OPTICS. 
294 
are extremely small, and by striking on ; 
our visual organs, give us the sensation of 
seeing.^ 
2. The particles of light are emitted from 
what are called luminous bodies, such as the 
sun, a lire, a torch, or candle, &c. &c. It is 
reflected or sent back by what are termed 
opake bodies, or those which have no power 
of affording light in themselves. 
3. Light, whether emitted or reflected, al- 
ways moves in straight or direct lines, as may 
easily be proved by looking into a bent tube, 
which evidently obstructs the progress of the 
light in direct lii.es, 
4. By a ray of light, is usually meant the 
least particle of light that can be either inter- 
cepted or separated from the rest. A beam 
of light is generally used to express some- 
thing of an aggregate or mass of light greater 
than a single ray. 
5. Parallel rays are such as proceed equally 
distant from each oilier through their whole 
course. The distance of the sun from (he 
earth is so immense, that rays proceeding 
from the body of that luminary are generally 
regarded as parallel. 
o. Converging rays are such as, proceed- 
ing from any body, approach nearer and 
nearer to each other, and tend to unite in a 
point. The form of rays thus tending to an 
union in a single point has been compared to 
that of a candle-extinguisher; it is in facta 
perfect cone. 
7. Diverging rays are those which, pro- 
ceeding from a point, continue to recede 
from each other, and exhibit the form of an 
inverted cone. 
8. A small object, or a small single point 
of an object, from which rays of light diverge, 
or indeed proceed in any direction, is some- 
times called the radiant, or radiant point 
9. Any parcel of rays, diverging from a 
point, considered as separate from the rest, is 
called a pencil of rays. 
10. The focus of rays is that point to 
which converging rays tend, and in which 
they unite and intersect, or cross each other. 
It may be considered as the apex or point of the 
cone; and it is called the focus (or fire-place), 
because it is the point at which burning- 
glasses burn most intensely. 
1 1. The virtual or imaginary focus js that 
supposed point behind a mirror or looking- 
glass, where the rays would have naturally 
united, had they not been intercepted by the 
mirror. 
12. Plane mirrors or speculutns are those 
reflecting bodies, the surfaces of which are per- 
fectly plain op even, such as our common 
looking-glasses. Convex and concave mir- 
rors are those the surfaces of which are 
.curved, 
13. An incident ray is that which comes 
from any body to the reflecting surface; the 
reflecting ray is that which is sent back or 
reflected. 
14. The angle of incidence is the angle 
which is formed by the Hue which the inci- 
dent ray describes in its progress, and a line 
drawn perpendicularly to the reflecting sur- 
face: and the angle of reflection is the angle 
formed by the same perpendicular and the 
reflected ray; thus, (Plate I. Optics, fig. 1) 
if bu is a reflecting surface, and d c an inci- 
dent ray, then d c P is the angle of incidence, 
grid cc'P the angle of reflection. 
1 5. By a medium opticians mean any thing 
which is transparent, such as void space, air, 
water, or glass, through which consequently 
the rays of light can pass in straight lines. 
Id. The refraction of the rays of light is 
their being bent, or attracted out of their 
course, in passing obliquely from one medium 
to another of a different density, and which 
causes objects to appear broken or distorted 
when part of them is seen in a different me- 
dium. It is from this property of light that a 
stick or an oar which is partly immersed in 
water, appears broken. 
17. A lens is a transparent body of a dif- 
ferent density from the surrounding medium, 
commonly of glass, and used by opticians to 
collect or Asperse the rays of light. They are 
in general either convex, that is, thicker in 
the middle than at the edges, which collect, 
and by the force -of refraction converge the 
rays, and consequently magnify; or concave, 
that is, thinner in the middle than at the edges, 
which by the refraction disperse the rays of 
light, and diminish the objects that are seen 
through them. 
18. Vision is performed by a contrivance 
of this kind. The crystalline humour, which 
is seated in the fore-part of the human eye, 
immediately behind the pupil, is a perfect 
convex lens. As therefore every object is 
rendered visible by beams or pencils of light, 
which proceed or diverge from every radiant 
point of the object, the crystalline lens col- 
lects all these divergent rays, and causes 
them to converge on the back part of the 
eye, where the retina or optic nerve is spread 
out ; and the points where each pencil of 
rays is made to converge on the retina, are 
exactly correspondent to the points of the 
object from which they proceed. As, how- 
ever, from the great degree of convergence 
which this contrivance will produce, the pen- 
cils of light proceeding from the extreme 
points ot the object will be made to cross 
each other before they reach the retina, the 
image on the retina is always inverted. (See 
Plate II. fig. 23.) 
19. The magnitude of the image painted 
on the retina will also, it is evident, depend 
on the greatness or obtuseness of the angle 
under which the pencil of rays proceeding 
from the extreme points of the object enters 
the eye. For it is plain, that the more open 
or obtuse the angle is, the greater is the ten- 
dency of these rays to meet in a point and 
cross each other ; and the sooner they cross 
each other after passing the crystalline lens, 
the larger will be the inverted image painted 
on the retina. (See Plate If. fig. 24.) The 
visual angle, therefore, is that which is made 
by two right lines drawn from the extreme 
points ot any object to the eye; and on the 
measure of that angle, the apparent magni- 
tude of every visible object will depend. 
20. The prism used by opticians is a tri- 
angular piece of fine glass, which has the 
power of separating the rays of light. 
History of discoveries. The mostantient 
hypothesis which leads to the true theory of 
light and colours, is that of the Platonics, viz. 
that light, from whatever it proceeds, is pro- 
pagated in right lines ; and that when it is 
reflected from the surfaces of polished bo- 
dies, the angle of reflection is equal to the 
angle of incidence. To this may be added 
the opinion of Aristotle, who supposed that 
rainbows, haloes, and mock, suns, were occa- 
sioned by the reflection of the sun’s beams in 
different circumstances. We have reason to 
believe, that the use of convex glasses, both 
as magnifiers and as burning-glasses, was not 
unknown to the antients, though the theory 
was not understood. The magnifyingpower 
of glasses, and some other optical pheno- 
mena, were also largely treated of by Allia- 
zen, an Arabic philosopher of the twelfth 
century. These observations were followed 
by those of Roger Bacon, who demonstrated 
by actual experiment, that a small segment 
of a glass globe would greatly assist the sight 
of old persons ; and from the hints afforded 
by these two philosophers, it is not unrea- 
sonable to conclude, that the invention of 
spectacles proceeded. Concerning the ac- 
tual author of this useful invention, we have 
no certain information ; we only find, that it 
was generally known about tire beginning of 
the fourteenth century. 
In the year 1375, Maurolycus, a teacher 
of mathematics at Messina, published a trea- 
tise on optics, in which lie demonstrates, that 
the crystalline humour of the eye is a lens, 
which collects the rays of light proceeding 
from external objects, and throws them on 
the retina, or optic nerve. From this prin- 
ciple he was led to discover the reason of 
what arc called short and imperfect sights. 
In the one case, the rays converge too soon ; 
in the other, they do not converge soon 
enough. Plence short-sighted persons are 
relieved by a concave glass, which causes 
the rays to diverge in some degree before 
they enter the eye, and renders it more diffi- 
cult for them to converge so fast as they 
would have done after entering the crystal- 
line humour. Hence too he proves that a 
convex lens is of use to persons who have 
weak but long sight, by causing the rays to 
converge sooner, and in a greater quantity, 
than would otherwise happen. lie was the 
first also that solved a problem which had 
caused much perplexity in the antient 
schools, respecting the sun’s image appear- 
ing round, though the rays that form it are 
transmitted into a dark room through an an- 
gular aperture. He considered, that as the 
rays of light are constantly proceeding, in 
every direction, from every part of the sun’s 
disk, “ they must be crossing, each other 
from the extreme part of it in every point of 
the aperture ; so that every such point will 
be the apex of two cones, of which the base 
of the one is the sun’s disk, and that of the 
other his image on the opposite wall.” The 
whole image, therefore, consists of a number 
of images, all of which are circular ; the 
image of the sun formed of those images 
must be circular also ; and it will approach 
the nearer a perfect circle, the smaller the 
aperture, and the more distant the image. 
Nearly about the same time Johannes Bap- 
tistaTorta, of Naples, invented the camera 
obscura ; and his experiments upon’ that in- 
strument convinced him that light is a sub- 
stance, by the intromission of which into the 
eye, vision is performed ; for it is proper to 
mention, that before his time the opinion was 
almost general, that vision depended upon 
what was termed visual rays, proceeding 
from the eye. In this the system of Porta 
corresponds nearly with that of Maurolycus : 
but it ought to be remarked, that the disco- 
veries of each of these two philosophers were 
