34
The point o f union of the rays o f light, formed by means o f
a g l a f s lens, & c . is called the f o c u s .
Now, as each ray carries with it the image o f the objeft from
whence it proceeded, it follows, that i f thofe rays, after inter-
fefting each other, and having formed an image at their inter-
feftion, are again united by a refraftion or refleftion, they will
form a new image, and that repeatedly, as long as their order is,
not confounded or difturbed.
It follows alfo, that when the progrels o f the luminous ray is;
under confideration, we may look on the image as the object,,
and the objeft as the image ; and confider the fecond image, as
i f it had been produced by the firft as an objeft, and fo on.
In order to gain a clear idea o f the wonderful effects produced
by glafles, we mull proceed to fay fomething o f the principles o f
refraftion.
Any body, which is lo eonftituted as to yield a paflage to the
rays of light, is called a m e d i u m . Air, water, glafs, &c. are-
mediums o f light. I f any medium affords an ealy paflage to the
rays o f lights it is called a r a r e m e d i u m ; but if it does not
afford an eafy paflage to thefe rays, it is called a d e n s e
m e d i u m .
Let Z, Fig. 2, Plate I. be a rare medium, and Y a denle one;
and let them be feparated by the plane lurface G H. Let I K.
be a perpendicular to it, and cutting it in C.
With
With the center C, and any diftance, let a circle be defcribed.
Then let A C be a ray o f light, falling upon the denfe medium.
This ray, i f nothing prevented, would go forward to L ; but be-
caufe the medium Y is fuppofed to be denfer than Z, it will be
bent downward toward the perpendicular IK , and defcribe the
line C B .
The ray A C is called the incident ray ; and the ray C B,
the refracted r a y .
The angle A C l is called the angle of incidence, and the
ansle B C K is called the O angle of refract‘ ion.
I f from the point A, upon the right line C I, there be let fall
the perpendicular A D, that line is called the line o f the angle of
incidence. .
In the fame manner, if from the point B, upon the right line
IK , there be let fall the perpendicular B E, that line will be the
fine o f the angle of refraftion.
. The lines o f the angles are the meafures of the refraftions, and
this meafure is conftant; that is, whatever is the line o f the angle
of incidence, it will be in a conftant proportion to the fine of the
angle o f refraftion, when the mediums continue the fame. A
general idea o f refraftion may be formed from the following
experiments.
Let A B C D, Fig. 3, Plate I. reprefent a veflel fo placed, with
refpeft to the candle E, that the lhadow of the fide A C may fall
E 2 at