1000000031

"'ere equally bent upward., the piaure would be a round

one upon

tht:

paper

1\1N,

after the rays have beco re·

fra fud, as well as wheo they paffed Uraight forwards aod

fell upon a paper at

But this refraaed piaure PT is

found to beobloll g. The horizontal di.meter or breadtb

of this oblong piaure is equal to the diameter of the cir–

cular ene Y : but

lhe

perpendicular díameter

height

of ¡he piaure PT is mueh greater thao its breadth. T he

refraétion is made upwards, and

nOI

in a bori2.ontal diree..

tion: thererore

alteration ought to be made in tbe

breadth or horizontal di.meter of the piaure; beeaufe

no rcfrl é\.ion,

that

nOI iD

the direétioo

that di:\me"

ter, can make the piélure either broader

narrower.

Aod if all the rays were equally refroaed upwards, Cueh

a refraBion would not ehange the length of the piaure ;

is round when

falls

Y, fo

would be round

\Vheo it i. ref.aaed upwards by the pri{m anrl f.lls at PT.

This oblong piéture

confin,

thercforc of rays, which. are

dlfferently refrangible: they all f.1I at equal angl<s of

obliouilY upon BC the hrll fide of the priCm , but io the

tcfnaion fome

are

more turned

OUt

of (he

way

than

others : thoCe rays which go tO

the opper part of the

piéture, are the man refrangible; and thofe which go

T, the lower pan of

it,

are leart refrangible: ,he

Ten,

whieh fall betwoco P alld

h..e iotermediare degrees of

refransibility.

Tbis oblong piaure is of ¡¡ilferent coloo.. io dilferent

parlS of it. The mol. refrangible rays

are violet, the

leall refrangible at

are red; lhe rays of intermediate

refrangibility from the violet dowowards tO the red are

iodigo-coloured. blue, greC"n, yellow, and orange. So

thar the whole piaure is made up of rayo of theCe {eveo

differeot coloon. We may (rom heoce Cee the reaCo.n

wby tbe coloured piélure eoofilliog of differenlly refran–

gible rayo fhould be oblong, in Cueh a man.oer tbat the two

lides of it are right line., and the two ends Cemicircles.

For it conull. (as inNo. H .) offeveneircle., the highell

of which

PAG~is

violet, the lowell STN is red, the

flve iotennediate ones, BH,

el,

D'K,

EL, OM, are

índigo coloured, blue, gre••, yellow, and oraose. The

",hite round piaure

(No.

3'·.)

i. formed by hetera·

,eneous

ray',

that are of feven dl1Ferent

fons,

dilliD·

guifhed from ooe another by Iheir diff<rent degree, of

refransibility and differcot coloun. The refraétion of

the priCm Ceparates theCe rays from o.e anolher by re–

rraaing Come of thcm more and olhers IcCs. And conCe–

'1uently the reCraaed piaure \ViII confill of · Ceven round

piaures one below anothér. . TheCe round piBur.. are

Dear

eaeh olher, that the highell of them APGR will

mix itCelf with fome of tboCe bolo..", it, as wi,h BH and

CQ!,

This nearneC. of theCe Ceveral round piBures to

cach other will prevent theireolours from beingdillinBl y

feen; it wil1likewiCe make the fideo AS, GN, which

are compofed of Cmall are, of circlel very clofe to one a–

Mther, appear like right lines; but the two eod, P aod

\ViII be Cemicircles.

the centres of theCe circle. cODlinue

the Came dif–

tance from one ¡ooIher. and ,he tircles themrdvcs are

made leCs, as

I1Pt. bh,

di,

~/.

(1m, J(n,

they

will

then be dillinét or will

nOI

míx with eaeh otber; and

the colaura of the fevera! par" will by tbis means

be kept feporate,

the «fraaed rays, inll, .d of form–

ing

O~:

continued oblong' piéhae, will form Ceven

[mall

circular cnes placed in a lioe perpendicu-!,H

the norizon.

T his Ceparation of the. C.veral parts in ¡ho refraé1:ed pie–

t~re

from eaeh olher is broogbt abour, (as in No.

34.)

by making the hale F io the window-fhurter vory Cmall,

and by colleéting Ihe rays thar come tn rough il with a coa–

veK len. MN. For this will make a very Cm.U white

pidure of tbe

fUD al

there is

pr¡Cm

abe:

but the

refraétion of tbis

prirm.

it is pl8ccd a liule beyond [he

lens..

will

fep.u'i\te

the. ~eterogeneous

nlys

rdraéling

them upwards '; aod in(lead of oae fmall round and

whitc

piBure at

there will be Ceven CmaU rouod p,flure.

PT,

which

wiJl

be víotot,

indigo,

bluc,

greca

/ yellow, ,orange, and

red.

That the priCon ABC (No. 32 .) does not

m.ke

the

nys diverge, fo as to fprcad over the [pace PT,

upon any

other.accouot bUI their difFerent refrangibility, \ViII be

evident, if (as in No.

35 .)

a Ceeond priCm

i. pi••

eed beyond the firll

"be.

For if Ihe rayo that come

from S and p,C. throus" ,he hale F of the window /ho,–

ter EG, were by the Dril F ifm

abe

made to·di.erge aod

fornl the obloog piBure PT upoo .ny otber aeeounr be'

fides their diff<rent refrangibility; then, Cupp ofing. Ce.

cond priCm

be pl.ced at right angles to tbe for–

mer, the e!Fea mull be this; th. firll priCm

a"e

make.

the rays diverge (rom ene aDother

a liDe PT perpen..

dicul" to the horizon, and eonCequeotly the feeond priCm

mufi

make them diverge froln

ODe

another in a lino

paralld. to the horizoni

rha, the feeond priCm

IVo.ld

increafe the bre.dih ot ,he piBure,

much as the 6rft

increartd its length; and as one prirm alone makes tho

piBure a loog one, bOlh of them together would make ir

rquare. as'

prll.

But

this recond refraétion does not 3"ltet

the figure of the piBure. but ooly the pofidoo of, i, ,

the Cecood priCm refraas the pla urc fidelv,y,; and thoCo

rays, which fe ll the highell

P after the 6rll refrae–

tion, are reCraBed lideway' Ihe moll by the feeond prifm;

thoCe whieh fell the lowell at Tare refraéled fidewaystho

lean; by which

means lhe piéture,

thGuph it

continues

ob–

long, \ViII

Dot

be perpeodicular tothe horizon as PTwas, bt:t

will be inclined

as to lieió tbe p<1filionpt. This m.kes

e–

vident, that the Cpreading of the rays by the 6rll refraBloo

was owing

theirdifferent refrangibility

and to

otber

cauCe.

mull be owing to their different refrangíbi–

lity, becau/;: thoCe whieh were mon refraaed upward.

by the firll priCm are moll refraaed fideways by tho

fecond. It cannot be owing to any other caure. bl!clpfe

if it was, the Cecond prifm would fpread the rays in

bread.h as much as the 6rn priCm Cpreads diem in

Ieo~th.

and botb priCm, ,"ould make the piaur. {quare.

'l'hOft raJi

/ighl, 'Whirh

IIrt mo/l rifrangiblti

aTl

W""'ift m.¡i rtfltxiblt.

When a beam of lig!>t

admitted into a

dar k

charltber

through the hole F in the window-!huuer EG. (No.

3ó.)

and Ihis oeom falls upon a priCm ACB, rhe fides of whicb

AC and AB are equal, .nd .the angle at A a right one ;

when the obliqui.y of theCe ray', .. they are to paCs out

of the prifOl at

liS

baCe BC, i. leCs than

4 0

degre", the

Gr~aleU

p,rt of <he he.m will paC, OUt. but Come few

Tll"

wlII