1000000031

the <Y' .t

fo tb.t none of the ray' that fan abovc

or belo\Y

can be refleéled to the eye at

and the dif–

tanee betweeo

.nd

is equal tO half the leogth of the

objeél

AB.

Heoee it

appe.rs

, that if a

"laO

fees his whole image

in a

pl.ne 100kiog-gl.Cs

, ,he pan of ,he elafs ,ha, rellells

his image mua be jua half as long aod half as broad as

himCelf, 1 .. him aond at any diaance from it wha,cver ;

and ,h., his image muO appear jua as far behind ,he

elafs as he is before ir. Thus, ,he man

(No.

30.)

viewing himfelf in the plane mirrour

which is

jull

b.lf

as loog

himCelf, fees his whole image as at

EF,

behiDd ,he glaCs, e"allly equal ' o hi. own fize. For,

ray

AC,

proeeediog from bis eye at

A ,

and f.lliDg

perpendieuhrly upoo ,he furfaee of Ihe glaCs al

is re·

Belled baek 'o his eye, in Ihe fame lioe'

CA;

and the

eye of his image will appear at

io Ihe fame line pro–

dueed 10

beyond Ihe gla:s. And a ray

BD,

1l0wiDg

from his fOOl, and falling obliquely on Ihe glaCs al

",iII

be rellelled as obliquely 00 Ihe other fide of Ihe

perpeodieular

abD,

iD Ibe direllioD

DA;

and Ihe (oot

of his image win appear al

in Ihe dircllion of the re·

Ilelled ray

AD,

produced 10

where il is CUI by Ihe

righl line

BCF,

drawn parallel

Ibe righl line

ACE.

JuO the Carne as if Ihe glafs were takeo away, aod a real

",an aood at

equal in Gze 10 ,he man aaoding a'

for 10 his eye al

Ihe eye of Ihe olher ruan at

",ould

be feeD in Ihe direllioo of ,he liDe

ACE;

aod Ihe foot

of Ihe mao al

would be Ceeo by Ihe eye

iD Ihe di–

rellioD of Ihe line

A DP.

the glaCs be broughl Dearer Ihe maD

AO,

as fuppofe

eb,

he win fee his image as at

CDG:

for Ihe reBec–

ted ray

(beiog perpendicular 10 Ihe glafs) will Ibew

Ibe eye of the image as al

aud Ihe incideol "y

Ob,

being reAelled io the line

bA,

will Ibew Ihe fool of his

image

Ihe aogle of leAeétion

abA

beiog al

\V.ys

equal 10 Ihe aogle of iocideneo

Bba:

and fo of all Ihe

inlermediate rays from

Heoce, if Ihe man

advances lowards Ihe glaCs

CD,

his image will approach

to",ards it ; aod if he recedes from the glaC., his image

will alCo recede from il.

Of Ihr

MACIC LA"T ER N.

ABCD (No.

31.)

i. a lin lanlero, wilh a lube

nUm

6xed iD the

C.de

of il. This tube coofiOs of IWO joints,

one of which (Jips iOlo Ihe other: and by drawiog Ihi.

joint out, or pufitiog

in, the tube may be made looger

or Iborter. Al

in Ihe end oí Ihe mov<able joiot of

the tube a convex lens is fixed, and an objel.'l painted

wilh Iranfparenl colours upoo a piece of Ihin glafs i. pla.

ced at

fomewhere in the immoveable joint of tbe tube;

fo .ha!

Ihe lube i. leoelheoed or Iborteoed, Ihe leos

\viII be eilher

a greater or a lefs diaaoce from this

tranfparen. objea. In Ihe fide of .he laolero th"e is a

very

convex

lens

hhc,

\l..hich

ffrV~S

c3rt

a ver'Í llrong

lighl from Ihe caodle wilhin Ihe Iaolero upoo rhe objell

d~.

Now when

the rays,

whích

lhine through the

ob~

jea

de,

diverge from the

reveral

poiots as

&c. in

.he objell, aod (all upoo Ihe leos

JI,

Ihcy will be made

to converge to as many poiols

Sec. on lhe olher fide

of Ihe leo., and will paiot aD ioverted pio!lure of Ihe

VOL,

1Il.

NP 87.

objeé! at

j'g

upoo a while wall, a /hecI or a Cercen of

while paper, provid<d Ihe objell is farlher (roOl Ihe len,

Ihao its priocipal foeus. To make this pic.ure appeac

diOiocl and brighl, i. mua have 00 olher lighl fall up–

on il bUI what comes Ihrough Ihe lens

il;

aod (or .his

r<afoo Ihe whole apparatus is 10 be placed io a dark

room EFGH. The leos

mua be very convex, fo Iha.

the object

may be very near to it, and yet not be near·

er (han its principal focus: for by this means, as the

objea is oear 10 Ihe leos, .he pillure

¡g

\ViII be

great diOaoce from il, and coo(equently Ihe pillure will

be much bigger Iban Ihe objea. Sioce Ihe pillure is

ioverted iD re(pell of .he

~bjecI,

iD order- 10 make Ihe

picture appear with the right ead upwards,

necef–

(ary Ihat Ihe objecl

Ibould be placed wilh tbe wroog

end upward•.

OflhrDITFERENT REFRANGIBILITY .fLICHT.

have hitherto fuppoCed ,ba< a partícle of light, as

it comes from Ihe fun, is Ihe lea

particle iOlO whieh Iigbt

cao be Ceparaled. BUI we mua oow correcuhis fup–

pofition, by Ibewiog, Iha. a particle of light, as il comes

from .he fUD, is, properly fpeakiog, a buodle of "ys,

\Vhich may be feparaled from ODe aDolher. There(ore,

for Ihe fUlu re, by a "y of lighl we mua be uoderaood 10

mean, not thac. collection of particJes which we have

hilherto called by .his oame, bUI Ihe leaa par!icle. io–

to which ligbl caD be feparaled.

Ra)'l o( lighl arf foid

hr di.ffúml" rrj'r""giblr,

when al/he

fam,

or egua/

al/g/el

o/ incidence

fl"'~

are

m~re

lurned oul

Ihe

wo)' Ihan

o/heN,.

Ra)'l are faid lo hr difffrml" Nflexihlr,

jo",r .re

mON rift" r'fldhd Iha" olhers.

Lighl is cal/ed hom.gmrous, 'Whm "l/1M rays aN r–

qua!" rifra"giblr : il is cal/<d hrluogmrous, 'Wh".

fl1fU

ra)'1 are more refrangible Ihan

olherl.

r;'r e%urs

homogmrous lighl

tlN

cal/.d pri11lary or

filllplr coloun; and Ih..

0fhti<rogmrous lighl IIrr

eal/rtl f<condpry or mix.d.

r he

r~)"

0f Ihr fun arr "01

all

equal" rtfra"gihlt:

antl Ihoft ra)", 'Whieh bavr a dijfuml tltgrre ofrr–

j'rangibililJ, ha", liAe'Wifl a diff<rml eolour.

lf a beam of lighl SF, (No.

32.)

Ihat comes from

Ihe CUD,

~a(fes

iOlo a dark room Ihrough Fa rouod hole

io a wiodow Ibulter EG; this beam proceediog ftraight

forwards, aod falline upoo a paper

would there mak.

rouod pillure of Ihe fun . Thi. pillure would be

coofuCed ooe indecd, if Ihe hole is a 13rgeooe aod Ihere i.

00 leos io Ihe hole. However, as Ibis rouod fpoloflight

is a pillure of Ihe Cuo, we Iball hereafler, notwilhOand–

io~

ilS eonfufion, c. 1I il

Ihis oame. Now if • glafs

priCm ABC is placed belweeo the hole iD the wiodow_

/hulter aod Ihe paper

Ibe rayo of this beam, by Ihe

refraélioo which Ihey fufFer io Ihe prifm, will be beot

from Iheir Orail courCe, aod ioOead of goiog 00

a. 10

fall upon Ihe paper al

Ihey will be luroed upwards,

aod .he pillure of Ihe Cuo produced by them will fal1 up-

00 a paper MN Ihal is placed above

lf.1I the rayJ

5 P

....