at D. Suppofe the veffel to be now filled with water, and 'the
fhadow will withdraw to d ; the ray o f light, inftead of proceeding
to D, being refraaed or bent to d. And there is no doubt
but that an eye, placed at d, would fee the candle at e, in the
direaion o f the refraaed ray d A. This is alfo confirmed by the-
following plealing experiment.
Lay a fhilling, or any piece o f money, at the bottom o f a
bafon; then withdraw from the bafon, till you lofe fiofit o f the-
{hilling; fill the bafon nearly with water, and the fhilling will be
feen very plainly, though you are at the fame diflance from it.
Experiment 3. Place a flick over a bafon which is filled with
water; then reflea the fun’s rays, fo that they may fall perpendicularly
on the furface o f the water; the fhadow o f the flick will
fall on the fame place, whether the veffel be empty or full.
What has been laid o f water, may be applied to any tranf-
parent medium, only the power o f refra&ion is greater in fome
than in others. It is from this wonderful property, that we
derive all the curious effeas of glafs, which make it the fubjea o f
optics. It is to this we owe the powers o f the microfcope and
the telefcope.
T o produce thefe effeas, pieces o f glafs are formed into given,
figures, which, when fo formed, are called lenfes. The fix under
mentioned figures, are thofe which are moll in ufe for optical
purpofes.
x. A
1. A plane glass, one that is flat on each fide, and of an
equal thicknefs throughout. F, Fig. 13, Plate I.
2. A double convex glass, one that i - more elevated towards
the middle than the edge. B, Fig. 13, Plate I.
3. A double concave is hollow on both fides, or thinner
in the middle than at the edges. D, Fig. 13, Plate I.
4. A plano convex, flat on one fide, and convex on the
other. A, Fig. 13, Plate I.
r A plano concave, flat on one fide, and concave on the
other. C, Fig. 13, Plate I.
6. A meniscus, convex on one fide, concave on the other.
E , Fig. 13, Plate I.
It has been already obferved, that fight proceeds invariably
from a luminous body, in flrait lines, without the leaft deviation
; but if it happen to pafs from one medium to another, it
always leaves the direction it had before, and aflumes a new one.
After having taken this new direftion, it proceeds in a flrait
fine, till it meets with a different medium, which again turns
it out o f it’s courfe.
A ray o f fight, paffing obliquely through a plane glafs, will go
out in the fame direftion it entered, though not precifely in the
fame line. T h e ray C D, Fig. 4, Plate I. falling obliquely
upon the furface of the plane glafs A B, will be refracted towards .
the