1000000031

pl.te

'of air be,ween ,hem. Bu, ,he le(, ,he dillance

from .he poin' of conraét A, ,he c1ofe. ,be glalfe. are

to one anolhe.r,

and

con{equently the rhinner will be the

pIafe that lies between them;

confequently

tbar p'art

a place of water where this yello\V appears. is thin–

ner lhan tbat part

'Of

ate

of water where the fame

coloar appears. And ,he fame holds good iD any olher

colour. But water ¡s. more denfe "than air: therefore.

.he more deofe ,he fubllance is ou, of which a ,hin pla,e

i, made, ,ne lef. is ',he ,hickoefs of ,he pla,e. where

refteás

aoy

cenaia cólour.

Th. [orl

eolour, 'lJJhieh h r.jldled from finy part

of a Ihin plate, dependl only upon Ih. Ihielnefl

¡h. pllJe iljelf in Ihal part : but Ihe fa"" eblour

'lJJ11I be mad. lefl vivid by inereajing Ihe denjily

Ihe 111<dium 'lJJilh 'lJ)hieh·lh. plal.

ene01llp"ffid.

The eolours upon any part of ,; ,hin pla,e of Mufeo·

vy glaf, are ,he famé in fOr!, whe.her .he pla.e i, dry

or weued with water.

'I'herefoTe

the

fOr[

colour

llny pa" depends no' upon .b. medium .ha. eneompafl'e.

.he pla.e, bu, upoo .he tbicknefs of the pla.e il[elf;

./ince the eolours are the fa me when {he pl••e is dryand

encompaífed

wj.th

air, or wet and fo encompa'fTed with

water.

BUI

the fáme colour! are more faint when "tbe

ate

wet, than

W}lCD

it is dry; and confequeotly,

1he

brigh.nef.

0[.

the !oJours doe, depend upon .he 'medium

that

'encompaff~s

die plate; and tbe denfer thar medium

¡SI

tbe fainter will be the colours ; they

ar~

are more

faint

wbeo the pJate

covered ",ilb water

Ihan

wh~n

dry and [o i, [urrounded wi.h airo

Th< ray{ .of light hav< 'allernale

I11

óf <afy "jlec·

lim and eafy IranjmiJ!i' /J, 'lJJ6i<h relurn al egual

;nlerva/J.

Let

GF.

(No. 41 . ) be a beam of homogoneou, Iight

conGI!ing all of one fort of ray', as fuppofe all .he rayo

lha. compofe thé beam were red ones. Then, if ,hefe

ray" fall

UPO"

a thin plate of ai r ,between .the .wo glalfes

BAC, DAE, a' A there will' ue a da,k (pot, aod all

the ray' wíll he tranfmitted : round .his [po••here

will be 'a red riog, where .11 .he rayo are rcBeéted;

round .his red ring .here will be a d.,k ring, wbere

all.•he rays are .tranfmiued . Aod if .he . •hicknefs of

of the

pl.te

whtre all .he rayo are reBeéted lo the ring

nearell 'o A is caJled

1 ,

.he thickoefs where the dark

Tiog appears'and. aJJ the rays are tran[mitted will be •.

.Again, at .bat part of .he plate, where .he thickne[,

3, .11 the rays \ViII be tranfmiued: at ,he .hicknef, 4,

•hey will be aJl refleéted. Aod ,h., afternately, a, ex·

prerred by .he line. in the figure, .he rayo will be reBee–

,'ed iJl all parts of .he pla.e where the. ,hicknefs is ex;;

prefl'ed by any of 'the uneveo numbers

7, 9,

.,1:1<.

:and' will be tran!mitted where ,he thicknef. is ex–

pr'erred by any of the even numbers

4, 6, 8.

10,

&,.

Now 'as .he piate

.he fame in all parts. the eau fe

of this alternare

r~fl~élioJ)

and traofmiffion muO: be in tbe

rays themrelves ; and their difpofitioDS to be thus 'al·

terDately

rdll t1:~d

and tranrrhitted

are whal we call 6t5

of eafy refleétioo and eafy tranfmillion .

The rays lhat are in a 6t of ealy ..Océtion penetrate

fa, a. lhe fecoJld furface

the plate. for

,he

k·

43.1'

eond furfaee of a .hin piate of Mu[tovy glafs

weued.

the colollrs callred by

t~e

ahernate reHeétion grow fttin:cr;

wherea's if (he refleaion was made al the firfi [ur(ace,

wetting ,he [econd eould oo. a!feét .he eolours. B UI

fince .hofe ray' which

~ave

parred from the 6rll fUrface

of the pl••e

.he fecond where the .hicknef. of i, i,

are refteéted, aod .hofe which have parr. d from tbe úrll

Cllr(ace to the Cecend where the thicknef, of it is

are

tranfmitted; and .hen again ,hofe which ha,e thu. paf.–

from one Curface to rhe other, where .he thickoefs

3, are refieéted ; and thof. which have parred in the fame

manner, wbere the thickoeC, is'

are traDCmitted;

foJlow" that .he 6.. of eafy reBcétion and tranfmiffion·,·e–

turn at equal.iotervals . So that, ir

ray was to fetout

from

in the line AB, (No. 42.) aod waSlo be io a 6.·

of eafy refieétion whea i, had moved from

'0',

ít

would be in a 6. of eafy tranfmillion when it Dad mo–

ved

to twice toat difiance from

or when it was gOl to

d :

a. " onhe' diOanee 3 from A,

\viii

be in a 6t of ea–

fy reBeaion; a.

or .he dillanee 4, in a fi. of eafy

tranfmiffion; at

51 iD

flt of eaCy rtAcélion;

B , or 6, in o

t.of

eafy .ran[million : aod thus, io .he far–

.her progre[s of the ray, .he [ame 6.. wiJl returo a. e'l0a!

iptervals,

Th~.

if:.the .hickoefs of .he plate of air, where

i'ays .of any homogcneous coloúr are a1l retlelled,

'ís cqua!

and .he ray. ?re io a 6t of eafy reBeétioa

when ,hey.come to .he

[e~~nd

furfaee af .he plote ; {heo,

where .he .hicknefs of

tlí'é

plate is

.he ray'

_will ,be in a fi. of eafy tranfmillioo when '.hey come·

,he fecond furface, and confequently wiJl aJl pafs throagb

.hat furface.

Ag~in,

where lh••hieknefs is A, or 3,

,he

,ays ,when they come

.he fecood furfaee, will beia aJi.

eafy refieétion•. aod will' .JI be reJJea. d; where ,he

tbicknefs is

or 4, .he 6, of eafy. tranflnillioo wiIL b.

re.urn,ed when lhe rayo come to the [ecood furface,

reí

.ha, all of them wiJl be tranfmitted . Aod in like man,

oer,

(ueh

6ts returoiog at equal ¡nterval" the taya

:wiJl he re"eélec! where the ,hiek.nef, is exprefl'ed by.b..

number ' ,3,

1, 9,

&c.

and wiJl be tranfmitted wher..

exprerred by

2 ,

4, 6, 8,

10,

él,.

Whm a Ihi11 eoloured plate

vie'1Jud oblig,ui",

,n,

eolouN

every parl in th. pial. 'lJJil/ b< Illle"J;

When a bubble of

\~aler. or

a pla,. of air betweco 'wo

glarres BAC, DAE, ·(No. 40. ) i••

ie\~ed

obliqutJy, .he

eolnured rings

díl~re

therofdve.: and.confeq"ently. a ring

of any one colour, by belng diibttd, gcts ioto that pare

.he plale

w~ere

a ring of fome other colour_ appeared

when

tb~

pla,e was viewed direétly .

Ihe 'p/¡Ie

ÍJ

d~r.fir

Ihan Iht

rnr'ium

IbOI

t1fC~"I.

poJlo

il, Ihe

colour/ofü,

'When

vi~'Wed

fJ!J'iglltl)'''

ehal/g_

Iif'

~ht1n

Ihey 'W"dd if"tbe pll1l' 'lJJal rIJ–

rr,. Ihan 1/;; nudium Ibal tncr"mpnffil il,

bub'ble of

wat~¡'

i, a

,hi~ pl~.e

denfer ·.han .he ai.

that encompalfes it"": a.nda

pille

of air berween the

t~vo

'gl.rre. BAC" DAE, (No. 48 . ) i. rarer ·than .he gl.f.

.har eneompafl'es i•. , Upon 'iewing eaeh of .hefe thin

piates obliquely, the eoloared riog'

the pla!e of

W"ec

dilate lef, .hao thofe on tbe pla.e of air. Tberefor<

fince it is by Ibis dila.ioo of ,he ring' that a rioR

one.