G. P. (George Payn) Quackenbos.

A natural philosphy: embracing the most recent discoveries in the various branches of physics .. online

. (page 26 of 42)
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minute bodies at distances which render them invisible to ordinary sight.

697. ADAPTATION OF THE EYE. One of the most re-
markable properties of the eye is its power of adapting
itself to different intensities of light and different dis-
tances. The pupil, by expanding and contracting, regu-
lates in a measure the supply of light ; still, the difference
of intensity in the light admitted to the eye under different

Visual angles of the arrows in Fig. 255. On what does the apparent size of an object
depend? Illustrate this with the Figure. When does an object become invisible?
"When is a bird said to go out of sight f C96. In the case of familiar objects, what pre-
vents us from being misled as to their size ? Give some familiar examples. What
color must an object be, to be distinguished at the greatest distance ? How is an ob-
ject most distinctly brought out? What is said of dark-colored eyes? COT. What ia
12



266 OPTICS.

circumstances is very great. We can read by the light of
the moon and by that of the sun ; yet the latter is 800,000
times as intense as the former.

698. Again, the eye adapts itself to different distances.
If we look at a remote object through a telescope, we have
to pull out the tube to a certain length, according to the
distance, before we can see it to advantage. No such arti-
ficial adjustment is necessary with the eye. We look suc-
cessively at objects 1, 5, 10, and 20 feet off; and in each
case the eye instantly adapts itself to the distance, and we
see without an effort.

699. An object may move with such velocity that we
can not see it, as is the case with a cannon-ball. This is
because the image formed on the retina does not remain
sufficiently long to produce an impression. When an image
is once formed, it remains from one-sixth to one-third of a
second after the object has disappeared. Hence a burning
stick whirled rapidly round seems to form a circle of fire,
and a meteor or a flash of lightning, instead of appearing
in a succession of luminous points, produces a continuous
train of light in the heavens.

Optical Instruments.

TOO. Several of the more important optical instruments
remain to be described. They are for the most part com-
binations of the different lenses and mirrors already men-
tioned.

701. THE CAMERA OBSCURA. "We have seen that, when
rays from an object brilliantly illuminated are admitted
through an aperture into a dark room, an inverted image
is formed. This image is apt to be indistinct. We may
give it a sharper outline by placing a double convex lens in
the aperture, and receiving the image on a white ground

one of the most remarkable properties of the eye ? Give an example of the differ-
ence of intensity in the light admitted to the eye. 69S. Show how the eye adapts
itself to different distances. 699. Why is it that an object moving with very great
velocity is not seen ? When an image is once formed, how long docs it remain after
the object has disappeared? Give examples. 700. Of what are optical instruments
for the n^ost part combination,? ? 701. What ia meant by the Camera Obscura ? How



THE CAMERA OBSCUKA.



2G7



at its focus. Such an arrangement is called the Camera
Obscura, or dark chamber.

For practical purposes, the camera obscura must be
portable. A close box, painted black on the inside, is
therefore substituted for the darkened room. This instru-
ment "enables the draughtsman to sketch material objects
or natural scenery with great ease and accuracy, and is in-
dispensable to the daguerreotypist and photographer.



Fig. 25G.




DRAUGHTSMAN'S CAMERA.



702. DraugJitsman's Camera. Fig. 256
represents the camera as used by draughts-
men. To be conveniently traced, the image
must be thrown pn a horizontal surface, and
this is effected by making the opening in the
top of the box and receiving the rays on a
mirror, A, inclined at an angle of forty-five
degrees. From this mirror they are reflect-
ed to a meniscus, B, which crosses the aper-
ture, and are by it refracted to the horizontal
surface, C D, where, on white paper placed to
receive it, is formed a distinct image, which
can be readily traced with a pencil. The up-
per part of the draughtsman's person is ad-
mitted through an opening in the side of the
box, over which a dark curtain must be
drawn, so as to exclude all light except what enters from above.

Fig. 257. 703. Daguerreotypist' s Camera. As used in

the process of taking daguerreotypes and photo-
graphs, the camera has the form shown in Fig.
257. A is a brass sliding tube,
containing two achromatic dou-
ble convex lenses,which is drawn
out far enough to
bring the focus at
the right spot. The
image is received
on a piece of
ground glass, fit-
ted into a frame,
which slides in a
groove in the back
of the camera.
DAGUEHUEOTYPIST'S CAMERA. When a daguerrc-

is the camera made portable ? Ey -whom is the camera used ? 702. Describe tho
draughtsman's camera. 703. Describe the (laguerreotypist's camera. How is th.o plato




268 OPTICS.

otype is to be taken, the ground glass is withdrawn, and another frame, C,
containing a prepared plate, carefully shielded from the light, is introduced
in its place. A door in front of C is then raised, and the image formed by
the lenses is thus allowed to fall on the plate.

The plate is of copper, covered on one side with a thin sheet of silver,
which is rendered sensitive by exposure to the vapor of iodine. The rays
transmitted through the camera, by that property inherent in ther5 which
we have called actinism, in a few seconds produce a chemical effect on the
sensitive surface, and the plate is then removed to a dark room. No change
is visible on its surface ; but, as soon as it is exposed to the vapor of mercu-
ry, the picture begins to appear and soon becomes distinct. It is produced
by the adhesion of small globules of mercury to those parts of the plate that
have been affected by light, to the exclusion of the rest ; and this adhesion is
owing to some chemical change in the parts so affected. After being washed
in a weak solution of hyposulphite of soda, and then in water, the plate is
allowed to dry, and the image is fixed.

The photographic process is similar, except that the image is received on
paper rendered sensitive by different preparations, instead of on a metallic
plate.

704. THE MICROSCOPE. The Microscope is an instru-
ment which enables us to see objects too small to be dis-
cerned by the naked eye. This is the case with objects
whose visual angle is less than -^^ of one degree ; the mi-
croscope enables us to see them by increasing their visual
angle.

Microscopes are divided into two classes, Single and
Compound. A Single Microscope is one through which
the object is viewed directly. With the Compound Mi-
croscope a magnified image of the object is viewed, instead
of the object itself.

705. The Single Microscope. The single microscope
consists of a double convex lens (or sometimes more than
one), through which we look at the object to be mag-
nified. The principle on which it operates is shown in
Fig. 258.

The arrow b c would be seen by the naked eye under the visual angle
b A c. When the lens m is interposed, the rays are so refracted as to form

prepared ? Give an account of the daguerreotype process. How does the photo-
graphic process differ from it ? 70-1. What is the Microscope ? How does it enable
us to seo minute objects? Name tho classes into which microscopes are divided
What is a Single Microscope ? What is a Compound Microscope ? T05. Of what does
Hie single microscope consist? With Fig. 253, explain the principle on which tho



THE MICROSCOPE.



269




the visual angle DAE, and the arrow n . Fi S- 258 -

appears to be of the size D E, much
larger than it really is. Sometimes
an exceedingly minute object becomes
visible when brought very near the
eye, but in that position the rays en-
ter the eye with such divergency that
a confused image is produced. The
microscope corrects this excessive divergency, and presents a clear and mag-
nified image.

706. The Compound Microscope. The compound mi-
croscope is a combination of two, three, or four convex
lenses, through which we view a magnified image of an
object instead of the object itself. The lenses are fixed in
tubes moving one within the other, and suitable apparatus
is provided for adjusting them, for holding the object un-
der examination, and throwing on it a strong light. When
but two lenses are employed, they are arranged as repre-
sented in Fig. 259.




D E is the object, and B, the lens nearest to it, is called the object-glass.
C, the lens nearest the eye, is called the eye-glass. A magnified image of the
arrow is formed at II I by the lens B. This image is viewed through the
lens C, and is thus still further magnified, being seen under an increased
visual angle at F G. If the magnifying power of B is 20, and that of C 4, the
image seen will be SO times the size of life.

707. Solar and Oxy-Jiydroyen Microscopes. These mi-
croscopes are used for throwing magnified images on a
white screen in a darkened room.



single microscope operates. TOG. Describe the compound microscope. With the aid
of Fig. 259, name the parts and show the operation of the compound microscope.
707. For what are the Solar and the Oxy-hydrogen Microscope used ? Describe tho



270 OPTICS.

In the case of the Solar Microscope, an aperture is made
in one of the shutters. Outside of this a mirror is placed,
in the sun, at such an angle as to reflect the rays that fall
on it through a horizontal tube towards the object to be
magnified. They first fall on a convex lens, and then on a
second, which brings them to a focus on the object, and
thus illuminates it brilliantly. Another lens, at the oppo-
site extremity of the instrument, produces the magnifying
effect. A screen, from ten to twenty feet off, receives the
image, which increases in size with the distance. If the
screen is too far removed, the image becomes faint ; but
so powerful is the light concentrated on the object that a
very great magnifying effect may be produced without any
lack of distinctness.

In the Oxy-hydrogen Microscope, the principle is the same, but the bril-
liant light produced by burning lime in a current of oxygen and hydrogen is
substituted for the rays of the sun. Accordingly, with this instrument, the
aperture in the shutter and the mirror on the outside are unnecessary. Fig.
2GO shows the operation of the oxy-hydrogen microscope.

Fig. 260. B represents an intense

white light produced by the
_ ^ burning of a cylinder of lime
^ in a current of oxygen and hy-
drogen combined. This light
falls on the reflector A, by
which it is thrown back on the double convex lens C, and this brings it to a
focus on the object D. E is an achromatic lens, which throws a magnified
image on the screen.

708. The microscope introduces us to new worlds, of the very existence
of which we would otherwise have been ignorant. It reveals to us, in every
drop of water in which vegetable matter has been infused, swarming myriads
of moving creatures, miniature eels, infinitesimal lobsters, ravenous mon-
sters with distended jaws preying on their feebler fellows, all endowed with
the organs of life, and so minute that their little drop is to them a world nearly
as large as ours to us. It shows us the feeding apparatus of the flea magni-
fied to frightful dimensions, and his body arrayed in a panoply of shining
and curiously jointed scales, studded at intervals with long spikes. The
mould on decaying fruit it magnifies into bushes with branches and leaves,

solar microscope, and its operation. What is the effect of removing the screen to a
greater distance from the instrument ? What light is employed in the oxy-hydrogen
microscope ? With Fig. 260, show how this microscope operates. 708. What is said
f the revelations of the microscope ? What difference does it exhibit between th




THE MAGIC LANTERN.



271



displaying all the regularity and beauty of the vegetable creation. It dis-
closes to us many striking facts connected with physiology and chemistry.
It shows us the imperfection of the finest works of art, when compared with
those of nature; The edge of the sharpest razor, viewed through a micro-
scope, is full of notches ; the point of a needle is blunt, and its surface is cov-
ered with inequalities. The magnified sting of a bee, on the other hand, is
perfectly smooth, regular, and pointed. The finest thread of cotton, linen,
or silk, is rough and jagged : whereas in the filament of a spider's web not the
slightest irregularity can be detected. In a word, the revelations of the mi-
croscope are in the highest degree wonderful and interesting ; and, to what-
ever we direct it, we always find abundant matter to reward our labor and
stimulate us to further researches.

709. THE MAGIC LAOTEEN. The Magic Lantern is an
instrument for throwing on a screen magnified images of
transparent objects. It operates on the same principle as
the oxy-hydrogen microscope, but for its illuminating power
has an ordinary lamp instead of the intense light produced
by burning liine.

Fig. 261.




THE MAGIC LAKTXBK.



Fig. 2G1 represents the magic lantern. L is the lamp. M N is the re-
flector, which throws the light on the lens A. This lens brings it to a focus
on the picture, which is painted on a glass slider and introduced into the
opening C D. The lens B receives the rays from the slider, and throws a
magnified image on the screen F.

710. Phantasmagoria. When a powerful light is used,
and the tube containing the magnifying lens or lenses is
capable of being drawn out or pushed in, so as to bring
them at different distances from the object, we have what is
called a Phantasmagoria Lantern.



works of art and those of nature ? 709. What is the Magic Lantern ? How does it
differ from the oxy-hydrogen microscope? With Fig. 261, describe tho magic Ian-



272 OPTICS.

To exhibit the Phantasmagoria, a transparent screen is suspended, on one
side of which is the exhibitor with his lantern, on the other the spectators.
Having brought the lantern close to the screen and drawn out the tube till
the image (which will be quite small) is perfect, the exhibitor walks slowly
back. He thus gradually increases the size of the image, while he preserves
its distinctness by pushing in the tube as he recedes. The effect on the
spectators is startling. The room being dark, they can not see the screen,
but only the illuminated image, which, as it grows larger, appears to be
moving towards them ; even those who are familiar with the instrument can
hardly disabuse their minds of this impression. When the exhibitor ap-
proaches the screen and pulls out the tube, the image becomes smaller and
appears to recede.

711. Dissolving Views. Dissolving Views, in which
one picture appears to melt into another, are produced by
two magic lanterns, inclined so as to throw their images on
the same spot. An opaque shade is made to revolve in
front of the instruments, in such a way as gradually to in-
tercept the rays from one and uncover the tube of the other.
The first picture fades, and a new one takes its place, be-
coming more and more distinct as the other disappears.

712. THE TELESCOPE. The Telescope is an instrument
for viewing distant objects. It appears to have been in-
vented by Metius, a native of Holland, in 1608. The fol-
lowing year, Galileo, hearing of the new instrument, con-
structed one for himself, and was the first to make a
practical use of the invention. To the Telescope, Astron-
omy is indebted for the important advances it has made
during the last two centuries.

Telescopes are of two kinds, Refracting and Reflecting.
In the former, which were the first constructed, lenses are
used ; in the latter, polished metallic mirrors.

713. Refracting Telescopes. The simplest form of the
telescope is that devised by Galileo. It is a tube contain-
ing a convex object-glass and a concave eye-glass. By the
former parallel pencils are made to converge towards a
focus, where they would form an inverted image ; but be-

tcrn. 710. What is the Phantasmagoria Lantern ? How are the phantasmagoria pro-
duced ? What is said of their effect ? 711. What are Dissolving Views ? How are
they produced ? 712. What is the Telescope? By whom was it invented? Who
first made a practical use of the invention? Name the two kinds of telescopes.



THE TELESCOPE. 273

fore reaching the focus they fall on the concave lens, and
have their convergeney so far corrected that an object is
distinctly seen by an eye at the extremity of the tube. The
Opera-glass consists of two Galilean Telescopes combined.
The night-glass used by sailors is on the same plan.

In the instrument called the Astronomical Telescope, both object-glass and
eye-glass are convex. The former produces an inverted image at its focus ;
the latter, which is so placed that its focus falls at the same spot, refracts the
rajs diverging from this image, and thus renders it visible to the eye. The
inversion of the image is of no consequence in observing the heavenly bodies ;
but, when objects on the earth are viewed, we want an erect image, and there-
fore in the Terrestrial Telescope two additional lenses are introduced to cor-
rect the inversion.

714. Reflecting Telescopes. In Reflecting Telescopes,
a speculum, or mirror, takes the place of the object-glass.
These instruments appear in several different forms. The
principle on which Herschel's is constructed, will be under-
stood from Fig. 262.

The mirror SS is Fig. 262.

placed at the farthest
extremity of the tube,
inclined so as to make
the rays that fall upon
it converge towards the
side of the tube in which
the eye-piece a b is fixed to receive them. The observer at E, with his back
towards the heavenly body, looks through the eye-piece, and sees the reflect-
ed image. His position is such as not to prevent the rays from entering the
open end of the tube. The advantage gained with this instrument depends
in a great measure on the size of the mirror ; for all the rays that fall on it
are concentrated and transmitted to the eye.

715. The largest telescope ever constructed was made by the Earl of Rosse.
The great mirror is six feet in diameter, and weighs four tons. The tube, at
the bottom of which it is placed, is of wood hooped with iron. It is fifty-two
feet long and seven feet across. It is computed that with this instrument
250,000 times as much light from a heavenly body is collected and transmit-
ted to the eye as ordinarily reaches it.

713. Describe the Galilean Telescope. Of what does the Opera-glass consist ? De-
scribe the Astronomical Telescope. How does the Terrestrial Telescope differ from
the Astronomical ? 714 In reflecting telescopes, what takes the place of the object-
glass ? "With Fig. 262, explain the principle on which Herschers Telescope operates.
On what does the advantage gained with this instrument depend ? 715. Describe tha
telescope of tho Earl of Eosse. IIow great is tlio advantage gained with it ?




274 OPTICS.



EXAMPLES FOR PRACTICE.

1. (See 594.) How long does it take a ray from the moon to reach the earth,

the moon's distance being 240,000 miles ?

2. The planet Jupiter is 496,000,000 miles from the sun. How long does it

take a ray of light from the sun to reach the planet ?

3. A ray of light from the sun is about 12,326 seconds longer in reaching the

newly discovered planet Neptune than in reaching Jupiter. About how
many miles farther from the sun is Neptune than Jupiter ?

4. (See 595.) A holds his book 1 foot, and B holds his 3 feet, from a certain

candle. How much more light does A receive than B ?

5. The planet Uranus is twice as far from the sun as the planet Saturn.

How does the light received at Saturn compare in intensity with that re-
ceived at Uranus ?

6. (See 650.) How many times is the ordinary heat of the sun increased by

a burning glass with an area of 10 square inches, the focus of which has
an area of ^o of a square inch ?

7. A convex lens has a focus 1 / 5 of a square inch in area, and increases the

heat of ordinary sun-light 200 times; what is the area of the lens?



CHAPTER XV.

ACOUSTICS.

71G. ACOUSTICS is the science that treats of sound.

717. NATURE AND ORIGIN OF SOUND. Sound is an im-
pression made on the organs of hearing by the vibrations
of elastic bodies, transmitted through the air or some other
medium. These vibrations may be compared to the mi-
nute waves which ripple the surface of a pond when a stone
is thrown in, spreading out from a centre, but growing
smaller and smaller as they recede, till finally they are no
longer perceptible. They are produced by percussion, or
any shock which gives an impulse to the particles of the
sounding body. There is no sound that can not be traced
to mechanical action.

718. Bodies whose vibrations produce clear and regular



SOUND PRODUCED BY VIBRATIONS. 275

Bounds are called Sonorous. Bell-metal, glass, the head of
a drum, are sonorous.

719. That sound is produced by vibrations is proved in various ways. A
person standing near a piano-forte or an organ, when it is played, feels a
tremulous motion in the floor of the apartment, as well as in the instru-
ment itself if he touches it. We perceive the same tremor in a bell when
in the act of being rung. In like manner, if we strike a tumbler so as to pro-
duce a sound, and then touch the top, we feel an internal agitation ; and,
when the vibrations are stopped, as they are by contact with the finger, the
sound ceases with them. If we put water in a glass and produce a sound by
rubbing the top with the finger, the liquid is agitated, and its motion contin-
ues until the sound dies away. Place some fine sand on a square piece of
glass, and, holding it firmly with a pair of pincers, draw a violin-bow along
the edge. The sand is put in motion, and finally settles on those parts of
the glass that have the least vibratory movement. If a tuning-fork be struck
and applied to the surface of mercury, minute undulations may be observed
in the metal.

That these vibrations are communicated to the air and by it transmitted
to the ear, also admits of easy proof. The rapid passage of a heavy cart or
stage shakes the walls of a house. The discharge of artillery sometimes breaks
windows. These effects are due to the vibrations suddenly produced in the
air. If there is no air or other medium to transmit the vibrations to the ear,
no sound is heard. We have already seen ( 439) that a bell rung in an ex-
hausted receiver can hardly be heard ; if the air could be entirely removed,
it would be wholly inaudible. Sound, therefore, does not leap from point to
point, but is transmitted by vibrations communicated from one particle to
another.

720. All sonorous bodies are elastic, but all elastic bodies
are not sonorous.

Soft bodies are generally non-elastic, and consequently not sonorous.
This is the case with cotton, for example, which yields little or no sound
when struck by a hammer. It is on this account that music loses much of its
effect in rooms with tapestried walls or curtained windows. Hence, also, a
speaker finds it more difficult to make himself heard in a crowded room than
in one that is empty.

721. TRANSMISSION OF SOUND. All the sounds that or-



71C. "What is Acoustics ? 717. What is Sound ? How are sound- waves produced ?
To what is every sound traceable? 718. What bodies are called Sonorous? Give
examples. 719. How is it proved by familiar experiments that sound is produced by
vibrations? If a tuning-fork be struck and applied to the surface of mercury, what
may be observed ? How is it proved that these vibrations are communicated to the
air and by it transmitted to the ear? 720. What property belongs to all sonorous
bodies ? What bodies are, for the most part, not sonorous ? Give examples. What
follows from the fact that soft bodies are not sonorous ? 721. By what are the sounds



276 ACOUSTICS.

dinarily reach our ears are transmitted to them by the air.
Any material substance, however, that connects our organs
of hearing with a vibrating body, may transmit the vibra-
tions in the same way. Thus, with our heads immersed in
water, we can hear a sound produced under the surface at
a considerable distance. Here water is the transmitting
medium.

722. Liquids are better conductors of sound than aeri-
form bodies, and solids than liquids.

Persons in boats can converse with each other at a great distance, be-
cause water is a good conductor of sound. When the ear is applied to one



Online LibraryG. P. (George Payn) QuackenbosA natural philosphy: embracing the most recent discoveries in the various branches of physics .. → online text (page 26 of 42)