well; but I cannot now conceive, how any sight can be perfect: for, if
the crystalline humour is of a proper degree of convexity, to bring the
image of distant objects to a focus on the retina, it will not represent
near objects distinctly; and if, on the contrary, it is adapted to give
a clear image of near objects, it will produce a very imperfect one, of
distant objects.
_Mrs. B._ Your observation is very good, Emily; and it is true, that
every person would be subject to one of these two defects, if we had it
not in our power to adapt the eye, to the distance of the object; it is
believed that this is accomplished, by our having a command over the
crystalline lens, so as to project it towards, or draw it back from the
object, as circumstances require, by means of the two muscles, to which
the crystalline humour is attached; so that the focus of the rays,
constantly falls on the retina, and an image is formed equally distinct,
either of distant objects, or of those which are near.
_Caroline._ In the eyes of fishes, which are the only eyes I have ever
seen separate from the head, the cornea does not protrude, in that part
of the eye which is exposed to view.
_Mrs. B._ The cornea of the eye of a fish is not more convex than the
rest of the ball of the eye; but to supply this deficiency, their
crystalline humour is spherical, and refracts the rays so much, that it
does not require the assistance of the cornea to bring them to a focus
on the retina.
_Emily._ Pray, what is the reason that we cannot see an object
distinctly, if we place it very near to the eye?
_Mrs. B._ Because the rays fall on the crystalline humour, too divergent
to be refracted to a focus on the retina; the confusion, therefore,
arising from viewing an object too near the eye, is similar to that
which proceeds from a flattened crystalline humour; the rays reach the
retina before they are collected to a focus, (fig. 4.) If it were not
for this imperfection, we should be able to see and distinguish the
parts of objects, which, from their minuteness, are now invisible to us;
for, could we place them very near the eye, the image on the retina
would be so much magnified, as to render them visible.
_Emily._ And could there be no contrivance, to convey the rays of
objects viewed, close to the eye, so that they should be refracted to a
focus on the retina?
_Mrs. B._ The microscope is constructed for this purpose. The single
microscope (fig. 5.) consists simply of a convex lens, commonly called a
magnifying glass; in the focus of which the object is placed, and
through which it is viewed: by this means, you are enabled to place your
eye very near to the object, for the lens A B, by diminishing the
divergence of the rays, before they enter the pupil C, makes them fall
parallel on the crystalline humour D, by which they are refracted to a
focus on the retina, at R R.
_Emily._ This is a most admirable invention, and nothing can be more
simple; for the lens magnifies the object, merely by allowing us to
bring it nearer to the eye.
[Illustration: PLATE XXIII.]
_Mrs. B._ Those lenses, therefore, which have the shortest focus will
magnify the object most, because they enable us to place it nearest to
the eye.
_Emily._ But a lens, that has the shortest focus, is most bulging or
convex; and the protuberance of the lens will prevent the eye from
approaching very near to the object.
_Mrs. B._ This is remedied by making the lens extremely small: it may
then be spherical without occupying much space, and thus unite the
advantages of a short focus, and of allowing the eye to approach the
object.
There is a mode of magnifying objects, without the use of a lens: if you
look through a hole, not larger than a small pin, you may place a minute
object near to the eye, and it will be distinct, and greatly enlarged.
This piece of tin has been perforated for the purpose; place it close to
your eye, and this small print before it.
_Caroline._ Astonishing! the letters appear ten times as large as they
do without it: I cannot conceive how this effect is produced.
_Mrs. B._ The smallness of the hole, prevents the entrance into the eye,
of those parts of every pencil of rays which diverge much; so that,
notwithstanding the nearness of the object, those rays from it, which
enter the eye, are nearly parallel, and are, therefore, brought to a
focus by the humours of the eye.
_Caroline._ We have a microscope at home, which is a much more
complicated instrument than that you have described.
_Mrs. B._ It is a double microscope, (fig. 6.) in which you see, not the
object A B, but a magnified image of it, _a b_. In this microscope, two
lenses are employed; the one, L M, for the purpose of magnifying the
object, is called the object-glass, the other, N O, acts on the
principle of the single microscope, and is called the eye-glass.
There is another kind of microscope, called the solar microscope, which
is the most wonderful from its great magnifying power: in this we also
view an image formed by a lens, not the object itself. As the sun
shines, I can show you the effect of this microscope; but for this
purpose, we must close the shutters, and admit only a small portion of
light, through the hole in the window-shutter, which we used for the
camera obscura. We shall now place the object A B, (plate 23, fig. 1.)
which is a small insect, before the lens C D, and nearly at its focus:
the image E F, will then be represented on the opposite wall, in the
same manner, as the landscape was in the camera obscura; with this
difference, that it will be magnified, instead of being diminished. I
shall leave you to account for this, by examining the figure.
_Emily._ I see it at once. The image E F is magnified, because it is
farther from the lens, than the object A B; while the representation of
the landscape was diminished, because it was nearer the lens, than the
landscape was. A lens, then, answers the purpose equally well, either
for magnifying or diminishing objects?
_Mrs. B._ Yes: if you wish to magnify the image, you place the object
near the focus of the lens; if you wish to produce a diminished image,
you place the object at a distance from the lens, in order that the
image may be formed in, or near the focus.
_Caroline._ The magnifying power of this microscope is prodigious: but
the indistinctness of the image, for want of light, is a great
imperfection. Would it not be clearer, if the opening in the shutter
were enlarged, so as to admit more light?
_Mrs. B._ If the whole of the light admitted, does not fall upon the
object, the effect will only be to make the room lighter, and the image
consequently less distinct.
_Emily._ But could you not by means of another lens, bring a large
pencil of rays to a focus on the object, and thus concentrate upon it
the whole of the light admitted?
_Mrs. B._ Very well. We shall enlarge the opening, and place the lens X
Y (fig. 2.) in it, to converge the rays to a focus on the object A B.
There is but one thing more wanting to complete the solar microscope,
which I shall leave to Caroline's sagacity to discover.
_Caroline._ Our microscope has a small mirror attached to it, upon a
moveable joint, which can be so adjusted as to receive the sun's rays,
and reflect them upon the object: if a similar mirror were placed to
reflect light upon the lens, would it not be a means of illuminating the
object more perfectly?
_Mrs. B._ You are quite right. P Q (fig. 2.) is a small mirror, placed
on the outside of the window-shutter, which receives the incident rays S
S, and reflects them on the lens X Y. Now that we have completed the
apparatus, let us examine the mites on this piece of cheese, which I
place near the focus of the lens.
_Caroline._ Oh, how much more distinct the image now is, and how
wonderfully magnified! The mites on the cheese look like a drove of pigs
scrambling over rocks.
_Emily._ I never saw any thing so curious. Now, an immense piece of
cheese has fallen: one might imagine it an earthquake: some of the poor
mites must have been crushed; how fast they run--they absolutely seem to
gallop.
But this microscope can be used only for transparent objects; as the
light must pass through them, to form the image on the wall?
_Mrs. B._ Very minute objects, such as are viewed in a microscope, are
generally transparent, but when opaque objects are to be exhibited, a
mirror M N (fig. 3.) is used to reflect the light on the side of the
object next the wall: the image is then formed by light reflected from
the object, instead of being transmitted through it.
_Emily._ Pray, is not a magic lanthorn constructed on the same
principles?
_Mrs. B._ Yes, with this difference; the objects to be magnified, are
painted upon pieces of glass, and the light is supplied by a lamp,
instead of the sun.
The microscope is an excellent invention to enable us to see and
distinguish objects, which are too small to be visible to the naked eye.
But there are objects, which, though not really small, appear so to us,
from their distance; to these, we cannot apply the same remedy; for when
a house is so far distant, as to be seen under the same angle as a mite
which is close to us, the effect produced on the retina is the same: the
angle it subtends is not large enough for it to form a distinct image on
the retina.
_Emily._ Since it is impossible, in this case, to make the object
approach the eye, cannot we by means of a lens bring an image of it,
nearer to us?
_Mrs. B._ Yes; but then the object being very distant from the focus of
the lens, the image would be too small to be visible to the naked eye.
_Emily._ Then, why not look at the image through another lens, which
will act as a microscope, enable us to bring the image close to the eye,
and thus render it visible?
_Mrs. B._ Very well, Emily; I congratulate you on having invented a
telescope. In figure 4, the lens C D, forms an image E F, of the object
A B; and the lens X Y, serves the purpose of magnifying that image; and
this is all that is required in a common refracting telescope.
_Emily._ But in fig. 4, the image is not inverted on the retina, as
objects usually are: it should therefore appear to us inverted; and that
is not the case in the telescopes I have looked through.
_Mrs. B._ When it is necessary to represent the image erect, two other
lenses are required; by which means a second image is formed, the
reverse of the first, and consequently upright. These additional glasses
are used to view terrestrial objects; for no inconvenience arises from
seeing the celestial bodies inverted.
_Emily._ The difference between a microscope and a telescope, seems to
be this:--a microscope produces a magnified image, because the object is
nearest the lens; and a telescope produces a diminished image, because
the object is furthest from the lens.
_Mrs. B._ Your observation applies only to the lens C D, or
object-glass, which serves to bring an image of the object nearer the
eye; for the lens X Y, or eye-glass, is, in fact, a microscope, as its
purpose is to magnify the image.
When a very great magnifying power is required, telescopes are
constructed with concave mirrors, instead of lenses. These are called
reflecting telescopes, because the image is reflected by metallic
mirrors. Concave mirrors, you know, produce by reflection, an effect
similar to that of convex lenses, by refraction. In reflecting
telescopes, therefore, mirrors are used in order to bring the image
nearer the eye; and a lens, or eye-glass, the same as in the refracting
telescope, to magnify the image.
The advantage of the reflecting telescope is, that mirrors whose focus
is six feet, will magnify as much as lenses of a hundred feet: an
instrument of this kind may, therefore, possess a high magnifying power,
and yet be so short, as to be readily managed.
_Caroline._ But I thought it was the eye-glass only which magnified the
image; and that the other lens, served to bring a diminished image
nearer to the eye.
_Mrs. B._ The image is diminished in comparison with the object, it is
true; but it is magnified, if you compare it to the dimensions of which
it would appear without the intervention of any optical instrument; and
this magnifying power is greater in reflecting, than in refracting
telescopes.
We must now bring our observations to a conclusion, for I have
communicated to you the whole of my very limited stock of knowledge of
Natural Philosophy. If it enable you to make further progress in that
science, my wishes will be satisfied; but remember, in order that the
study of nature may be productive of happiness, it must lead to an
entire confidence in the wisdom and goodness of its bounteous Author.
Questions
1. (Pg. 195) What is the form of the body of the eye? fig. 1, plate 21.
2. (Pg. 195) What is its external coat called?
3. (Pg. 195) What is the transparent part of this coat denominated?
4. (Pg. 195) What is the second coat named?
5. (Pg. 195) What opening is there in this?
6. (Pg. 195) What is the coloured part which surrounds the pupil?
7. (Pg. 195) The pupils dilate and contract, what purpose does this
answer?
8. (Pg. 196) How could you observe the dilatation and contraction of the
pupils?
9. (Pg. 196) What purpose is the choroid said to answer?
10. (Pg. 196) In what animals is the change in the iris greatest?
11. (Pg. 196) What are the three humours denominated, and how are they
situated?
12. (Pg. 197) What is the part represented at _i i_, and of what does it
consist?
13. (Pg. 197) What are the respective uses of the humours, and of the
retina?
14. (Pg. 197) Why is it necessary the rays should be refracted?
15. (Pg. 197) How is this illustrated by fig. 3 and 4, plate 21?
16. (Pg. 198) What causes a person to be short-sighted? fig. 5, plate
21.
17. (Pg. 198) Why does placing an object near the eye, enable such, to
see distinctly? fig. 6.
18. (Pg. 199) A concave lens remedies this defect; how? fig. 1, plate
22.
19. (Pg. 199) What is the remedy, when a person is long-sighted? fig. 2.
20. (Pg. 199) Why does holding an object far from the eye, help such
persons? fig. 3.
21. (Pg. 200) How is the eye said to adapt itself to distant, and to
near objects?
22. (Pg. 200) Why are objects rendered indistinct, when placed very near
to the eye? fig. 4, plate 22.
23. (Pg. 200) What is the single microscope, fig. 5, and how does it
magnify objects?
24. (Pg. 201) How may objects be magnified without the aid of a lens?
25. (Pg. 201) Why can an object, very near to the eye, be distinctly
seen, when viewed through a small hole?
26. (Pg. 201) Describe the double microscope, as represented in fig. 6,
plate 22.
27. (Pg. 202) How does the solar microscope, (fig. 1 plate 23.) operate?
28. (Pg. 202) Why may minute objects be greatly magnified by this
instrument?
29. (Pg. 202) In its more perfect form it has other appendages, as seen
in fig. 2, what are they? and what their uses?
30. (Pg. 203) What is added when opaque objects are to be viewed? fig.
3.
31. (Pg. 203) In what does the magic lanthorn differ from the solar
microscope?
32. (Pg. 203) What are the use and structure of the telescope, as shown
in fig. 4?
33. (Pg. 204) When terrestrial objects are to be viewed, why are two
additional lenses employed?
34. (Pg. 204) What part of the telescope performs the part of a
microscope?
35. (Pg. 204) In what does the reflecting, differ from the refracting
telescope?
36. (Pg. 204) What advantages, do reflecting, possess over refracting
telescopes?
GLOSSARY.
ACCELERATED MOTION. Motion is said to be accelerated, when the velocity
is continually increasing.
ACCIDENTAL PROPERTIES. Those properties of bodies which are liable to
change, as colour, form, &c.
ACUTE.--See ANGLE.
AIR. An elastic fluid. The atmosphere which surrounds the earth, is
generally understood by this term, but there are many kinds of air. The
term is synonymous with _Gas_.
AIR PUMP. An instrument by which vessels may be exhausted of air.
ALTITUDE. The height in degrees of the sun, or any heavenly body, above
the horizon.
ANGLE. The space contained between two lines inclined to each other, and
which meet in a point. Angles are measured in degrees, upon a segment of
a circle described by placing one leg of a pair of compasses on the
angular point, and with the other, describing the segment between the
two lines. If the segment be exactly 1-4th of a circle, it is called a
_right_ angle, and contains 90 deg. If more than 1-4th of a circle, it
is an _obtuse_ angle. If less, an _acute_ angle. See plate 2.
ANGLE OF INCIDENCE, is the space contained between a ray which falls
obliquely upon a body, and a line perpendicular to the surface of the
body, at the point where the ray falls.
ANGLE OF REFLECTION. The space contained between a reflected ray, and a
line perpendicular to the reflecting point.
ANGLE OF VISION, or visual angle. The space contained between lines
drawn from the extreme parts of any object, and meeting in the eye.
ANTARCTIC CIRCLE. A circle extending round the south pole, at the
distance of 23 1-2 degrees from it. The same as the south frigid zone.
APHELION. That part of the orbit of a planet, in which its distance from
the sun is the greatest.
AREA. The surface enclosed between the lines which form the boundary of
any figure, whether regular or irregular.
ARIES. See SIGN.
ASTEROIDS. The name given to the four small planets, Ceres, Juno,
Pallas, and Vesta.
ASTRONOMY. The science which treats of the motion and other phenomena of
the sun, the planets, the stars, and the other heavenly bodies.
ATMOSPHERE. The air which surrounds the earth, extending to an unknown
height. Wind is this air in motion.
ATTRACTION. A tendency in bodies to approach each other, and to exist in
contact.
ATTRACTION OF COHESION. That attraction which causes matter to remain in
masses, preventing them from falling into powder. For this attraction to
exist, the particles must be contiguous.
ATTRACTION OF GRAVITATION. By this attraction, masses of matter, placed
at a distance, have a tendency to approach each other. Attraction is
mutual between the sun and the planets.
AXIS OF THE EARTH, OR OF ANY OF THE PLANETS. An imaginary line passing
through their centres, and terminating at their poles; round this their
diurnal revolutions are performed.
AXIS OF MOTION. The imaginary line, around which all the parts of a body
revolve, when it has a spinning motion.
AXIS OF A LENS, OR MIRROR. A line passing through the centre of a lens,
or mirror, in a direction perpendicular to its surface.
BALLOON. Any hollow globe. The term is generally applied to those which
are made to ascend in the air.
BAROMETER. Commonly called a weather-glass. It has a glass tube,
containing quicksilver, which by rising and falling, indicates any
change in the pressure of the atmosphere, and thus frequently warns us
of changes in the weather.
BODY. The same as _Matter_. It may exist in the solid, liquid, or
aeriform state; and includes every thing with which we become acquainted
by the aid of the senses.
BURNING-GLASS, OR MIRROR. A lens, or a mirror, by which the rays of
light, and heat, are brought to a focus, so as to set bodies on fire.
CAMERA OBSCURA, a darkened room; or more frequently a box, admitting
light by one opening, where a lens is placed; which, bringing the rays
of light, from external objects, to a focus, presents a perfect picture
of them, in miniature.
CAPILLARY TUBES. Tubes, the bore of which is very small. Glass tubes are
usually employed, to show the phenomenon of _capillary attraction_.
Fluids in which they are immersed, rise in such tubes above the level of
that in the containing vessel.
CENTRE OF A CIRCLE. A point, equally distant from every part of its
circumference.
CENTRE OF GRAVITY. That point within a body, to which all its particles
tend, and around which they exactly balance each other. A system of
bodies, as the planets, may have a common centre of gravity, around
which they revolve in their orbits; whilst each, like the earth, has its
particular centre of gravity within itself.
CENTRE OF MOTION. That point about which the parts of a revolving body
move, which point is, itself, considered as in a state of rest.
CENTRE OF MAGNITUDE. The middle point of any body. Suppose a globe, one
side of which is formed of lead, and the other of wood, the centres of
magnitude and of gravity, would not be in the same points.
CENTRAL FORCES. Those which either impel a body towards, or from, a
centre of motion.
CENTRIFUGAL. That which gives a tendency to fly from a centre.
CENTRIPETAL. That which impels a body, towards a centre.
CIRCLE. A figure; the periphery, or circumference of which, is every
where equally distant, from the point, called its centre.
CIRCLE, GREAT. On the globe, or earth, is one that divides it into two
equal parts, or hemispheres. The equator, and meridian lines, are great
circles.
CIRCLE, LESSER. Those which divide the globe into unequal parts. The
tropical, arctic and antarctic circles, and all parallels of latitude,
are lesser circles.
CIRCUMFERENCE. The boundary line of any surface, as that which surrounds
the centre of a circle; the four sides of a square, &c.
COMETS. Bodies which revolve round the sun, in very long ovals,
approaching him very nearly in their perihelion, but in their aphelion,
passing to a distance immeasurably great.
COHESION. See ATTRACTION.
COMPRESSIBLE. Capable of being forced into a smaller space.
CONCAVE. Hollowed out; the inner surface of a watch-glass is concave,
and may represent the form of a _concave mirror_, or _lens_.
CONVEX. Projecting, or bulging out, as the exterior surface of a
watch-glass, which may represent the form of a _convex mirror_, or
_lens_.
CONE. A body somewhat resembling a sugar-loaf; that is, having a round
base, and sloping at the sides, until it terminates in a point.
CONJUNCTION. When three of the heavenly bodies are in a straight or
right line, if you take either of the extreme bodies, the other two are
in conjunction with it; because a straight line drawn from it, might
pass through the centres of both, and join them together. At the time of
new moon, the moon and sun are in conjunction with the earth; and the
moon and earth, are in conjunction with the sun.
CONSTELLATION, OR SIGN. A collection of stars. Astronomers have imagined
pictures drawn in the heavens, so as to embrace a number of contiguous
stars, and have named the group after the animal, or other article
supposed to be drawn; an individual star is generally designated by its
fancied location; as upon the ear of _Leo_, the Lion, &c.
CONVERGENT RAYS, are those which approach each other, so as eventually
to meet in the same point.
CRYSTALS. Bodies of a regular form, having flat surfaces, and well
defined angles. Nitre, and other salts, are familiar examples. Many
masses of matter, are composed of crystals too minute to be discerned
without glasses.
CURVILINEAR, consisting of a line which is not straight, as a portion of
a circle, of an oval, or any curved line.
CYLINDER. A body in the form of a roller, having flat circular ends, and
being of equal diameter throughout.
DEGREE. If a circle of any size be divided into 360 equal parts, each of
these parts is called a degree. One quarter of a circle contains ninety
degrees; one twelfth of a circle, thirty degrees. The actual length of a
degree, must depend upon the size of the circle. A degree upon the
equator, upon a meridian, or any great circle of the earth, is equal to
69-1/2 miles.
Straight lines are sometimes divided into equal parts, called degrees;
but these divisions are arbitrary, bearing no relationship to the
degrees upon a circle.
DENSITY. Closeness of texture. When two bodies are equal in bulk, that
which weighs the most, has the greatest density.
DIAGONAL. A line drawn so as to connect two remote angles of a square,
or other four-sided figure.
DILATATION. The act of increasing in size. Bodies in general, dilate
when heated, and contract by cooling.
DISCORD. When the vibrations of the air, produced by two musical tones,
do not bear a certain ratio to each other, a jarring sound is produced,
which is called discord.
DIVERGENT RAYS. Those which proceed from the same point, but are
continually receding from each other.
DIVISIBILITY. Capability of being divided, or of having the parts
separated from each other. This is called one of the _essential
properties_ of matter; because, however minute the particles may be,
they must still contain as many halves, quarters, &c. as the largest
mass of matter.
ECHO. A sound reflected back, by some substance, so situated as to
produce this effect.