reverfe to its natural pofition with

rerpeét:

otller

objetts whieh \Ve Cee andocompare il Wilh.-Ir we lay

bold of an uprighl niek in lhe dHk, we eao ,ell whieh

is the upper or lower pan of

it,

moving

OUT

hand

downward or upward;· and knoW'

very

wel l that we

cannOl

{<el

lhe upper eod by moviog our hand dowowlrd. J ull

fa we find

experience, that upon direéting

OUT

eyes

towards a

tatl

objelt , "ve

cannOl

fee

jls

top

lurning

o'Jr eyes down

Nard,

n:)r irs

fool

by'turning

OUT

eyes up–

ward ; bUl mufl trace lhe obj,tt lh: Came way by ,he eye

Cee il from head 'o fOOl, as we do by lh e hanel ' o

feel

it;

and as lhe

udgment í9 informed

lhe motion

of ,he hand in ooe t aCe, fo i, is",ICc> by lh. mOlion of lhe

eye

the other.

lo (N o.

'3 .)

js exhibi,ed ,he maoner of C<eiog ,he Came

objett

ABC,

by bo,h lhe eyes

and

a' ooee.

Wheo .ny par! of ,he image

cba

f.lIs upoo ,he opde

n"ve

,he eorreCpoodiog par' of lhe objett beeomes in_

"ifible. On whieh aeeouo" oalure has wiCcly plaeed lhe

optic nerve of e¡ch eye, nol in the middle of lhe bottom

of lhe eye, bUl lowards ,he fide oex' ,he oofe ;

thal

whale~er

parl of ,he image fa lls.upoo lhe op,ie oerve of

ooe eye, may

00'

faJl upoo ,he op,ie oerve of ,he o,her.

T hus the POiol

of ,he image

cba

fall. upon lhe op,ie

nerve of lhe eye

bUl 00' of ,_he eye

aod lhe poi n'

falls upoo

l~e

op,ic oerve of lhe eye

bu, Mt of lhe

eye

D :

and lherefore, ' o bo,h eye. takeo ,ogether, ,he

",hole objett

AOC

is vi6ble..

The oearer l ha' aoy objett i. ' o ,he

ey~,

,he larger i.

,he angleunder wnich il is feen,

30d-

the magnimde

UD–

der whieh·i, ' ppear" Thu. 'o lhe eye

D ,

(No .

'4.)

,he objett

ABC

i. Ceeo under ,he aogle

A PC;

and ilS

image

,ha

is very Jarge

~pon

the

re~ina:

but to the eye

al a double diflanee, ,he fame ohjett is Ceeo uoder

, he aogle

ApC;

whieh i. equa! ooly 'o half ,he aogle

APC,

as i. eviden, by ,he figure.

Th~

image

tba

1.lc'

ewife , wiee as large io ,he eye

a. ,he o,her image

cba

i. io ,he eye

E .

lo ho,h IheCe repreCeo,a,ion" a

par! of ,h'; image fall. 00 ,he. apde oerve, aod lhe objett

,he eorreCpondiog par! i. ioviuble.

A . ,he CeoCe of feeing i. aJlowecj 'o be oceafioned by

,he impulfe of the ray' trom ,he

viubl~

objett upoo ,he

reliDa of ,he eye, aDd formlOg ,he !t¡1age of ,he objett

,hereoD, aod ,ha, ,he re,ina i. only lhe expaofioo

.he

~pli¿

nerve aH over the

choro~des;

fhould fc:em

furpd ing, Iha' ,he par, of lh.. imase "hieh f.lls 00 ,he

op,ie .nerve {}nuld render ,he ·like

parl

of lite objett in–

vifible; .cpecially a. lha! oerve is allowod.o be 'lhe io–

lIrum! o, by whieh ,he impulCe aod image are 'eooveyed

' o lhe eommoo feoCory iD ,he braiD. Bu, ,his d.ffieulty

vanifhes, when we confider that there is an artery within

lhe trunk of [he optic

nerve~

wh-ich entirely ob[cures (he

il:nage

that parto ahd conveys no fcnfation to the brain.

T hat ,he pan o{ ,he irAage whieh falls upoo ,he middle

of theoplienerveis Ion, aod eoorequeotly lhe corr.Cpood- .

iog p'" of lhe objett is rendered invifible, is

pl.in

t'xperim!nt. For, if a perron fixes thrce palches.

A.B

,e,

(No . 15.) upoo a whi,e \VolI , al ,he heigh ' of ,he· cye',

aod the dj{hoce of

aOOllL:1

foo t from each otha, aad

placcs him(elf befare! lhe01, fh uuing the -righ t eye, aod.

dlreiling: [he Jeft towards (he piuch

he wi ll fee lhe

p\tehes

aod

bUllhe middle paleh

will

di ~

ppear.

Or, ir he

!hUt5

his left fye, and direéh the right towards

A ,

he will Cee bo,h

aod

bUlO wi ll diCapRear; and,

if he direél. his eye ,owards

he w.1I C.e bOlh

and

bUl 00'

For wh.,ever pareh is dirett(y oppoGle

to [he optic nen"e N, vimifhes. This requires ' a little

prdtlice, after which he will rindoit eafy t:> direét hiseye,

as lo I"Ce ,he lighl of whiehever pateh he pleaCes.

are no, eommon ly fenGole of ,his diCappearaoee,

becaufe the'o1ot,ions of the eyc'are fo quick and inflanta–

neous, tlia[ \Ve no foo nrr Jofe the figh[ of aoy part of añ

objett, than \Ve recover it agilin ; much the fame as in

t~!

twinkliog of our eyes

for at

caen

twinkling we are

blindc.>d; bUI il is fo foon over, thar we are fcarce ever

fenGble of i, .

Some eyes requi re the :lffifhace of convex glalfes

h ..-

make lhem Cee objeéls dinir.aly, and o,hers of eooe've.

ei,~er

,he eorn. a

abe,

(No,

16.)

or ehryl!alline hu–

mOU! e, or both of them, be tOO Aar, as in the eye

their fOCU9 will not be on the retina. as at

ti,

where

ii:

ought to be. in order tO render viCión dillinét ; but

be.

yood ,he eye, as a'

¡:

Aod ,herefo¡e, lbofe ays whid\

60w' from ,he objett

and paC, lhrough ,he bUlDour'

of the eye, are not con\'erged enough to uni(e at

d ;

and

[herefore the obferV'tr'can ha\re but a

very

indilli nlt vie w

of ,he obj ett. T his is rcmedied b.y p).eing a eoove"

glafsgh before ,he eye, whieh m,kes ,he rayo

eooverg~

iOQner, ' and imprints the image 'duly on tbe retina at

either ,he cornea, or ehryflallioe humour, or bo,¡i

of theOl, be too convex , as in the eye

the rays rhat

eOler il fron! , he objett

will be eooverged ' o a foeu.

in the vitreous humour, as at.f, and by divc:rging fr0rI!

thence tO the retina,

wiJI

form a very confufed image

thcreon :. and fo, of courfe, 'the obferver will have as

eoofu[ed a view of ,he objeél, a. if hi, eye had been '00

fl.lt

T his inconvenience is remedied by plaeiQg a

con~

cave glaCs

before ,he eye; whieh

gl.Cs,

by el uGng

[he rays to diverge between

and tbe eye, leDgthens the

focal di!lanee

Co ;

,hal if ,he glaC, be properly ehoCeo, lhe

rays

~'iJl

unite at the retina, 2nd form a difiinét p;(ture

of ,he objett .upon it.

Such eyés as have tht>ir humours of a due convexity,

canoo' Cee aoy objea diflinttly a, a leC, diflaoee ,hao G"

ioehes; . od ,here are oumberlef, obj!tl. '00 fmall 'o be

feen at

fha~

difiance, becaufe they caonoe appear uoder

aAy CeoGble aogle . T he me,hod ofviewiog Cueh mioulC

objetts(i, by a

mieroftop, ..

of whieh lhere are lpree Corrs,.

"iz.

,hejing/:,

lhe

double,

aod tbe

fola"

Oj'

M,ea oseopss.

T H S

fingle microlcope

i, oDly a Cmall eoo"'"

gl.Cs.

l .

(t'I

17 .)

haviog lhe objetl d plaeed in ilS foeu.,

and lhe eye al ,he Came diflan:e .on ,be olher fide;

fa ,hal lhe rays of eaeh peoeil, 6owiog from every poin'

of lhe ohj<tt 00 lhe fide n:x' lbe glafs, may go 00 pa·

ralle!