Copyright
Richard Green Parker.

A school compendium of natural and experimental philosophy : embracing the elementary principles of mechanics, hydrostatics, hydraulics, pneumatics, acoustics, pyronomics, optics, electricity, galvanism, magnetism, electro-magnetism, magneto-electricity, astronomy : containing also a description of online

. (page 22 of 38)
Online LibraryRichard Green ParkerA school compendium of natural and experimental philosophy : embracing the elementary principles of mechanics, hydrostatics, hydraulics, pneumatics, acoustics, pyronomics, optics, electricity, galvanism, magnetism, electro-magnetism, magneto-electricity, astronomy : containing also a description of → online text (page 22 of 38)
Font size
QR-code for this ebook


* Mr. John A. Whipple, of this city, has given several exhibition!* of
this kind, with great success. A summer scene seemed to dissolve into tlie
same scene in mid-winter ; a daylight view was gradually made to faint
successively into twilight and moonshine; and many changes of a most in-
teresting nature showed how pleasing an exhibition might be made by o
skiiful combination of science and art



OPTICS.



scope differ j rorr. tion tc the lenses; and the image oi the
object, reflected from a >concave mirror, is
seen, instead of the object itself.

906. Each of these kinds of telescope has its respective advan-
tages, but refracting telescopes have been so much improved that
they have in some degree superseded the reflecting telescopes.

What is an ^07. Among tn e improvements which have

Achromatic Tele- been made in the telescope, may be mentioned,
sc P e - as the most important, that peculiar construc-

tion of the lenses by which they are made to give a pencil of
white light, entirely colorless. Lenses are generally faulty in
causing the object to be partly tinged with some color, which is
imperfectly refracted. The fault has been corrected by employ-
ing a double object-glass, composed of two lenses of different
refracting power, which will naturally correct each other. The
telescopes in which these are used are called Achromatic. Com-
mon telescopes have a defect arising from the convexity of the
object-glass, which, as it is increased, has a tendency to tinge
the edges of the images. To remedy this defect, achromatic
lenses were formed by the union of a convex lens of crown
glass with a concave lens of flint glass. Owing to the difference
of the refracting power of these two kinds of glass, the images
became free from color and more distinct; and hence the glasses
which produce them were called Achromatic, that is, free from
color. (See pars. 1509-1511.)

Lenses are also subject to another imperfection, called spheri-
cal aberration, arising from the different degrees of thickness
in the "/nitre and edges, which causes the rays that are refracted
through them respectively, to come to different focuses, on ac-
count of the greater or less refracting power of these parts, con-
sequent on their difference in thickness. To correct this defect,
tenses have been constructed of gems and crystals, &c., which
have a higher refractive power than glass, and require less
sphericity to produce equal effects.

What is the sim- 908 - T ^ e simplest form of the telescope coii-
pkst form of the sists of two convex lenses, so combined as to
ttkscove? increase the angle of vision under which th



248



NATURAL PIIILOSC PIIY.



Object-glass, and
which the Eye-
glass, of a iele-



object is seen. The lenses are so placed that the distance
between them may be equal to the sum of their focal distances

Which is the f\ n ^

909. The lens nearest to the eye is culled

the Eye-glass, and that at the other extrem-
ity is called the Object-glass.

910. Objects seen through telescopes of tlip
construction (namely, with two glasses only)
are always inverted, and for this .reason this
kind of instrument is principally used for as-
tronomical purposes, in which the inversion of

the object is immaterial.

What is the dif- 91L ThQ common day telescope, or spy-
ference between glass, is an instrument of the same sort, with

a day and a fo e addition of two, or even three or four
night telescope?

glasses, lor the purpose ol presenting the object

upright, increasing the field of vision, and diminishing the aber-
ration caused by the dissipation of the rays.

912. Fig. 139 represents the parts of an
Explain Fig. as t rO nomical telescope. It consists of a tube

lot/*

A B C D, containing two glasses, or lenses.
The lens A B, having a longer focus, forms the object-glass ;
the other lens D C is the eye-glass. The rays from a very



Pig. 139.



scope ,



How are objects
teen throu ^h tel-
escopes of the
simplest con-
struction *







distant body, as a star, and which may be considered parallel to
each other, are refracted by the object-glass A B to a focus at
K. The image is then seen through the eye-glass D C, magni-
fied as many times as the focal length of the eye-glass is con-
tained in the focal length of the object-glass. Thus, if the focal
length of the eye-glass D C be contained 100 times in that o f



OPTICS.



the object-glass A B, the star will be seen magniLed 100 times.
It will be seen, by the figure, that the image is inverted ; for
the ray M A, after refraction, will bs seen in the direction O,
and the ray N B in the direction D P. (See par. 1508.)

913. Fig. 140 represents a day-glass, or ter-
restrial telescope, commonly called a spy-glass.
This, likewise, consists of a tube A B H G,
containing four lenses, or glasses, namely, A B, C D, E F, and
G H. The lens A B is the object-glass, and G H the eye-glass.
The two additional eye-glasses, E F and C D, are of the same
size and shape, and placed at equal distances from each other,

fig. 140.



Explain Fig.
140.




H



in such a manner that the focus of the one meets that of the
next lens. These two eye-glasses E F and C D are introduced
for the purpose of collecting the rays proceeding from the in-
verted image M N, into a new upright image, between G H and
E F ; and the image is then seen through the last eye-glass G H,
under the angle of vision P Q. (See par. 1511.)

Opera Glasses are constructed on the prin-
era Classes * ^~ c ^ e ^ ^ e refracting telescope. They are in
fact, nothing more than two small telescopes,
united in such a manner that the eye-glasses of each may be
moved together, so as to be adjusted to the eyes of different
persons. (See par. 1512.)

Of what does the ^14. THE REFLECTING TELESCOPE. The Re-
fafating Tel- fleeting Telescope, in its simplest form, con-
escope consist? gisted of ft concave mirror and a convex

eye-glass. The mirror throws an image of the object, and the
s views that image under a larger angle of vision.



J50



NATURAL PHILOSOPHY.



This instrument was subsequently improved by Newton, and
since him by Cassegrain, Gregory, Hadley, Short, and th<
Herschels.

915. Fig. 141 represents the Gregorian
l & Telescope. It consists of a large tube, con
taining two concave metallic mirrors, and two
plano-convex eye-glass 3S. The rays from a distant object are
received through the open end of the tube, and proceed from r t

Fig. 141.



Explain
141.




> B



to r r, at the large mirror A B, which reflects them to a focus
at <7, whence they diverge to the small mirror C, which re-
flects them parallel to the eye-glass F, through a circular aper-
ure in the middle of the mirror A B. The eye-glass F col-
lects those reflected rays into a new image at I, and this image
Is seen magnified through the second eye-glass G.

It is thus seen that the mirrors bring the object near to the
eye, and the eye-glasses magnify it. Reflecting telescopes are
attended with the advantage that they have greater magnifying
power, and do not so readily decompose the light. It has
already been stated that the improvements in refractors have
given them the greater advantage. (See par. 1514.)

How does the 916. The Cassegrainian telescope differs from

iaTteSco n e that which haS been described ' in havin g tkp
differ from smaller mirror convex. This construction is at-
the Gregorian ? tended with two advantages ; first, it is superior
in distinctness of its images, and, second, it dispenses with the
necessity of so long a tube.



OPTICS. 251

917. The talescopes of Herschel and of Lord
WJiat peat-
liaritus are -Kosse dispense with the smaller mirror. This is

there in the done by a slight inclination of the large mirror, so
Herschel and as * tnrow tne i ma g e on on e side, where it is viewed
the Earl of by the eye-glass The observer sits with his back
Rosse? towards the object to be viewed. Herschel's gigan-
tic telescope was erected at Slough, near Windsor, in 1789. The
diameter of the speculum or mirror was four feet, and the mir-
ror weighed 2118 pounds ; its focal distance was forty feet. (See
par. 1514.)

918. The telescope of Lord Rosse is the largest that has ever
been constructed. The diameter of the speculum is six feet, aud
its focal distance fifty-six feet. The diameter of the tube is seven
feet, and the tube and speculum weigh more than fourteen tons.
The cost of the instrument was about $60,000.

The telescope now belonging to Harvard University is a refractor.
It is considered one of the best instruments ever constructed.

What is 919. CHROMATICS. That part of the sci-

Chromatics? ence O f Optics which relates to colors is
called Chromatics.

Of what is light 920. Light is not a simple thing in its
composed? nature, but is composed of rays of different
colors, each of which has different degrees of refrangibility,
and has also certain peculiarities with regard to reflection.

Of what color ^^' ^ ome substances reflect some of the
are bodies rays that fall upon them and absorb the others,
composed? gome appear to reflect all of them and absorb
none, while others again absorb all and reflect none. Hence,
bodies in general have no color of themselves, independent
of light, but every substance appears of tint color which it-
reflects.

What are ^^* White ^ s a ^ ue mixture of all colors in

white and nice and exact proportion. When a body re-
flects all the rays that fall upon it, it will ap-
pear white, and the purity of the whiteness depends on the
perfcc tiiess of the reflection.
11



NATURAL PHILOSOPHY.

923. Black is the deprivation of all col( r, and,

body reflects none of the rays that fall upan it, it will
appear black.

924. Some bodies reflect two or more colors either partially
or perfectly, and they therefore present the varied hues which
we perceive, formed from the mixture of rays of different
colors.^

What are the 925. The colors which enter into the composi-
2^??i f tion of light, and which possess diiferent degrees
of refrangibility, are seven in number, namely,
red, orange, yellow, green, blue, indigo, and violet.
What is a 926. A Prism is a solid, triangular piece of
Prism? highly- polished glass.

927. A prism which will answer the same purpose as a solid one
may be made of three pieces of plate glass, about six or eight inches
long and two or three broad, joined together at their edges, and
made water-tight by putty. The ends may be fitted to a triangular,
piece of wood, in one of which an aperture is made by which to till

* When the eye has become fatigued by gazing intently on any object,
of a red or of any other color, the retina loses, to some extent, its sensitive-
ness to that color, somewhat in the same manner that the ear is deafened for
a moment by an overpowering sound. If that object be removed and
another be presented to the eye, of a different color, into the composition of
which red enters, the eye, insensible to the red, will perceive the other
colors, or the compound color which they would form by the omission of the
red, and the object thus presented would appear of that color. The truth
of this remark may be easily tested. Fix the eye intently for some time on
a red wafer on a sheet of white paper. On removing the wafer, the white
disk beneath it will transmit all the colors of white ligh but the eye,
insensible to the red, will perceive the blue or green colors at the other end
of the spectrum, and the other spot where the red wafer was will appear
of a bluish-green, until the retina recovers its sensibility for red light. Th
colors thus substituted by the fatigued eye are called the accidental color.

The accidental colors of the seven prismatic colo -8, together with blacfc
and white, are as follows :

Accident il Coltr

Red . Bluish Green.

Orange Blue.

Yellow Indigo.

Green Violet reddisn.

Indigo Orange red.

Violet Orange yellow.

Black White.

White . . Bkick



OPTICS.



25?.



it with water, and thus to give it the appearance and the refractive
power of a solid prism.



928. When light is made to pass through a
prism, the different-colored rays are refracted
or separated, and form an image on a screen or
wall, in which the colors will be arranged in
the order just mentioned.

929. Fig. 142 represents rays of light passing from
f* m ' the aperture, in a window-shutter A B, through the
prism P. Instead of continuing in a straight course to E, and
there forming an image, they will be refracted, in their passage
through the prism, and form an image on the screen G D. But,

Fig. 142. A



What effect
has a prism
on the light
that passes
through it ?



Explain




as the different-colored rays have different degrees of refrangi-
bility, those which are refracted the least will fall upon the
lowest part of the screen, and those which are refracted the most
will fall upon the highest part. The red rays, therefore, suffer-
ing the smallest degree of refraction, fall on the lowest part of
the screen, and the remaining colors are arranged in the order
of their refraction. (See par. 1491.)

930. It is supposed that the red rays are refracted the least, on
account of their greater momentum ; and that the blue, indigo and
violet* are refracted the most, because they have the least momentum.
The same reason, it is supposed, will account for the red appoar-
ance of the sun through a fog, or at rising and setting. Ihe in-
creased quantity of the atmosphere which the oblique rays must
traverse, and its being loaded with mists and vapors, which are
usually formed at those times, prevents the other rays from reach-
ing us.

A similar reason will account fir the blue appearance of the ?!;/.



254 NATURAL PHILOSOPHY.

As these rays hive less momentum, they cannot traverse the atino*-

Ehere so readily as the other rays, .and they are, therefore, reflected
ack to our eyes by the atmosphere. If the atmosphere did not
reflect any rays, the skies would appear perfectly black,

931. If the colored rays which have been sepa-
How can the . r

rar* refract- rated by a pnsi: fall upon a convex lens, they

ed by a prism will converge to a focus, and appear white. Hence
it appears that white is not a simple color, but if-
produced by the union 01 several colors.

932. The spectrum formed by a glass prism being divided
in+o 360 parts, it is found that the red occupies 45 of those parts,
the orange 27, the yellow 48, the green 60, the blue 60, the
indigo 40, and the violet 80. By mixing the seven primitive
colors in these proportions, a white is obtained ; but, on account
of the impurity of all colors, it will be of a dingy hue. If the
colors were more clearly and accurately defined, the white thus
obtained would appear more pure also. An experiment to prove
what has just been said may be thus performed : Take a circular
piece of board, or card and divide it into parts by lines drawn
from the centre to the circumference. Then, having painted the
seven colors ir, the proportions above named, cause the board to
revolve rapidly around a pin or wire at the centre. The board
will then appear of a white color. From this it is inferred
that the whiteness of the sun's light arises from a due mixtiu-e
of all the primary colors. (See par. 1492.)

933. The colors of all bodies are either the simple colors, as
refracted by the prism, or such compound colors as arise from a
mixture of two or more of them. (See par. 1498.)

934. From the experiment of Dr. Wollaston,
What are the .
three simple xt appears that the seven colors formed by the prism

colors? ma y be reduced to four, namely, red, green, blue,

and violet ; and that the other colors are produced by combina-
tions of these, but violet is merely a mixture of blue and red,
and green is a mixture of blue and yellow. A better division
of the simple colors is blue, yellow, and red. (See par. 1502.)
935. Light is found to possess both heat and chemical udioa.



OPTICS. 255

The prismatic spectaim presents some remaikable phenomena with
regard to these qualities : for, while the red rays appear to be tna
seat of the maximum of heat, the violet, on the contrary, are the
apparent se,\t of the maximum of chemical action.

036. Light, from whatever source it proceeds, is of the same
nature, composed of the various-colored rays ; and although some
substances appear differently by candle-light from what they appear
by day, this rtault may be supposed to arise from the weakness or
want of purity in artificial light.

037. There can be no light without colors, and there can be no colors
without light.

938. That the above remarks in relation to the colors of bodies
are true, may be proved by the following simple experiment. Place
a colored body in a dark room, in a ray of light that has been re-
fracted by a prism ; the body, of whatever color it naturally is, will
appear of the color of the ray in which it is placed ; for, since it
receives no other colored rays, it can reflect no others.

939. Although bodies, from the arrangement of their particles,
have a tendency to absorb some rays and reflect others, they are
not so uniform in their arrangement as to reflect only pure rays of
one color, and perfectly absorb all others ; it is found, on the con-
trary, that a body reflects in great abundance the rays which deter-
mine its color, and the others in a greater or less degree in propor-
tion as they are nearer or further from its color, in the order of
refrangibility. Thus, the green leaves of a rose will reflect a few of
the red rays, which will give them a brown tinge. Deepness of
color proceeds from a deficiency rather than an abundance of reflect-
ed rays. Thus, if a body reflect only a few of the green rays, it
will appear of a dark green. The brightness and intensity of a
color shows that 8 great quantity of rays are reflected. That bodies
sometimes change -their color, is owing to some chemical change
which takes pla^e in the internal arrangement of their parts,
whereby they looe their tendency to reflect certain colors, and
acquire the power of reflecting others.

How is a rain- 940. The rainbow is produced by the re-
bow produced ? fraction of the sun's rays in their passage
through a shower of rain ; each drop of which acts as a
prism in separating the colored rays as they pass through it.

941. This is proved by the following considerations: First,
a rainbow is pcver seen except when rain is falling and the sun
shining at the same time ; and that the sun and the bow are
always in opposite parts of the- heavens ; and, secondly, that the
game appearance may be produced artificially, by means of water
thrown into the air, when the spectator fc placed in a proper



ICATUllAL PHILOSOPHY.

position, with his back to the sun ; and, thirdly, that a simitar
bow is generally produced by the spray which arises from large
cataracts or waterfalls. The Falls of Niagara afford a beautiful
exemplification of the truth of this observation. A bow is
always seen there when the sun is clear and the spectator's back
is towards the sun. (See par. 1501.)

942. As the rainbow is produced by the refraction of the sun s
rays, and every change of position is attended by a corresponding
change in the rays that reach the eye, it follows that no two persona
can see exactly the same rainbow, or, rather, the same appearance
from the same bow.

943. The Polarization of Light is a change effected during reflec-
tion or refraction, by which the etlier vibrations on one side of the
ray are stopped. (See par. 1478, Appendix.) This property of light
was first discovered by Huygens in his investigations of the cause
of double refraction, as seen in the Iceland crystal. The attention
of the scientific world was more particularly directed to it by the
discoveries of Malus, in 1810. The knowledge of this singular
property of light has afforded an explanation of several very intri-
cate phenomena in Optics, and has afforded corroborating evidence
in favor of the undulatory theory ; but the limits of this volume
will not allow an extended notice of this singular property.

9-14. OF THE THERMAL, CHEMICAL, AND OTHER NON-OPTICAL
EFFECTS OF LIGHT.* The science of Optics treats particularly of
light as the medium of vision. But there are other effects of this
agent, which, although more immediately connected with the sci-
ence of Chemistry, deserve to be noticed in this connection.

945. The thermal effects of light, that is, its agency in the excita*
tion of heat when it proceeds directly from the sun, are well known.
But it is not generally known that these effects are extremely un-
oqual in the differently Colored rays, as they are refracted by the
prism. It has already been stated that the red rays appear to
possess the thermal properties in the greatest degree, and that in the
other rays in the spectrum there is a decrease of thermal power
towards the violet, where it ceases altogether. But, on the contrary ,
that the chemical agency is the most powerful in the vi'olet, from
which it constantly decreases towards the red, where it ceases alto-
gether. Whether these thermal and chemical powers exist in all
light, from whatever source it is derived, remains yet to be ascer-
tained. The chromatic intensity of the colored spectrum is greatest
in the yellow, from whence it decreases both ways, terminating
almost abruptly in the red, and decreasing by almost imperceptible
shades towards the violet, where it becomes faint, and then wholly
indistinct. Thus it appears that the greatest heating power resides
where the chemical power is feeblest, and the greatest chemical

'* See also pars. 1492-1494.



OPTICS. 25'

jw)v\pr \vht-re tha heating power is feeblest, and tnat the optiooJ
power is the strongest between the other two.

946. The chemical properties of light are shown in this, that the
light of the sun, and in an inferior degree that of day when the sun
is hidden from view, is a means of accelerating chemical combina-
tions and decompositions. The following experiment exhibits the
chemical effects of light :

Place a mixture of equal parts (by measure) of chlorine and hy-
drogen gas in a glass vessel, and no change will happen so long as
the vessel be kept in the dark and at an ordinary temperature ; but,
on exposing it to the daylight, the elements will slowly combine
and form hydrochloric acid ; if the glass be set in the sun's rays,
the union will be accompanied with an instantaneous detonation.
The report may also be produced by transmitting ordinary dayliglif
through violet or blue glass to the mixture, but by interposing a re^
glass between the vessel and the light all combination of the elements
is prevented.

947. The chemical effects of light have recently
What is , . x . .

meant by Pho- " een employed to render permanent the images ob-

tograpky., or tained by means of convex lenses. The art of thua
Heliographytfc^ thera is terraed Photography, or Heliography.
These words are Greek derivatives ; the former meaning " writing
or draining by means of light," the latter " writing or draw-
ing by the aid of the sun." (See par. 1491.)

I/I/A 4i 948. The mode in which the process is performed

Who is the . ,. !-. c ,, m, r . , c f , ,

th f PI ls essen * ;ia ^y as follows: ihe picture, formed by a
it & h i camera obscura, is received on a plate, the surface of
which has been previously prepared so as to make it
as susceptible as possible of the chemical influence of light. After
the lapse of a longer or shorter time, the light will have so acted on
the plate that the various objects the images of which were pro-
jected upon it will appear, with all their gradations of light and
shade, most exactly depicted in black and white, no color being

Kesent. This is the process commonly known by the name of
iguerreotype, from M. Daguerre, the author of the discovery
Since his original discovery, he has ascertained that by isolating and
electrifying the plate it acquires such a sensibility to the chemical
influence of light that one-tenth of a second is a sufficient time to
Dbtain the requisite luminous impression for the formation of the
picture.

949. The chemical effects of light are seen in the varied colors of
the vegetable world. Vegetables which grow in dark places are either
<vhite or of a palish-yellow. The sunny side of fruits is of a richer
tinge than that which grows in the shade. Persons whose daily
employment keeps them much within doors are pale, and more or
less sickly, in consequence of such confinement.



NATURAL PHILOSOPHY.

From what has now been detailed with regard to the nature, the



Online LibraryRichard Green ParkerA school compendium of natural and experimental philosophy : embracing the elementary principles of mechanics, hydrostatics, hydraulics, pneumatics, acoustics, pyronomics, optics, electricity, galvanism, magnetism, electro-magnetism, magneto-electricity, astronomy : containing also a description of → online text (page 22 of 38)