What is the 853. The word meniscus is derived from the
f a Greek language, and means literally a little
moon. This term is applied to a concavo-convex
lens, from its similarity to a moon in its early appearance. To
this kind of lens the term periscopic has recently been applied,
from the Greek language, meaning literally viewing on all sides.
When the concave and convex sides of periscopic glasses are
even, or parallel, they act as plane glasses ; but when the sides
are unequal, or not parallel, they will act as concave or convex
lenses, according as the concavity or the convexity is the greater,
What is the axis 854. The axis of a lers is a line passing
yf a lens? through the centre : thu? F G, Fig. 131, is
the axis of all the five lenses.
85D. The peculiar form of the various
lenses ? kinds of lenses causes the light which passes
through them to be refracted from its course
according to th) laws of Dioptrics.
It will be remembered that, according to these laws, light, in
passing from a rarer to a denser medium, is refracted towards
the perpendicular ; and, on the contrary, that in passing from a
denser to a rarer medium it is refracted further
c. ti T U fh W f ^ rom ^ e P er P en dicular. I* 1 order to estimate
feet of a lens J the effect of a lens, we must consider the situa-
tion of the perpendicular with respect to the
surface of the leas. Now, a perpendicular, to any convex or
concave surface, must always, when prolonged, pass through
the centre of sphericity ; that is, in a lens, the centre of the
sphere of which the lens is a portion. By an attentive observa-
tion, therefore, of the laws above stated, and of the situation of
the perpendicular on each side of the lens, it will be found, in
(1.) That convex lenses collect the rays into
**>**,* magnify objects at a certain dis-
cave lenses re- tance.
*P ec (2.) That concave lenses disperse the rays,
and diminish objects seen through them.
What is the fr- 856. The focal distance of a lens is the
cal distance of distance from the middle of the glass to the
focus. This, in a single convex lens, is equal
to the diameter of the sphere of which the lens is a portion,
and in a double convex lens is equal to the radius of a
sphere of which the lens is a portion.
857. When parallel rays * fall on a corx^
What rays will , , , , . , . ., . . ..
pass through a vex ^ ens ? those only which fall in the direc-
lens without re- tion of the axis of the lens are perpendicular
to its surface, and those only will continue
* The rays of the sun are considered parallel at the surface of the earth.
They aie not so in reality, but, on account of the great distance of that
luminary, their divergency is SJ small that it is altogether inappreciable.
on in a straight line through the lens. The other rays,
falling obliquely, are refracted towards the axis, and will
meet in a focus. (See par. 1484)
858. It is this property of a convex lens
ci ^/e W are ^sun w ^ c ^ gives it its power as a burning-glass, or
glasses, or sun-glass. All the parallel rays of the sun
burning-glasses, w hich pass through the glass are collected to-
conslructed ? ,, . ,, , . 7 7
gether in the focus ; and, consequently, the heat
at the focus is to the common heat of the sun as tJie area of the
glass is to the, area of the focus. Thus, if a lens, four inches in
diameter, collect the sun's rays into a focus at the distance of
twelve inches, the image will not be more than one-tenth of an
inch in diameter; the surface of this little circle is 1600 times
less than the surface of the lens, and consequently the heat
will be 1600 timeb greater at the focus than at the lens.
859. The following effects were produced by a large lens, or burn-
ing-glass, two feet in diameter, made at Leipsic in 1691. Pieces of
lead and tin were instantly melted ; a plate of iron was soon ren-
dered red-hot, and afterwards fused, or melted ; and a burnt brick
was converted into yellow glass. A double convex lens, three feet
in diameter, and weighing two hundred and twelve pounds, made by
Mr. Parker, in England, melted the most refractory substances'.
Cornelian was fused in seventy-five seconds, a crystal pebble in six
seconds, and a piece of white agate in thirty seconds. This lens
was presented by the King of England to the Emperor of China.
860. If a convex lens have its sides ground
down into several flat surfaces, it will present
as many images of an object to the eye as it
has flat surfaces. It is then called a Multiplying-glass. Thus,
if cne lighted candle be viewed through a lens having twelve
flat surfaces, twelve candles will be seen through the lens. The
principle of the multiplying-glass is the same with that of a
crnvex or concave lens.
801. The following effects result from the laws of refraction
FACTS WITH REGARD TO CONVEX SURFACES. (1.) Parallel rays
passing out of a rarer into a denser medium, through a CONVEX sur-
face , will become converging.
'2.) Divevginor rays will be made to diverge less, to become por-
236 NATURAL PHILOSOPHY.
allel, or to converge, according to the degree of divergency before
refraction, or the convexity of the surface.
(3.) Converging rays towards, the centre of convexity will sufibr
(4.) Rays converging to a point beyond the centre of convexity
will be made more converging.
(5.) Converging rays towards a point nearer the surface thac
tne centre of convexity will be made less converging by refraction.
[When the rays proceed out of a denser into a rarer medium, the
reverse occurs in each case.]
862. FACTS WITH REGARD TO CONCAVE SURFACES. (1.) Parallel
rays proceeding out of a rarer into a denser medium, through a
CONCAVE surface, are made to diverge.
(2.) Diverging rays are made to diverge more, to suffer no
refraction, or to diverge less, according as they proceed from a
point boyond the centre, from the centre, or between the centre and
(3.) Don verging rays are made less converging, parallel, or diverg-
ing, accosting to their .degree of convergency before refraction.
803. The above eight principles are all the necessary consequence
of the operation of the three laws mentioned as the fundamental
laws of Dioptrics. The reason that so many different principles are
produced by the operation of those laws is, that the perpendiculars
to a convex or concave surface are constantly varying, so that no
two are parallel. But in flat surfaces the perpendiculars are paral-
lel ; and one invariable result is produced by the rays when jpaes-
ing from a rarer to a denser, or from a denser to a rarer medium,
having a flat surface.
[When the rays proceed out of a denser into a rarer medium, the
reverse takes place in each case.]
864. Double convex, and double concave
What kinds of , , , . ,
glasses are used glasses, or lenses, are used in spectacles, to
in spectacles, remedy the defects of the eye : the former,
and for what -. . a , i
purpose? when by age it becomes too flat, or loses a
What kinds of portion of its roundness: the latter, when
erall * worn^b V an J otner cause it assumes too i ound a
old persons? form, as in the case of short-sighted (or, as
"I \ Tl 7 * J 7, O\?
young ? m y tne y are sometimes called, near-tighted)
persons. Convex glasses are used when the
eye is too flat, and concave glasses when it is too round.
These lenses or glasses are generally numbered, by opticians,
according to their degree of convexity or. concavity ; so thai, by
knowing the number that fits the eye, the purchaser can generally
bo accommodated without the trouble af trying many glajaes.
806 THE EYE. The eyes of all animals are constructed on the
same principles, with such modifications as are necessary to adapt
them to the habits of the animal. The knowledge, therefore, of the
construction of the eye of an animal will give an insight of the con-
struction of the eyes of all.
~? 866. The eye is composed of a number of
U/ what is \ ...
tke eye com- coats, or coverings, within which are enclosed
posed? a j enSj an( j certain humors, in the shape and
performing the office of convex lenses.
What are the different 86 ^. The different parts of the eye
par Is of the eye ? are :
(1.) The Cornea.
(2.) The Iris.
(3.) The Pupil.
(4.) The Aqueous Humor.
(6.) The Vitreous Humor
(7.) The Retina.
(8.) The Choroid.
(9.) The Sclerotica.
(5.) The Crystalline Lens. | (10.) The Optic Nerve.
Explain 868 ' Fi S' 132 re P resents Fig- 132.
Fig. 132. a front view of the eye, in
which a a represents the Cornea, or, as
it is commonly called, the white of the
eye ; e e is the Iris, having a circular
opening in the centre, called the pupil,
p, which contracts in a strong light, and
expands in a faint light, and thus reg-
ulates the quantity which is admitted
to the tender parts in the interior
of the eye.
Explain 869 ' *ig- 13S re P"
Fig. 133. resents a side view of
the eye, laid open, in which b b
represents the cornea, e e the iris,
i d the pupil, //the aqueous hu-
aaor, g g the crystalline lens, 'i h
the vitreous humor, i i i i i the retina, c c the choroid, a a a
a a the scl erotica, and n the optic nerve.
Describe the $7 '0. The Cornea forms the anterior portion
Cornea. the eye. It is set in the sclerotica in the same
nanner as the crystal of a watch is set in the case. Its
degree of convexity varies in different individuals, and in
different periods of life. As it covers the pupil and the
iris, it protects them from injury. Its principal office is to
cause the light which reaches the eye to converge to the
axis. Part of the light, however, is reflected by its finely -
polished surface, and causes the brilliancy of the eye.
Describe the 871. The Iris is so named from its being
l> of different colors. It is a kind of circular
curtain, placed in the front of. the eye, to regulate the quan-
tity of light passing to the back part of the eye. It has a
circular opening in the centre, which it involuntarily en-
larges or diminishes.
872. It is on the color of the iris that
What causes a the color of the eye depends. Thus a person
person's eyes to . .,"< ^ t ^ i ^ ^ i
be black blue or ls sai( * * nav ^ black, blue, or hazel eyes
gray, <%c. .-' according as the iris reflects those colors
What is the ^73. The Pupil is merely the opening in the
Pupil? iris, through which the light passes to the lens
behind. It is always circular in the human eye, but
in quadrupeds it is of different shape. W nen the pupil i&
expanded to its utmost extent, it is capable of admitting ten
times the quantity of light that it does when most con-
874. In cats, and other animals which are paid
Borne 'animals to see * n ^ e dark, *he power of dilatation and eon-
M* in the traction is much greater; it is computed that their
pupils may receive one hundred times more ligh<
at one time than at another. That light only which passes the
pupil can be of use in vision ; that which falls on the iris, being
reflected, returns through the cornea, and exhibits the color of
When we come from a dark place into a strong light, our eyee
suffer pain, because the pupil, being expanded, admits alarger quan
tity of light to rush in, before it has had time to contract. And,
when we go from a strong light into a faint one, we at first imagine
ourselves in darkness, because the pupil is then contracted, and does
not instantly expand.
875. The Aqueous Humor is a fluid as clear
Aqueous Hu- as the purest water. In shape it resembles a
meniscus, and, being situated between the cor-
nea and the crystalline lens, it assists in collecting and
transmitting the rays of light from external objects to that
876. The Crystalline Lens is a transparent
[Wiat ts the J r
Crystalline body, in the form of a double convex lens,
Lens? placed between the aqueous and the vitreous
humors. Its office is not only to collect the rays to a focus
on the retina, but also to increase the intensity of the light
which is directed to the back part of the eye.
T*-/, , ,; 877. The Vitreous Humor (so called from its
\\hat is the ^
Vitreous Hu- resemblance to melted glass) is a perfectly
transparent mass, occupying the globe of tho
eye. Its shape is like a meniscus, the convexity of which
greatly exceeds the concavity.
878. In Fig. 134 the shape of the
aqueous and vitreous humors and the crys-
tal] ine lens is presented. A is the aqueous
Humor, which is a meniscus, B the crystal-
line lens, which is a double convex lens,
and C the vitreous humor, which is aide a
meniscus, whose concavity has a small ir radius than its con-
240 flATUKAL PHILOSOPHY.
What is tto &7S. The Retina is the seat of vision. The
Retina ? ra j s O f light, being refracted in their passage by
the other parts of the eye, are brought to a focus in the
retina, where an inverted image of the object is represented
What is the ^80. The Choroid is the inner coat or cover-
Choroid? j n g O f the eye. Its outer and inner surface
is covered with a substance called the pigmentum nigrum
(or black paint). Its office is, apparently, to absorb the
rays of light immediately after they have fallen on the retina.
It is the opinion of some philosophers that it is the choroid,
and not the retina, which conveys the sensation produced
by rays of light to the brain.
Describe the $81. The Sclerotica is the outer coat of the
Sclerotica. e ye. It derives its name from its hardness.
Its office is to preserve the globular figure of the eye, and
defend its more delicate internal structure. To the sclero-
tica are attached the muscles which move the eye. It re-
ceives the cornea, which is inserted in it somewhat like a
watch-glass in its case. It is pierced by the optic nerve,
which, passing through it, expands over the inner surface
of the choroid, and thus forms the retina.
882. The Optic Nerve is the organ which
what is the carr j es the impressions made by the ravs of
Optic Nerve? J *
light (whether by the medium of the retina, or
the choroid) to the brain, and thus produces the sensation
What optical 883. The eye is a natural camera obscura
instrument r ? o n r i 11 f-n-i-
foes the eye i see o. 805J, and the -images of all objects
resemble? seen by the eye are represented on the 'retina
in the same manner as fne forms of external objects are
delineated in that instrument.
Explain 884. Fig. 135 represents only those parts of the eye
l **' ' which are most essential foi the explanation of the
phenomenon of vision. The image is formed thus : The : ay?
from the object c d, diverging towards the eye, enter the cornea
c, and cross one another in their passage through the crystalline
lens d, by which they are made to converge on the retina, where
they form the inverted image / e. (See par. 1488.)
Hew i.s the 885. ^ ue convexity of the crystalline humor is
convexity of increased or diminished by means of two muscles,
thecryttaltinefo M h it at t ac hed. By this means, the focus
lens altered, m J
and for what of the rays which pass through it constantly falls
purpose? on the retina; and an equally distinct image is
fcrmed, both of distant objects and those which are near.
How can you 886. Although the image is inverted on the re-
"he^wrent tma ' we see ^J ects erec ^ because all the images
position of formed on the retina have the same relative posi-
oojccts ? tion which the objects themselves have ; and, as the
rays all cross each other, the eye is directed upwards to receive
the rays which proceed from the upper part of an object, and
downwards to receive those which proceed from the lower part.
887. A distinct image is also formed on the re-
Win! do we
not "see double tma of each eye ; but, as the optic nerves of the
with two eyes ? two eyes unite, or cross each other, before they
reach the brain, the impressions received by the two nerves are
united, so that only one idea is excited, and objects a?e seen
single. Although an object nay be distinctly seen with only
one eye, it has been calculated that the use of both eyes makes
u difference >f about one-twelfth. From the description now
given of the eye, it may be seen what are the defects wnich art
remedied by the use of concave and convex lenses, and how the
use of these lenses remedies them.
What defects 888. When the crystalline humor of the eye is
of the eye are too roun a tne rays of light which enter the eye
spectacles de- or
signed to converge to a focus before they reach the retina,
remedy ? an d ) therefore, the image will not be distinct ; and
when the crystalline humor is too flat (as is often the case with
old persons), the rays will not converge on the retina, but tend
to a point beyond it. A convex glass, by assisting the converg-
ency of the crystalline lens, brings the rays to a focus on the
retina, and produces distinct vision.
889. The eye is also subject to imperfection b\
For what de- J , A-
fects of the reason 0* the humors losing their transparency
eye is there either by age or disease. For these imperfection?
no remedy ? nQ g] asses O g- er a reme dy, without the aid of surgi-
cal skill. The operation of couching and removing cataracts
from the eye consists in making a puncture or incision through
which the diseased part may escape. Its office is then supplied
by a lens. If, however, the operator, by accident or want of
skill, permit the vitreous humor to escape, the globe of the eye
immediately diminishes in size, and total blindness is the inevi
What is a ^' ^ g i n o^ e microscope consists simply of
singlemic.ro- a convex lens, commonly called a magnifying-
scope. glass ; in the focus of which the object is placed
and through which it is viewed.
891. By means of a microscope the rays of light from an
object are caused to diverge less ; so that when they enter the
pupil of the eye they fall parallel on the crystalline lens, by
which they are refracted to a focus on the retina.
Explain 892, Fig. 136 represents a convex lens, or single
&' ' microscope, C P. The diverging rays from tbe object
A B are refracted in their passage through the leu* C P. aii'J
made to fall parallel on
the crystalline lens, by
which they are refracted
to a focus on the retina
R R, ; and the image is
thus magnified, because
the divergent rays are
collected by the lens and
carried to the retina.
893. Those lenses or microscopes which have
What glasses ,,,.' . - .
have the great- tne shortest focus have the greatest magnifying
est magnifying power ; and those which are the most bulging
or convex have the shortest focus. Lenses are
made small because a reduction in size is necessary to an increase
What is a double 894. A compound microscope consists
microscope? O f wo convex lenses, by one of which a
magnified image is formed, and by the other this image is
carried to the retina of the eye.
Explain 895. Fig. 137 represents the effect produced by the
Fig. 137. lenses of a compound microscope. The rays which
diverge from the object A B are collected by the lens L M (called
the object-glass, because it is nearest to the object), and form an
invertcd magnified image at C D. The rays which diverge from
this image are collected by the lens N O (called the eye-glass,
it is nearest to the eye), wWch acts- on the principle of
24A NATURAL PHILOSOPHY.
the single microscope, and forms still another image on the
IW / ' fk &$$ The solar microscope is a microscope
solar micro- with a mirror attached to it, upon a movable
wye ; joint, which can be so adjusted as to receive
the sun's rays and reflect them upon the object. It con-
sists of a tube, a mirror or looking-glass, and two convex
lenses. The sun's rays are reflected by the mirror through
the tube upon the object, the image of which is thrown upon
' t white screen, placed at a distance to receive it.
897. The microscope, as above described, is used for viewing
transparent objects only. When opaque objects are to be viewed,
a mirror is used to reflect the light on the side of the object ;
the image is then formed by light reflected from the object.
instead of being transmitted through it.
898. The magnifying power of a single mi-
%? them *S~ croscope is ascertained by dividing the least
of singie and distance at which an object can be distinctly
double micro- geen j^y tne na k e( i eyc b y t h e f oca l distance of
scopes ascer- . . mi . . ,
tained? ie ^ Gns ' This, in common eyes, is about seven
inches. Thus, if the focal distance of a lens
be only of an inch, then the diameter of an object will be
magnified 28 times (because 7 divided by is the same as 7
multiplied by 4), and the surface will be magnified 784 times.
The magnifying power of the compound microscope is found
in a similar manner, by ascertaining the magnifying power, first
of one lens, and then of the other.
The magnifying power of the solar microscope is in propor-
tion as the distance of the image from the object-glass is
greater than that of the object itself from it. Thus, if the dis-
tance of the object from the object-glass be of an inch, and
the distance of the image, or picture, on the screen, be *en feet.
.ir 120 inches, the object will be magnified in length 4bO times
or in surface 280,000 times,
A lens may be caused to magnify or to aiminish an object. If the
jbject be placed at a distance from the focus of a lens, and the im-
age be formed in or near the focus, the image will be diminished ;
but, if the object be placed near the focus, the image will be mag-
What is the Mag- The Magic Lantern is an instrument con-
k Lantern? structed on the principle of the solar micro-
scope, but the light is supplied by a lamp instead of the
899. The objects to be viewed by the magic lantern are gener-
ally painted with transparent colors, on glass slides, which are
received into an opening in the front of the lantern. The light
from the lamp in the lantern passes through them, and carries
the pictures painted on the slides through the lenses, by means
of which a magnified image is thrown upon the wall, on a white
surface prepared to receive it.
Fig. 138 represents the magic lantern. The
rays of light from the lamp are received upon
the concave mirror e, and reflected to the con-
vex lens c, which is called the condensing lens, because it con-
centrates a large quantity of light upon the object painted on
the slide, inserted at b. The rays from the illuminated object
at b are carried divergent through the lens a, forming an imagi
on the screen at/. The image will increase or diminish in sizo
in proportion to the distance of the screen from the lens a.
1Mb NATURAL PHILOSOPHY.
900. DISSOLVING VIEWS. The exhibition
Hair are" Dis- ca i} e d Dissolving Views " is made by means
srlving Views , . , ,
t "presented? * * wo rna g i c lanterns of equal power, so as to
throw pictures of the same magnitude in the
Kime position on the screen. By the proper adjustment of
sliding tubes and shutters, one picture on the screen is made
brighter while the other becomes fainter, so that the one seems
to dissolve into the other. In the hands of a skilful artist #
this is an exhibition of the most pleasing kind.
901. TELESCOPES. A -Telescope is an
What is a Tel- . t ,. . . ,. x , /
tscope? instrument tor viewing distant objects, and
causing them to appear nearer to the eye.
How are tele- ^2. Telescopes are constructed by placing
scopes construct- lenses of different kinds within tubes that slide
*"' within each other, thus affording opportunity
of adjusting the distances between the lenses within.
903. They are also constructed with mirrors, in addition to
the lenses, so that, instead of looking directly at an object, the
eye is directed to a magnified image of the object, reflected
from a concave mirror. This has given rise to
How many kinds ^ ne t wo distinctions in the kinds of telescopes
there? m common USG > called respectively the Refract-
ing and the Reflecting Telescope.
How is the Re- 904. The Refracting Telescope is con-
scopT^onstruci- structed with lenses alone, and the eye is
d? * directed toward the object itself.
905. The Reflecting Telescope is con-
How does a Re- -. . , . -, >
fleeting Tele- structed with one. or more mirrors, in addi-