DIOPTRICS. 
a lighted candle on ane fide of it, and a little above the up- 
per edge of it, as at G, and the edge of the fhadow of the 
me ot the veite B will be found to fall upon the bottom 
t E, fo that the line GBD may be a tlraight line, Now, 
Seal carne any part of the apparatus, let the veffel 
ABC filled with water, and when this has been done 
hee will find that ial edge of the fhadow falls at F, viz. at 
a place different from D, which proves bevond a doubt, that 
ay light which fale blues a. the furface of the water 
t B is refraGted, wz. bent ; for if it proceeded in a 
ftraight line, would coinesde with GBD, which is 
means bl ca 
thewing this remarkable property of 
3° 
light is as secllews Place a piece of money D, jig. 4. in the 
ottom of a common ae) on, and let a ie fituate 
oes e at 2 viz. at a whence he may juft fee the 
pola along ie ieee B of the vellel, In this 
‘alr. 
will immediately perceive the piece of m 
of light D B, in pafling from ae water into ie air at B is 
bent, viz. refraéted into the direction B 
At the point Bof fg. 3. raife BK perpendicular to the 
furface of the water, and let the line BI be a prolongation 
of that furface, or an horizontal line. Then i ray of light fine 
GB, which falls upon the furface of the water at B, 
called the incident ray ; that part of it BF, which after the 
bending at B, paffes through the water, is the i Miahced 
the water, or any other tranfparent fubftance, which 
duces a fimilar effe&, is called the refrading medium; the an- 
gle which the incident ray G B makes with the perpendicu- 
are the angles made e in- 
cident and the refra&ted oe with the furface or hfecuel 
line ABI. - 
Inftead of water, the veflel ABDC, Jig. 3. may be filled 
with oil, or with fpirit of wive, or with a lump of glafs, or 
in fhort with any other tranfparent medium different from 
air, and the ray of light G B wil! always be bent ; i 
difference, however, ‘that the ray will be bent, Vi%. Ye- 
fracted, more or lefs according as one tranfparent medium 
or another is employed ; which fh 
{ubltence has a pechliae — of refraGting power. 
the 
\ the sea dae — that have been employed, may be 
determined. Asa 
§ 
t-fubitances, together 
with the bet methods of steer ng them, will be found 
baa be article Rerractrio 
arkable property has been obferved with refpc& to 
the meee power of tran{parent fubftances, which is that 
8 
in the fame fubftance the fine of the angle of refraQion = 
ways bears the fame proportion (either precifely or 
nearly ) to the fine of the angle of incidence. This mee | 
iHluftration by meansof a diagram. Let FGHZ (fy. 5. 
be a quantity of water; Bis a narrow tube through which 
the fun fhines obliquely upoa the water at C. Then, on ac- 
count of the refraCtive property of water, that Hght will 
not pafs through the water along the line CZ, which is in 
the fame ftraight direGtion with BC; but it will pafs in the 
direCtion C D; making the angle of relreGion DCE, with 
the line ACE (which is perpendicular to the furface of the 
water, or to the boundary of the two mediums, viz. wae . 
ter and air) fmaller than the angle of incidence A C B. 
Let a circle FH E be defcribed about the centre or point 
of incidence C, in the fame plane with the lines BC, C 
and from the interfetion H of the circle with the imeident 
ray, let a perpendicular H K be es on the line AE 
then H K is the fine of the angle BC A ‘fo, if on 
the interfeGtion I of the circle with the refragted ray, @ pers 
pendicular } L be dropped on ie fame line A E, then I L 
will - a fine of the angle D 
as been found that re fine J L is always nearly 
three-fourths of the fine H_ K, let that fine be what it may ; 
for inftance, if the io B be placed at M, thn OR 
will be the fine of the angle of incidence MCA; and 
wil oe i fine of the angle of refraction, or i the 
ngle in water Y : ud in this cafe it will be found, 
as ae yes the fine Y Q is nearly three-fourths of the 
line 
nifhes or becomes n 
fra€tion and its fine, Ate are va ios 5 viz. 
mult proceed ftraight elong ce line AC E; eee it is Bid 
that there is no refraGtion when the rays of rf ght enter a me- 
dium ina direétion perpendicular to its furface. 
If inftead of water, F G H Z be fuppofed to be ery at 
glafs, every thing elfe remaining as above; then e dif. 
ference aoe refult would be, that the ratio of ae fine of 
bee that of incidence world be that of 2 to 3 ; (in- 
ftead of being g that of 3 to 4, whichisthe cafe with water) viz. 
the angles of refraction will be refpeGtively fmaller when 
F 
It is evident, that when the eae a comes along the 
AC, the an its fine, va- 
GH Zis glafs than when it is water. And if inftead of 
water or gla{s, were a diamond, then the angle 
of refraction would be fmaller ftill; véz, the fine of the 
ee 
refractive property of tranfpare 
of fluids, is increafed a little, when t emperature is 
raifed. ‘wo other remarkable phenomena accompany the 
refraction of light, the firft of whtch is that the light is not 
only bent whenever it paffes obliquely from one medium into 
aoe: of different denfity, but is likewife enlarged in a {ec- 
toral, or fan-like, manner. The fecond is that this enlarged 
he of the is ag light is tinged with the colours of the 
ainbow. » IC repreients a fmall beam of folar 
cake which JSiee through the air, and enters another re- 
rating mediumat C. There the beam of light is Lager 
viz. 18 diverted from its ftraight line dire€tion IC, and is 
{pread, or prey into the feCtoral fhape vCr, which is 
called the angle of difoerfion, or of diffi aaa, and which is 
it sea divided sa {mailer feGtors of different colours ; viz 
i to the upper line Cr, the light appears red, and thence 
: peduly degenerates | into orange, then yellow, iS 
ue 
E 
rent saprucae and efpecially 
heir 
