OPTICS. 
%ards the discovery of telescopes. Other 
treatises on optics, with various and gradual 
improvements, were afterwards successively 
published by several authors, whose names, 
with the titles and brief accounts of their 
several w’orks would occupy a large space. 
We must, however, mention the excellent 
work on optics, by Dr. Smith, 2 vols. 4to. ; 
an abridgment of which was made by Dr. 
Kipling, for the use of the students at the 
universities, entitled “ Elementary Parts 
of Dr. Smith’s Optics,” &c. 1778 ; and an 
elaborate History of the Present State of 
Discoveries relating to Vision, Light, and 
Colours, by Dr. Priestley, 4to. 1772 ; a 
work highly instructive and entertaining to 
persons who have a taste for physics. 
The laws of optics depending upon the 
properties of Light, the reader will do well, 
as introductory to this article, to refer to 
what has been said in our fourth volume on 
that subject. There will be found much 
curious speculation, and a variety of inte- 
resting facts relating to the nature of light, 
its velocity, and the direction which it takes 
in moving through free space and through 
our atmosphere. We shall in this place 
give a few definitions necessary to the mere 
student. 
By a ray of light, is meant the motion of 
a single particle ; and its motion is repre- 
sented by a straight line. Any parcel of 
rays proceeding from a point, is called a 
pencil of rays. By a medium, is meant any 
pellucid or transparent body, which suffers 
light to pass through it. Thus, water, air, 
and glass, are called media. Parallel rays, 
are such as move always at the same dis- 
tance from each other. If rays continually 
recede from each other, as from C to c d 
(Plate I. Optics, fig. 1.) they are said to di- 
verge. If they continually approach to- 
wards each other, as in'moving from cd to 
C, they are said to converge. The point at 
which converging rays meet, is called the 
focus. The point towards which they tend, 
but which they are prevented from coming 
to, by some obstacle, is called the imaginary 
focns. When rays, after passing through 
one medium, on entering another medium 
of different density, are bent out of their 
former course, and made to change their 
direction, they are said to be refracted: 
thus A C (fig. 2), is a ray which, when it 
enters the medium H G K instead of pro- 
ceeding in the same direction C L, it is 
made to move in the direction C S. When 
they strike against a surface, and are sent 
back again from the surface, they are said 
to be reflected. The incident ray, as A 
is that which comes from any luminous 
body, and falls upon the reflecting surface, 
as H K, and C M is the reflected ray. Tha 
angle of incidence, is that which is contain- 
ed between the incident ray A C and a 
perpendicular to the reflecting surface in 
the point of reflection, as the angle ACD. 
The angle of reflection, is that contained be- 
tween the said perpendicular D C, and the 
reflected ray C M, viz. the angle D C M. 
The angle of refraction, is that contained 
between the refracted ray C S, and the per- 
pendicular C N, viz. the angle F C K. The 
angle of deviation, is that which is contained 
between the line of direction of an incident 
ray A L, and the direction of the same ray 
C F after it is refracted ; thus the angle 
L C F is the angle of deviation. 
A lens, is glass ground into such a form 
as to collect or disperse the rays of light 
which pass through it. These are of dif- 
ferent shapes, and from thence receive dif- 
ferent names. A plano-convex, has one 
side flat, and the other convex, as A (fig. 3.) 
A piano concave is flat on one side, and 
concave on the other, as B. A double 
convex, is convex on both sides, as C. A 
double concave, is concave on both sides, 
as D. A meniscus, is convex on one side 
and concave on the other, as E. A line 
passing through the centre of a lens, as F G, 
is called its axis. 
Of Refraction. If the rays of light, after 
passing through a medium, enter another of 
a different density perpendicular to its sur- 
face, they proceed through this medium in 
the same direction as before. Thus the 
ray O P (fig. 2.) proceeds to K, in the same 
direction. But if they enter obliquely to 
the surface of a medium, either denser or 
rarer than what they moved in before, they 
are made to change their direction in 
passing through that medium. If the me- 
dium which they enter be denser, they 
move through it in a direction nearer to the 
perpendicular drawn to its surface. Thus, 
AC, upon entering the denser medium 
H G K instead of proceeding in the same 
direction A Lis bent into the direction CF, 
which makes a less angle with the perpen- 
dicular O P. On the contrary, when light 
passes out of a denser into a rarer medium, 
it moves in a direction farther from the 
perpendicular. Thus, if S C were a ray of 
light which had passed through the dense 
medium H GK, on arriving at the rarer 
medium it would move in the direction 
C A, which makes a greater angle with the 
