1000000031

When raJl

ligM rejlr[}edIrom a drop of rain come lo

Ih~

e)'c,

,lJofe

are

cal/a)

effe...'1u!1/ w hich are ah/I!

t'xcilc a /t'n/a lioll.

lVhcn

nl)'(

1~1f/¡t

CfJr7U

out

e-(

a drop o[

rain,

the)

""il/l1e/ be e./fe[}unl, uulejí Ihe) aY< paral/el ol1d con·

I¡guau/ .

TH ER E

:\re

hm few rays that caft come to

lhe eye at

• 11: for Ihe gre.left pan of thofe rays whieh en'" the drop

(N°

43.)

belween

and

par. ou' of the drop Ihro'

the hinder furfaee

pg;

only few a re refteaed from thenee

and come out through lhe nearer fu rface between

and

y .

Now fu ch

r~ys.

as emerge, or come

OUt

of the drop,

between

and). \ViII be ineffeaull. unlef. they are pa·

rallel to one another, as

and

are

becaufe fueh

r~ys

as come out di\'crging from one another

will be fa far

2(under when they come to {he eye, that

~.I1

of Ihem

cannot eoter lhe pupiJ ; and the very fcw thal can

en~

ter

will

nOl ue

fufficient

to excite any

Ccnration.

BUl

even rays, which are paraHel, as

r V,r¡I,

will nOl be ef_

feétual, unlefs there are ,Ceveral 'of them contiguolls or

very near ro one another. The .two rays

and

tlone

will nOl be perceived, though borh of them enter lhe

eye; for fo very few rays are not fuffi cient tOexcite a

fenfation.

Whm rtl)1 of lighl come oul of a drop of rain afl" one

rejldiioTl, Ihoft 'Wil/ be _./fc[}ua l 1uhieh are Y<jle[}cd

froUl Ihe

fam~

point, and w hich

~nt~nd Ih.~

drop lJear

OIJe

anolh~r.

rays. as

and

cd,

(No.

44. )

\Vhen Ihey have

paffed

Out

of the air

iOlO

a ,drop of wate r. will be refrac–

ted towards lhe perpendiculars

M, dI

and as the ray

IÓ

{"II. farlher from the axis

QfJ

than the ray

cd, lb

wi ll be

more ·refraéled than

ed;

fo Ihit thefe rays. though pa·

rallel to one anotht:r at their incidence, may defcribe the

lin"

anol

after' refraaion, and be both of th em

reflea ed from one and ,h e fame point

Now all' rays

whích are thus refleéled from one and the fame poinr,

when they have deferibed the lines

ef. eg.

and aft er re·

/lcaion emerge at

and

\ViII be fa refraéled. when

.they pars out of the d rop inlO the air, as to defcribe Ihe

lines fh,

gi,

parallellO one another.

thefe rayo \Vere

return froro

in the lines

~h, ~d,

and \Vere to emerge

and

they \Vould be refraaed iota Ihe lines of

théir ineidence

bl,

de.

But if thefe rays. inflead of be·

ing returned in lhe lines

~ó,

(d,

are refleéled from the

{ame point

in the lines

~g,

e.f,

lhe lines of reBeélion

and

\ViII be inclined bOlh to one another arid lO lhe

furfaee of the drop: j.,fl as mueh as ,he lines

and

are.. F,,(l

and

make ju(l the fame angle wilh the

furfaee of Ihe drop : for the angle

bex.

whieh

m.kes

with lhe fur face of the drop, is the complement or inci·

dence; and lhe

angle .g~J' whi~h ~g

makes wit h lhe fu r–

raee, is lhe complC!ment of.reflellion

and thefe t \VO are

~qual

one t\nother. In the fame manner

\Ve

might prove

tha<

, d

and ef make

eq~al

angles wilh Ihe furface of Ihe

d rop. Seeondly. the angle

bcd

is equal to Ihe angle

f 'g .

,he

reflt!éled rays

eg,

~f,

and the íncident rays

[¡(,

d~,

are equally inclincd to <oeh olher. For Ihe angle of in·

cidence

is equal

the angle of refle:¡ion

grl,

ard lhe

angle of ineidence

del

is <qual to u,c aogl: of

rdk_~.vn

f el;

eonfequently. the

d;fI'aen~e

belween the .ngles or

incidence is equal to lhe dirrcrence between the angles

reneaion. or

brl- dd=ge'- fel,

bu'=g'f.

Sonce

therefo re either th e lioe"

ej:

or the linc:s

ed,

are

equally inclined bOlh

one ano,her and

the furfaee

of the d rop; tbe rayo will be refraaed in the fame m.n.

ner, whether -they were to returo in lbe lines

eb.

~d,

are refleéled in the

lin~s ~g,

if.

But if they were

re–

turn

the lines

eb,

~d,

the refraélion, when lhey e.

merge at

and

would make Ihem parallel. Therefore.

if they

~re

refie{tcd from one and the fame point

in the

lines

eg , ef,

the refraélion,. when they emerge at

and

will likewife

m.ke

them par.llel.

But 'hough fu eh rays. as are refleaed frolO the fame

point in the hinder pan of a drop of rain, are parallel to

one anothcr, when

lh~y

emerge, and lo have ene conda.

tion that is requilite towards making them effeélual; yet

(here is another conditioo

[] e~drary;

for rí\yS, that are:

e/feél u. l. mu(l be conliguous. as well as parallel. And

though rays. which eme('. the drop in different place"

may be parallel ..,hen Ihe emerge. thofe only will be

contiguous which

enter

it ncarl)' al tLe fame place.

L e, .'\}'. (No .

43 ·)

be a drop of rain,

the axis or

d iameter of the drop. and

111

a ray of ligbt that cornea

from lhe fun .and eOlers lhe dl'op at the point

a .

This

ray

UI,

be'caufe ir 1S perpendicular to both lhe furfaces,

will p.lrs (lrait Ihrough the drap in the line

agh

wi,hout

b<iog refraaed ; but any eolla,eral rays tha, fall about

l b,

as Ihey paf. through ,he drop,

\Viii

be made to con·

verge tO thei r axis, and paffing out at

wdJ meet the

axis

rays which [al! fanher from the axis than

lb,

fueh as thofe whieh fall about

/C,

will likewife be made

converge; but theo lheir focus will

neuer tO lhe

dr'!.P than

S¡úppore ,hetefore

to be the focus to

whieh the rays tha< fall aboat

I.e

will converge, any ray

Je,

when

has defcribed the Jine

within rhe drop, and

is tending to the foeu,

",,¡JI

paf. out of the drop at the

poi nt

The rays thar fall upon the-drop about

Id,

more remote

flill

from the axis , \Viii converge to a focus

í1ill nearer than

as fuppofe at

T hefe rays there·

fore go ou t of the drop at

The rays, that faU Uill

more

remote

from lhe axis, as

u ,

wilJ

converge to a [o–

cus nearer than.!. as fuppofe

I~,

and the ray

Je,

when

il has deferibed the line

wi,hio the drop. and is tend–

ing

will pafs out at the point

The rays, thu

fdll rtiU more ·remote fr.:om lhe axis, will converge to a

fo eus Uill nearer. Thus the tay

Ifwill

after rdr.aion

converge tO a focus at

which is nearer than

and

having defcribed Ihe line

",ithin the dr"p. it will p.f.

out

lhe poiot

Now here we may obferve. rhat as

any rays

IÓ

le,

fall . fanher above rhe axis

la,

the

points

11,

where they pars Out behind lhe drop, will·

be farther aboveg; or that, as lhe incident ray riles from

the axis

la,

tbe

arcgno

¡ncreares, till we come tO fome

raY 'ld.

w!.ich parres out of the drop at

p ;

and this 1Sthe

higheH point where any ray that falls upon

lile

quadrant

or qu.\ner

can pars out: (or any rays

{f.

·th.. fall higber thao

",iJl no.. p.f. OUt in any poiot

aboye

bm at

l~e points~,

wbich are bclo\l,.· at.

Confequently. though Ihe are

gnop

inereafes. wh:lIllhe

diflance of the incidenr ray from the axis

increafcd,

till we come to th. ray

yet afterward" the. lligher the