1000000031

grC':lt,

~ppe:as

{rom

t~H~

light of a caodle; which,

Iherc be

ohnacle in Ih" \Vay to o!lClr"a Ihe

paff.go

jls I:\}'S,

will 611 all the

fp¿ce

withi n

(\Vo

miles

of lhe

cancHe: c'fery way with luminous

paniclcs,

before

h,l!

lofl l!le I<all f.ofible pan of ilS fubClaoec.

A ray of light is a continued (lream of thefe

panicle~1

llowio;: from aoy vifihle bcdy in a Clraigh, lioe: and

11",

th< part iclc:s thernrelves are incomprehcr.fibly fmall , is

m.r.ifelt from ,he following experimenl . M . ke a

fm.ll

pin-hole in

pieee of blaek paper, and hold Ihe p"pe<

uprighl on a Iable faeing a ro\V of eandles Clanding by one

aoolher; Ihen

pl.ee

a Iheet of paCleboard a' a little dif–

taoee behind Ihe paper, and fome of Ihe rayo whieh How

from all Ihe eandles lhrough lhe hole in lhe paper, will

form as many ípecks of

Jight

on lhe

paCleboard,

as there

. re eandles on Ibe Iable before Ihe plate: eaeh fpeek be–

ing

diClina . nd <Iear, as if ,here was only one fpee"

from one fingle eaodle: whieh fh ews, lhal Ihe pan icles

of lighl are exeeedingly fmall, olherwife ,hey eould

0 0 '

pafs lhrough Ihe hole from fo maoy ditfereol eandle.

WilhoUI eonfuGon.-Dr N ieweoly' has

eomp~led,

Iha,

there flows more than

6,000,000,000,000

times as many

partieles of ligbt (rom a candle in one

recond

of time, as

,here are graios of fand io lhe whole eanh, fuppofi ng

each cubic ¡neh of ir to

coolaio 1,000,000.

Thefe panicle., by faU iog direaly upon our eyes, ex–

cile in our minds ,he idea of ligh, . And wheo Ihey f. U

.pon bodies, and are lbereby refieaed ' o our eyes, lhey

eXclte

us the ideas of thefe bodies.

And

as every

point

of a vifible body reBeéls ,be rays of líghl in all maoner

of d¡reétions, every point will be v¡tible io every

pan

whieh lhe líghl is refleaed from i,. Thu. lhe objea

A BC

(OPT.eAL PLAT ES, fig. nO

l.)

is vifi.ble to an eye in any

par! \Vhere lhe rayo

Aa,

Ab,

A,', Ad, A" Ba, Bh, Bc.

Bd, B"

and

Ca , Cb, Cc, Cd,

Cf,

come. Here \Ve have

fhewn lhe rays as if ,hey ",ere only refleaed from lhe ends

and

aod from ,he n.iddle poin,

of lhe objea; e–

very ollter poin, being fuppofed

reHea rays in lhe fame

m2oner. So (hat,

where~ ever

a Cpeétator is placed with

regard 'o ,he body, every poio'

,ha! paTl of Ihe fur–

face which is towards hiOl

wiIJ

be vilible, 'when no ínter·

.eoing objea C1op. lhe parrage of Ihe lighl.

Sinee no objea eao

feen lhrough Ihe bore of a bend·

ed pipe, il is evideo, lballhe rays of ligh' move in Clraigh'

Jines, whiHI there is nothing to

refralt

or turo them out

their rellilineal courfe.

WhiHl ,he rays of ligh' eon,inue io any • medium of

ao uniform denfilY, Ihey are C1raighl; bllt when lhey pafs

obliquely out of one medium into anothcr which is eitbcr

more denfe or more rare, they are refra{led towards the

denfer

medium : and

[his refraétion is more

lefs,

the

rays rall more or lef. obliquely on ,he refraaing furface

which divides the mediums .

T o prove lhis by experimen', fe< Ihe emply verre!

A SCD

(No.

2.)

inlo any place where lhe fun fhines ob–

liquely. and obfer.e lhe P'r! where lhe fi13do\V of lhe

. dge

fa lis on lhe b<>ttom of Ihe vetfel

;

Ihen fill

the velfel with

W'-ter,

and the f'hadow

will

reach no f3r·

Iher than

which fhews, that the

ray

aBE ,

which carne

firaighl in the opcn air, jufl orer the c:dgc of lhe ve(f;l

lO ilS bono:n al

is refr.,'led by fa llior.

obli~ ltely

on the (u

rf.ce

of tbe wal: r

and inOcad of going

on in the reétiliocÓ4.1

direa ion

aBE,

UCIH

dO·.I,.'n

.v"rd

in the

w~ ter

{rom

to e. the wholc bend beinn al rhe

fUI

(¿ce of

lhe water:

anel

of al!

olher

rays

a6;.

a fiick be laid over lhe v,rrel, aod Ihe fun's rays b,

refieéled from a g!afs perpeodicularly inlo lh o verrd, lhe

filado \V of lhe Hiek wiJl

r.JI

upon Ihe fame pan of Ih.

bOllom, whelher lhe ve(fe! be cmp'y or full; \V1,ieh fh<w.

Ih al lhe "ys of lighl are nOI refraaed ",hen Ihey faJl

perp.odicu l~r1y

on lhe

furf.ee

of any medium:

The rayo of ligh, are as mueh refr.aed by palling ou,

of water iota air, as by paffing

OUt

of ai r iota

water.

T hus, if a ray of ligh' flo ws from Ihe poin,

f ,

uoder

water, in the direétion

eB ;

when

it comes to

the

{ur·

face of the water at

wilJ

OOl

go on lhence in the

rea ilioeal eourfe

Bd,

bu, will be refraaed inlO ,he liDe

Ba .

Ther.fore,

To an ey'- al

lookiog lhrough aplane glaf. io lhe

bouom

the empty velfel, the paiot

cannOl be fceo ,

beeaufe lhe fide

of lhe yetfd in..rpofes ; and lhe poine

\ViII juCl be feen over Ihe edge of lhe verrel

B .

Bue

if lhe verre! be filled \Vilh water, lhe poinl

will be leen

from e ;

and wilJ

appear

elevated

che d¡rtétion

of lhe ray

eB.

Hence a piece

mane,. Iying al e,

,he bOllom of.o emply verrel, eanoo' be feen by an eye

beeaufe lhe edge of ,he verrel inler. enes; bUI lee

,he Yerre! b. filled wilh wa,er, and lhe ray

being ,hen

refraaed

\ViII C1rike lbe eye

ti,

and fo render ,he

mooey viGble, whieh will appear as if i, \Vere raifed up–

'o.rin lhe line

aB}.

The ,ime of fun-rifing or fetting, fuppofing ilS

rays

fuffered DO refrac1ion, is eafil y found by-c.lculalion. Bu,

obfervalion proves, that the fun rifes {oaDer and

(eu

la–

'er every day Ihan lhe eáleulaled limoe; Ihe reafon of

whieh is plain, from wha, \Va. faid immedialely aboye.

For, lhough lhe fun's rays do no' come pan of lhe way

to us through waler, yet they do

through

[he

¡ir

or at–

ruofphere, which being a grolf<r medium , ha. lhe free

fpaee belweeo ,he funl and lhe ,op of ,he ..mofphere,

lhe rays, by cn'. ring obliquely inlo lhe a,mofphere, are

Ihere refraaed, .nd lhe. ee ben' down

Ihe eanh. And

a"bough Ihere afe many place, of lhe eanh

whieh lhe

fun is vertical at noon, aop confequently hiS' rays can fuf–

rer no refraétion al thal time, becaufe they come perpen..

dieularly

lhrough

the atmorphere; yet there is no place

whieh lh. fun 's rays do nOI fall obliquely on lhe

10 p

of Ihe a'mofph ere, a' his rifing .nd felling; and confe–

quentl)", no clear day in whieh lhe fun will no' be vifible

before

he rires ¡n.the

horizon,

aDd

after

he feu

it;

;¡,nd

the looger or fhorter. as lhe atmorphere is more or leCs'

replele ...ilh v.pou rs. F or, lel

ABC,

( No.

3.)

be par! of

the eanh's furface,

DEFthe

aunofph ere that co\'ers

it,

and

lhe I. ofible horizon of an obrerver a'

c–

very poin' of lhe fun's furfaee fends OUl rays of lighl in

all m.nner of direaions, fome of his

r.ys

will conlbntly

fall upon, and enligh..n, fome half of our almofph ere ;

and

t\ny

lhinS throllgh

which

lhc

ró),s of Jight can pafs, is callrcJ a mcdium;

air, water, glafs, diamond, or

en"n

vacuum.