1000000031

gas upon • een: r:, till they beeant: paralld to

th.

InciJcnt

T70 y.

Hut

either

lhere

lin~¡

or r2}'S \VeTe

rcrl'~a{'d

fo much fro:n

as to become paratlcl to

Jb,

the ray (o much refraéled \Vould, after emcr{jon, ' make

no .•ngle with

J1,

beeaufe it would be paralJel tO it. And

eonreql\ently Ihat ray whieh is mofi turoed round upon

,he po;nt

or tllat ray whieh i. mofi refraogible, will

after emerfion be

nearen

parallel to

th~

incidc..nt ray, or

will

make the lea(l angle \Vith ir. The f.m. may be pro–

ved of alJ other

r.ys

em:rging parallel lo

and

re ·

~peéti\'ely,

or of all dfeétual rayo ; ,hore, whieh are mofi

rcfraog;ble, will after e",erGon. make a lefs angle with

lhe ineident rays, thao .hofe do whieh are lea'! re–

frangible,

. BUI finee the e/feétual rays of di/ferent eolours

m.ke

different angles wi,h

at their emerfion, they will be

fepar.,ed from one .no,her: fo ,ha[ if ,he eye w.s pla–

ced ' io ,he beam

íghi,

it would reeelve only rayo of one

eolour from the drop

xagy;

aod if i. w.s plaeed io ,he

peam.fgnp,

itwould reeeiveooly rays offonte o,her'colou r.

The angle

,""p,

wbieh the lea!! refraogible or red rayo

make

witt.l

lhe

incident

ones, when they emerge fo

be e/feétuaJ, is found by e.lcula,ioo ro be 42 de–

grees

mioutes. Aod the aogJe

J~i,

whieb ,he mofi

r efr.ogibl. rays make wi,h ,he ioeideot ooe., when ,hey

enterge fo as to be effeauaJ, is fouod

degree.

minutes . The rays,

which

have [he intermediale de–

grees of reCrangibility, make w;th the incident ones in·

termediate angles

between

degrees

minutes

and 40

dtgrees

minutes.

line iJ fuppofed lo he dra'Wn from Ihe cenlN of

Ihe /un Ihrough Ihe eje o/ Ih, JpendlDr, Ihe angle

'1JJhich any

eJJ~nual

ra.], after t1J)O rifrl1{1ionJ anri one

re.fit{]ion, 11I01.-¿I wilh the il/cide'JI ray, 'luill

efJutll

l o Ihe angle 'Which

ma~a

'Wilh Ihalline,

L et the

eye

of ,he Cpeét.,or be at i, (No, 44 ' ) aod let

'11

be the line Cuppofed to be dra",o fro", , he centre of ,he

fun ,hrollgh the eyeof ,he fpeét.ror; ,he angle

git,

whieh

BOy ./feétual ray make. wi,h thi. Iioe, will be equal

,he

angle

ikl,

which the fame

ray

makes

with

~he

incident

ray

J~ _

is a ray eoming fron¡ ,he eeotre of ,he

fun, ,hen finee

is fuppofed to be drawo from Ihe fame

point, there t\Vo lines, upon account of the remoteners of

lhe poi", from whenee they are drawo, may be looked up–

or{ as p.rallcl toone .nother. But ,he right Jine

erofs·

¡ng ,hefe ' \VO par.lld line. \ViII make the alternare an·

gles equa!. Eue, b,

pr9P, 29 - Therefore

kil

gil

cqual to

J.fi,

Wh.n Ihe

jiu¡

l"ina upon Ihe dropJ o( rain aJ Ihey

/alling; Ibe ral' IhaJ come frD/IIl hofe dropJ lo Ihe 'Ye

!prIlalor, nfter onf rtjle{fion and l'Wo rifra{]ion/J

prDduce

Iht

primary rainbow.

If"the fun (hines upon the r"in as it falls, there are com·

monly fe eo ,wo bow., as AFB,

CHD,

(No, 46.) or if

. he cloud aod rain does not reaeh over ,hat whole fide of

t he O' y where the bows appear, ,heo oolya pan of one

.or of both bows is feen in that place where the rain falls .

thefe tWO bows-, the innermon AFB is tne more vivid

of ,he ,wo, and ,his ¡s ealled the primary bow . T he ou, –

pan TFY of tne primary bo\V i. red, ,be iooer pa"

VEX

viol et

t~e

infcrmed.iate

p3.:-ts,

r~cko:1ing

frcm

the red

the violet , 3re orange, yellow, greco, blue, and

indigo, Suppofe ,he fpeélMor'. eye to be

.;\t

O, and let

LO? be ao imagioary line drawo from ,he centre of ,be

fun ,hrough ,he eye of ,he fpeéta,or :

a beam of Jight

S eoming from tbe fun fall. upoo aoy drop F; aod ,he

rays ,hat emerge a, F io the lioe FO, fo as 'o be effeétual,

make ao angle FO? of 42 degree. 2 miou,es wi,h ,he Jine

L?; theo thefe e!feaual rayo

m.ke

ao aogle of 42 degree.

minutes with the incident rays, by the precediDg propo ..

litioo, and eonfequently ,hefe rayo will be red, fo ,ba, ,he

drop F will appear red. AII,he o,her

r.ys

, whieh emerge

a' F, aod would be e!fcélual if ,hey fell upon ,be eye, are re–

fraéted more ,han the red ooes, aod eoofequeotly will paf.

above the eye, If a beam of ¡ighl S f.11s upoo ,be drop

E";

aod the rayo ,ha[ emerge a'

in Ihe Jine EO, fo as

to be e!feétual, make an aogle EO? of

degree. [7 mi–

nutes with the lioe LP ; then ,befe e!feétual rayo

m.ke

likewife an aogle of

degree. [7 miout.. wi,h the ioci–

dentrays, aod the drop E will appearof a violet eolo.r. All

,he other ray', whieh emerge at E, and would be e/fec–

'ual if they eame 'o ,he eye, are refraéted Jef, !hao the

violet

00",

aod therefore

p.fs

belów Ihe eye. The io–

termedia'e drop. hetweeo F aod E will for the fame rea–

foo. be of the intermediare eolours,

Thus we have (bewD whya fel of drops from F lO E.

as ,hey are falling, Ihould appear of ,he primary eoJoon.

red, orange, yetlow

greeD, bJue. índigo, and violet.

001

oecetrary ,hal the f"eral drop., whieh produce thefe

colours, (bould all of them fall at exaétly the fame difiaoce

from ,he eye. The aogle FO?, for inllance, i. ,he fame

whether ,he difiaoee of the drop from the eye i. OF, or

whether it is iD .ny o,her par. of ,he Jine OF fomethiog

nearer to ,he eye. And whilfi ,he angle FOP is ,he fame,

the aogle

m.de

by ,he emergiog aod ioeideot ray'. and

eonfequeotly the eolour of the drop, wiJl he ,be fame.

Thi. i. equ.lly Irue of any o,ber drop, So ,h...al!hough

io ,he figure ,he drop. F aod E are

repr~feoted

a. faJliog

perpendieularly one uoder ,he o,her, ye, thi. i.

001

De–

eelrary io ord.r

produce ,he bow.

But ,he coloured Jine FE, whieh we have

alre.dy

ac_

couoted for, i. ooly the bread,h of Ihe bo\V.

fiill re·

maios ro be Ihewo, why

00'

oDly the drop F Ihould ap–

pear red, bu, why aJl the o,her drops qui,e from A

B in the are ATFYB Ihould

app.ar

of ,he'fame eolour.

No\V it

evident, (hat where-ever

drop of rainisplaced.

if ,he aogle, whieh ,he e/feaual rayo make with the lioe

LP i. equal to the aogle

F.oP,

that i., if ,he

.0~le

which [he effeélual nys make

with

the incident rays is

degrees

minute., any of thofe drop. will be red,

for the fame reafon that the,drop }' i. of,bi. eoJour,

If FO? was tO turo rouod upoo ,he Jioe OP, fo that

one eod of this Jioe Ihould always be at the eye, aod Ibe

other be a[ P oppofi.. tO ,he fuo; fueh a mo,ioo of ,hi.

figure would be like ,hatofa pair of eomparres lurning round

upoo ooe of ,he legs OP wi,h the opening FO?, In ,his

revoll"ion ,he drop F would de!"eribe a eirele, P would

be the centre, aDrl ATFYB \Vould be ao arc iD this

cirele . N ,,\V (ince, in this motinn ofthe Jine anddropOF,

,he angle made by FO wi,h OP, iha, is, ,he angle FO?

ccnlinues the fame ;

the fun was to Chine llpon this drop

a. it revolve., ,he ttfeétuaJ rayo wouJd ouke ,be fame

ao¡¡le