475 / 868
o
T
a"d therefore. when the
run
is
al
l.
below Ihe horizon
H,
thole rays whieh go
00
in the free fpace
lkK
pref<rve a
r,eéliJir.e.,J eourfe until they f. 1I upon the tOP of the ato
morpbere ; amI tbofe wbicb fall fo about
K.
are refraéld
at their entrance ioto fhe
atmofphere,
and beot down in
the line
¡(mB,
tO
the obfern."r's pJace
al
B :
;:¡nd there·
fare, to him tbe fuo ",ill appear at
L,
in the dirc,'!ioo of
the
(ay
B~nK,
above tbe horizon
BCH,
'wheo he is real!y
beJow it at
l .
The angle eontained between a ny of light, and a pero
pendicular to the refraéling furfaee, is ealled
Ih! ansl,
of
inci:lence.'
and che aogle coctained betwecn
lhe
(ame per–
pendicular, and the fame " y after refraéhon, is e.lled
,he
angle
ofrifraflion.
Thu. (No. 4.) let
LBfIil
be the
refraélin~
fu rfaee of a mediom (ruppofe water,) and
ABr:
a perpendicular tO that furfaee; let
J)
B
be a ray of light,
goi·ng <?ut of air loto water at
B
J
and thercin refraéted in
t·he line
BH;
tbe angle
ABD,
is th.e angle of ineideoee,
of whieh
DF
is the fine; aDd the angle
KBH
is the an–
gle of.refraélioo, 'waofe fin e is
KI.
Vvhen the refraéling mediuOl is water, Ihe fine of the
angle of ineidence is tOtbe fine of the angle of rdraélion
as 4 tO 3; whieh is eonfirmed by tbe following experi.
ruent, uken from Ooélor
SMITH '~
Optles .
D eferihe tbe eirele
DAEC
on aplane fquare board,
2nd
eroC,
it at ,i¡ht angles witb tbe !lraigbt liDes
ABC,
and
LBfIil;
then, from the interfeélion
A,
with aoy
0-
peniDg of the compalfes. fel off the eq'13\ ares
AD
and
AE,
and draw the right lioe
DFE:
then, takiog
Fa,
whieh i. three quarters of the leogth
FE,
from the poiot
IZ,
draw
al
parallel to.
ABK,
and join
KI
parallel to
BM:
fo
K!
\ViII be equal to three quarters of
FE
or of
DF.
This dooe;
6"
the board uprjght upoo the leadeo
peddhl
O,
and !lick tbree pios perpendieularly into the
board,
at
the points
D, B,
and
1:
then fet the board
upright into th.e velfel
T UV.
aod 611 up the velTeI with
water
to
the line
LB/lf.
When the water ba, fettled,
¡ook along the lioe
DB,
fo as you may fee the head of
the pin
B
over the head of the pio
D ;
and the pin
1
,viII appe" in the fame right line produeed to G, for i..
he.d will be feeo ju!l over the head of the pin at
B :
whieb file"'s tbat the ray
lB,
eomiog from the pin at
1,
is fo refnéted at
B)
as to proceed from theoce
in
the
line
BD
to the eye oftheobferver
¡
the 'ame as it would
do from any point
e
in the right line
DBC,
if there
were no water in the vefTd:
and airo
Ihews, (hat
KI,
the fine of
rerraélioD
in water, is to
DF,
tbe fine of io.
.cidence io air, as 3 to 4.
Henee, if
DBH
were a erooked fliek put obliquely
ioto (he water,
il
would appear a (haight one
al
DOG.
Therefore, as the line
B
H
appears at
BC,
Co~he
line
BC
will appear at
Bg;
and eonCequeotly, a flraight !liek
DBC
put oblrquely into water, will feem beot at tbe furface
of the water io
B,
aod erooked, as
DBg.
Wheo a ray of light palTes out of air ioto glaCs, the
(ine of
incidence
is to the fine of
n:fraétion
as 3 to
2 ;
and when
OUt
of air ioto a diamood, as 5 to
2.
OfGLASSES.
GL ASS may be grouod into eight diffcreot Chapes at
lean, for optiea) purpofe.,
"iz.
e
s.
.. A
plan,
glaft,
(No. 5.) whioh is Bal on both fid..,
and .of eqttal thicknefs in all pans, as
A.
2.
A
pltJnc-com.,'ex,
\fJhich is ilat
00
one lide. and con·
vex on lhe other, as
B.
3. A
d,)U{¡Ü-.&'fiV~X,
wbkh
is convex on
both fides,
as
C.
4. A
plano-concQtle,
which is fIat on one fid e, and
conca've on the other, as
D.
S.
A
doubl, ronca"e,
whieh is eoneave
00
both fide.,
as
E.
6.
AJIUnifouI,
which is coocave
00
one Cide, and
convex..
on [he
other, -as
F.
7. A
Jln'
plano'
con",,,,
·whofe eonvex fide is ground
into Ceveral liule Rat furfaees, as
e.
8. A
priJiJJ,
whieh has three
flat
fides; and when
viewed endwife, appears like an equilateral triaogle, as
H.
GlalTe••ground ioto any. of the Ibapes
B, C, D, E, F,
are gener.lly ealled
lenfu.
A right lioe
L/K,
.(ND. 6.) going pe'pendieularly
througb the rniddle of a leos, is calle<!
Ih, axil
of
,he
lent .
A ray 'of light
Ch,
falliog perpendieularly on aplane
glaC.
EF,
will.pafs through the glafs in the rame diree–
tion
hi,
and go
out of
it ioto the air in the [ame right
courfe
iH.
A ray of light
AB,
f.lling obliquely
00
a plaoe glafs.
will go out of tbe glafs in the
fa.ledireélioo, but not io
the Came right Iioe: for in touehing the glaf" it will
be r<fraéted in the I:ne
BC;
and in leaving the glaC" it
will be reFraéled io the line
CD.
A ray'
~f
light
CD,
(No. 7.) falling obliqucly on the
middle of a eonvex glafs, will go forward io tbe
f.medireélion
DE,
as if it had fallen with the fa me degree of
obliquity
00
aplane glafs; aod will go out of the glaC.
in
lhe fame
direétion with which it
entered
: for it will
be <quallr refr.é1ed at the points
D
and
E,
as if it had
p.rred through aplane fudace . But the rays
ce
and
el
will be fo reFnéled, as to meet agaio at the poiot
F.
Therefore, .11 the ráys wnieh Row from the point
C
r
fo as to go th,ough the glaCs, will meet again at
F;
aod
if they go farthe r onward , as to
L,
.hey erofs
al
F,
aod
go forwá rd
00
the oppofite lides of the middle ray
CDEF,
to what they ",ere in approaehing it io the direélioo.
HF
and
KF.
When paralle! ray', as
A
BC,
(No. 8.) fall direélly
upon a plano.eoovex glaCs
DE,
and pafs through it, they
will be fo refroéled, as tO unite io a point.f behiod it
¡
aod this poiOl is ealled the
principalfoClu;
the di nance of
whieh, from the middle of the glafs, is ealled the
f ocal
diJIan« ,
whieh is equal to twice the radius of the fphere
of the glafs's convexity. And,
When parallel rays, as
.1BC,
(No. 9.) fall direélly
upon a glaf.
DE,
whieh is equally eonvex
00
both fides,
and pafs through it ; they will be fa refraéled, a.
10
meel
in a point o, principal foeus
¡;
whofe diflanee is equal to
the radius or femidiameter of the fphere of the glaf,',
convexity. But if a glaf. be more eoovex
00
one fide
th.n
00
the other. the. rule for fioding the focal dillanee
is tlris: As the fu·m of the fernidiameters of both eon–
vexities is
10
the remidiameter of eitbel, fo is double the
fem idiameter