G. P. (George Payn) Quackenbos.

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

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spectrum, and paint them the corresponding shades. Then cause the card
to revolve rapidly. No separate color will be visible, but the whole card
will look white.

667. A prism decomposes white light into its seven component parts, be-
cause these parts are refracted differently, some more and some less. It will
be observed that red, which occupies the lowest part of the spectrum, is
turned from its course the least ; orange, a little more ; yellow, still more ;
then green ; then blue ; then indigo ; while violet, which is at the top of the
spectrum, is refracted the most. The colors, therefore, have different de-
grees of refrangibility. This fact was discovered by Sir Isaac Newton.

668. DIFFERENCE OF COLOK, EXPLAINED. According
to the Undulatory Theory, the color of light depends on
the size of the minute waves that produce it. The undula-
tions that excite in the eye the sensation of red light are
each 4 o"oT ^ an mc ^ m Breadth ; those that produce vio-
let, 6^oo > wn ^ e tne intermediate colors are produced by
undulations varying between these limits.

669. Color is not a property inherent in bodies, but in
the light that they reflect. A non-luminous body seems to
be whatever color it reflects to the eye.

An object lying in green light, looks green ; in red light, red, &c. This
is because green or red is the only light that falls upon it, and therefore it
can reflect no other to the eye. A body seen by ordinary light looks green,
when it absorbs all or most of the other colors of the spectrum, and reflects
or transmits green alone. It looks red when it absorbs the other colors, and
reflects or transmits red, &c. It looks white, when it does not decompose the
light that falls on it, but reflects all the colors combined. It looks black,
when it absorbs nearly all the light that falls on it, and does not reflect any-
particular color in preference to the rest.

670. What colors a substance absorbs and what it reflects, depends chiefly
3n its structure. The particles of some bodies are so arranged as to hav?
'. peculiar affinity for certain colors ; these they absorb, reflecting the rest

of this ? How may we re-unite the seven primary colors ? What other mode it
there of doing this ? 667. To what is it owing that a prism decomposes white light
into its seven component parts ? By whom was this fact discovered ? 668. Ac,
cording to the Undulatory Theory, on what does the color of light depend ? What
is the difference in the undulations that respectively produce red and violet light?
669. In what is the property of color inherent? Why does an object lying in green
light look green? When does an object seen by ordinary light look green ? When
does it look white ? When, black ? 670. What is it that determines what colora v



COMPLEMENTARY COLORS. 257

Changes of color are caused by changes of structure. We may show this
by an experiment with a substance called iodide of mercury. This mineral
is a bright scarlet; when heated and allowed to cool undisturbed, it be-
comes yellow ; but, the moment the surface is scratched, the particles re-
ai-range themselves, and the color turns back to scarlet. Here the same
particles undergo a marked change of color by simply being made to assume
a different arrangement.

671. COMPLEMENTARY COLORS. Any two colors are
said to be Complementary, when, if combined in due pro-
portion, they will produce white. Those colors are com-
plementary to each other which are distant half the length
of the spectrum ; as,

Red and green, Orange and blue,

Yellow and violet, White and black.

It is a curious fact that if we look intently at a bright object of any given
color and then close our eyes, we shall still see it, but tinged with the com-
plementary color. After gazing a few moments at a bright fire, everything
we look at seems to have a greenish hue. If we place a red wafer on a piece
of white paper and look at it intently, we shall soon see a circle of light green
playing around it. A blue wafer will have a similar circle of orange, and a
yellow wafer one of a violet tinge.

672. A color appears to the best advantage, when placed
beside its complementary color.

Thus red is set off by green ; blue, by orange, &c. A pale face appears
paler still when a black dress is worn. On white paper, black ink is plainer
and pleasanter to the eye than ink of any other color. In arranging bou-
quets, and selecting different articles of dress that are to be worn together,
the effect of each individual color is heightened by bringing it in immediate
contrast with its complementary col6r.

673. PROPERTIES OF THE SPECTRUM. Every ray of or-
dinary sun-light appears to have three distinct properties :
1. Brightness. 2. Heat. 3. Power of producing chem-
ical effects. This last property is called Actinism.

674. The chemical effects of sun-light are shown in various ways. Phos-
phorus and nitrate of silver undergo a marked change when exposed to the

substance absorbs, and what it reflects? By what are changes of color caused?
Prove this with an experiment. 671. When are two colors said to be Complementa-
ry ? Name four pairs of complementary colors. What curious fact is stated with
respect to complementary colors ? Give examples. 672. When does a color appear
to the best advantage? Give examples. 673. How many distinct properties has
every ray of ordinary sun-light ? Name them. 674. Instance some of the chemical



258 OPTICS.

solar rajs. Daguerreotypes and photographs are taken by means of the ac-
tion of light on sensitive chemical preparations. Almost all the colored vege-
table juices, when exposed to sun-light, undergo a change of hue. Hydrogen
and chlorine, which may be mixed without danger in the dark, combine with
a loud explosion in the light. Light, also, is essential to the chemical changes
which result in the healthy growth of plants. Hence plants kept in a dark
room become pale and sickly. A similar effect is produced on persons kept
away from the light of the sun.

675. Ordinary sun-light combines these three properties,
but the seven colors into which it is decomposed by the
prism do not possess them alike. Brightness belongs par-
ticularly to yellow ; heat, to red ; actinism, to violet and
indigo.

An object that is bright yellow makes a more vivid impression on the eye
than one of any other color. Hence soldiers dressed in yellow are more dis-
tinct objects of aim to an enemy and more apt to be shot than those dressed
in dark green or gray.

The red portion of the spectrum has the most heat. This is shown by
placing the bulb of a thermometer successively in each of the colors of the
spectrum. It will be most affected by the red, but will show a still higher
temperature, if brought a short distance below the red end of the spectrum,
where no light falls at all. This shows that the heat of a solar ray is re-
fracted as well as its light, but in a less degree.

Actinism is strongest in violet and indigo rays. If a seed be placed un-
der a dark blue glass, so that all the light that strikes it will be tinged with
that color, it will germinate in one-fourth of the time that it usually takes.
Placed under a red glass, it will hardly germinate at all, because red, al-
though it contains more heat than the other colors, has little or no actinism.

676. DARK LIXES IN THE SPECTRUM. If the solar spec-
trum be viewed through a telescope, a great number of
dark lines, parallel to each other but differing in breadth,
will be seen crossing its surface. Seven of these are par-
ticularly distinct, but with a powerful telescope as many
as 2,000 have been counted.

The position of these lines is always the same in the solar spectrum ;
but, when a ray of star-light is decomposed, their number and arrangement

affects of sun-light. 675. Do tho seven primary colors possess these three properties
in equal degrees ? To which does brightness particularly belong? To which, heat?
To which, actinism? What follows from tho peculiar brightness of yellow? How
Is !t proved that the red portion of the spectrum has the most heat ? How does the
refraction of solar heat compare with that of solar light ? Prove this. How may it
bo shown that actinism is strongest in violet and indigo rays ? C76. Describe the dark
Unes in the spectrum. What is said of the lines found in spectra produced from star*



ACIIH03IATIC LENSES. 259

are different, nor do they correspond in spectra formed by rays from different
stars. When rays produced by electricity or combustion are decomposed
with the prism, bright lines are found crossing the spectrum instead of dark
ones.

677. DISPERSION OP LIGHT. By the Dispersion of light
is meant the formation of a spectrum from a single ray.
Spectra formed by different refractive media are of differ-
ent lengths. Thus flint-glass forms a spectrum about twice
as long as crown-glass forms, and four times as long as wa-
ter. Flint-glass is therefore said to have twice the disper-
sive power df crown-glass, and four times that of water.

678. ACHROMATIC LENSES. Lenses, like prisms, refract
light, and produce spectra. Rays passing through a con-
vex lens, therefore, instead of coming to a focus at a single
point, are more or less dispersed, and form colored fringes
about the focus. This defect is called Chromatic Aberra-
tion. It was long a serious drawback in the use of optical
instruments ; but the difficulty is now remedied by com-
bining two lenses of such different materials that the dis-
peilsive power of the one may nullify that of the other.
Lenses combined on this principle are called Achromatic
Lenses.

AcJiromatic means colorless, and the lenses are so called because they do
not fringe their images with the colors of the spectrum. A double convex
lens of crown glass may be united with a plano-concave lens of flint glass.
The latter corrects the chromatic aberration of the former, without entirely
Nullifying its converging effect.

679. THE RAINBOW. The Rainbow is an arch composed
of the seven primary colors, which is visible in the sky
when the sun shines during a shower. It appears in the
opposite quarter to the sun, in the west in the morning,
and the east in the afternoon.

When the sun is in the horizon, the rainbow is a circle ; but the lower
part of it is intercepted by the earth's surface, and therefore we do not gen-

Jight ? In spectra produced from the light of electricity or combustion ? 677. What
is meant by the Dispersion of light? When are different media said to differ in dis-
persive power? 678. Wtyit is Chromatic Aberration ? How is it corrected ? What
docs achromatic mean ? "Why are achromatic lenses so called ? How may an achro-
matic lens bo formed ? 679. What is the Eainbow ? Where is it seen ? What is tha



200 OPTICS.

erally see more than a semi-circle. From the mast-head of a vessel or the
top of a mountain, more than a semi-circle is visible.

680. The rainbow is caused by the refraction and reflection of the sun's
rays by drops of falling rain. Each drop operates like a prism, decomposing
the light that strikes it. The observer's eye is so placed as to receive but
one of the colors from one drop, but from other drops it receives the other
colors, and thus has an arched spectrum formed complete. As no two per-
sons occupy exactly the same spot, no two can see exactly the same bow.

681. Sometimes two distinct bows are visible, one with-
in the other. The inner one, which is called the Primary
Bow, is the brighter of the two. The outer one is called
the Secondary Bow; the rays that form it undergo one
more reflection within the drop than those that form the
primary bow, and are therefore fainter. In the primary
bow, the arrangement of the colors is the same as in the
solar spectrum ; in the secondary bow, this order is re-
versed.

682. Whenever the air is filled with drops, and the sun shines on them at
a certain angle, rainbows are formed, which are visible to an observer in a
proper position. Hence they are often seen in the spray of water-falls and
fountains.

688. Bows are sometimes similarly formed by moon-light, but they are
faint and rarely seen. When so formed, they are called Lunar Rainbows.

684. HALOES. Haloes are luminous or colored circles
seen around the sun and moon under certain conditions of
the atmosphere. They are more frequently seen around
the moon, because the sun's light is so intense that they are
lost in its superior brightness. Haloes arise from the refrac-
tion and dispersion of light by small crystals of ice floating
in the higher regions of the atmosphere.



Vision.

685. THE EYE. The eye is the organ with which we
see. Nothing more strikingly displays the wisdom of the

form of the rainbow ? 680. Explain the principle on which the rainbow is formed.
681. "When two bows are formed, what is each called, and which is the brighter ? In
what order are the colors arranged in the rainbow ? CS2. By what besides rain may
bows be produced? 6S3. What are Lunar Rainbows? What is said of them?
684. "What are Haloes ? Where are they most frequently seen ? How are haloes pro-



THE EYE.



261



Creator than the nice adaptation of this wonderful instru-
ment to the purposes for which it is designed.

686. Parts of the Eye. The human eye is a spheroid,
about an inch in diameter, resting in a cavity below the
forehead, capable of being moved upward, downward, or
sidewise, by muscles attached to it behind. It consists of
ten parts :

1. The Cornea. 6. The Vitreous Humor.

2. The Iris. 7. The Ret'-i-na.

3. The Pupil. 8. The Choroid Coat.

4. The Aqueous Humor. 9. The Sclerotic Coat.

5. The Crystalline Lens. 10. The Optic Nerve.

687. When we look at an eye as set in the head (see
Figure 253), we see but three of these parts : the Cornea
(g] ; the Iris (i) ; and the Pupil (b).

The Cornea is a transparent coat, cov-
ering the whole front of the eye, and
more convex than the rest of the ball.
The Iris is the circular membrane in the
middle of the cornea, according to the color of which we
say that the eye is blue or black, hazel or gray. The Pupil
is a circular opening in the iris, through which light passes
into the interior of the eye. Fig. 254 Fig. 254.

represents a section of the eye. A A A
is the cornea. 1 1 is the iris, and the
opening in the centre is the pupil. In
the following description reference is
made to this Figure.

On passing through the cornea, a ray of light
enters the narrow apartment E, between the cor-
nea on one side and the iris and crystalline lens on the other. This is filled
with a transparent liquid resembling water, and called the Aqueous Humor.
Traversing this, the ray next enters a transparent body, L, called from its
shape the Crystalline Lens. Behind this is the Vitreous Humor, D, a trans-





duced? 685. What is the eye ? 6S6. Describe the eye. Of how many parts does it
consist? Name them. 687. Which of these parts do we see when we look at an eye
as set in the head ? What is the Cornea ? What is the Iris ? What is the Pupil ?
With the aid of Fig. 254, name and describe the various parts of the eye. By what U



262 OPTICS.

parent fluid which fills the greater part of the globe of the eye. This humor
is enclosed within the Retina, C C C, a delicate fibrous membrane resembling
net-work, formed by the expansion of the optic nerve, on which every image
seen by the eye is formed. The Optic Nerve, 0, passes through the back of
the eye to the brain, and conveys to that organ the impressions made on the
.retina.

The retina is surrounded by another coat called the Choroid, represented
in the Figure by a dotted line. The choroid coat is lined on its inner surface
with black coloring matter, to prevent any reflection of light from the inte-
rior of the eye. Outside of all is the Sclerotic Coat, B B B, a strong mem-
brane, to which the muscles that move the eye are attached. It envelopes the
whole ball except the portion in front covered by the cornea, which fits into
it just as the crystal of a watch fits into the case.

688. Uses of the Different Parts. The outer coats of
the eye protect the delicate parts within. The cornea re-
flects some of the light that falls on it, and this gives the
eye its brilliancy. It transmits the greater part, however,
and unites with the aqueous humor, the crystalline lens, and
the vitreous humor, in bringing the incident rays to a focus
and forming an image on the retina.

The iris intuitively regulates the supply of light admit-
ted into the eye, contracting and thus enlarging the pupil
in a faint light, expanding and thus diminishing it in a
strong one. These changes are not instantly made. Hence,
when we pass from a bright light into a room partially
darkened, we can hardly discern anything till the pupil en-
larges, so that more rays are admitted. When we go from
a dark room into a bright light, the eye is pained, because
the pupil, which had expanded to the utmost to accommo-
. date itself to the faint light, does not immediately contract,
and more light is admitted than the sensitive membrane
can endure.

The pupils of cats, tigers, and animals generally that prowl at night for
prey, are capable of being expanded to such a degree as to admit one hun-
dred times as much light as when they are most contracted. They can there-
fore see as well by night as by day. The owl's pupil is exceedingly large ;

the retina surrounded? With what is the choroid coat lined? "What is outside ot
all? What are attached to the sclerotic coat? 63S. What is the use of the outer
coats of the eye ? Of the cornea? Which parts unite with tho cornea in bringing
incident rays to a focus ? What is the use of the iris ? Give some familiar proofs that
the iris accommodates itself to the intensity of the light. What is said of tho pupil



DEFECTS OP VISION. 263

in the day-time, even when contracted to the utmost, it admits so much light
that the bird is nearly blinded, and has to remain stupidly on its roost.

689. DEFECTS OP VISION. In a perfect eye, the rays
that enter are brought to a focus on the retina, and an im-
age is there formed. If the rays are not brought to a focus
by the time they reach the retina, or come to a focus before
reaching it, no impression is made on the optic nerve or
communicated to the brain, and consequently no image
is seen.

Hence arise two defects of vision. When the cornea is
too convex, distant objects form images in front of the ret-
ina, and are not seen ; only such objects as are very near
the eye are visible, and hence persons with this defect of
vision are called near-sighted. When, on the contrary, the
cornea is not convex enough, the rays are not brought to
a focus by the time they reach the retina, and no image is
seen. The eyes of old people generally labor under this
defect, in consequence of the waste of a portion of the vit-
reous and the aqueous humor, so that the crystalline lens
and the cornea fall in. This falling in is just what the near-
sighted person needs ; accordingly it is often found that
those who are near-sighted in youth see perfectly well when
they grow old.

690. The two defects of vision mentioned above are remedied by the use
of spectacles, which consist of lenses of different shapes placed in frames be-
fore the eyes. A near-sighted person uses glasses just concave enough to
nullify the too great convexity of his eye. An old person uses glasses with
sufficient convexity to make up the deficiency of his eye in that respect.

691. Spectacles were first used about the end of the thirteenth century.
It is supposed that the world is indebted to Roger Bacon for their invention.
Before that time all near-sighted and most aged persons had to remain in a
state of comparative blindness.

692. Though all other parts of the eye be perfect, if the optic nerve does
not perform its functions, blindness is the result. Images arc formed on the
retina, but there is no communication with the brain, and no impression

of beasts that prowl at night? What is said of the owl's pupil? 689. Where are im-
ages formed in a perfect eye ? What will prevent an image from being seen ? De-
scribe the two defects of vision arising from images' not being formed on the retina.
690. How are these two defects of vision remedied ? What sort of glasses does a near-
sighted person use ? An old person ? 691. When were spectacles first used ? By
whom are they supposed to have been invented? C92. If the optic nerve does not



264



OPTICS.



is produced. For amaurosis, or paralysis of the optic nerve, there is no
remedy.

693. IMAGES FORMED ON THE RETINA. Images arc
formed on the retina, just as in a dark room, by light ad-
mitted through an aperture (see Fig. 235). In the latter
case, as we have already seen, the image is inverted, and it
follows that images formed on the retina must be inverted
also. Why then do we see them in their natural position ?
This question it is hard to answer. The explanation com-
monly given is this : That we see all things inverted, and
have always done so ; but, inasmuch as we know by expe-
rience that they are erect, the mind of itself, insensibly to
us, corrects the delusion that the inversion would other-
wise produce. We have no means of comparison ; we see
nothing erect, to serve as a standard and prove the general
inversion.

694. Another question is sometimes asked : Since we have two eyes, and
two images are formed, one on each retina, why do we not see two images of
every object ? The answer is, because both eyes are inclined to any given
object at nearly the same angle. The images produced on the retinas are very
nearly the same. The impressions transmitted to the brain by the two branches
of the optic nerve are identical and simultaneous, and but one perception is
the result. If we press on one of our eyes, so as to incline it towards an ob-
ject at a different angle from the other, we see two images. Drunken men
often see double, because they lose control of the muscles of the eye, and do
not direct both eyes to wards a given object at the same angle.

695. VISUAL ANGLE. The visual angle is the angle
formed by two lines drawn from the eye to the extremities
rig. 255. of a given object.

In Fig. 255, the vis-
ual angle of the ar-
rowBA is BEA;
that of the arrow
CDisCED.
A given object

perform its functions, what is the consequence ? 693. What kind of an image is
formed on the retina, and why ? Since an inverted image is formed on the retina,
why do wo see objects in an erect position ? 694. Since we hare two eyes, why do
we not see two images of every object ? How may Ave make two images visible ?
Why do drunken men often see double ? 695. What is the Visual Angle ? Show th




THE VISUAL ANGLE. 265

looks large or small, according to the visual angle that it
forms. Two equal arrows held up before the eye at differ-
ent distances, as in Fig. 255, form different visual angles,
and therefore seem to be of different size. If we measure
their apparent lengths with an interposed rod, we shall find
the nearer one to measure the distance a #, the farther one
only about half as much, c d. A small object placed near
the eye may form as great a visual angle as a very large
distant object, and may therefore entirely hide the latter
when interposed between it and the eye.

Accordingly, the nearer an object is brought to the eye, the larger it ap-
pears to be, and the farther it is removed the smaller it looks. When the
visual angle is less than Vaoo of a degree, an object becomes invisible. A
bird flying from us grows smaller and smaller, till its visual angle dimin-
ishes so that it can no longer be seen, and we say that it has gone out of sight.

696. In the case of familiar objects, experience prevents us from being
misled by their apparent size. Insensibly to ourselves, we make allowance
for their distance, of which we judge by the distinctness of their outline and
by intervening objects. A man at work on a lofty steeple may not look more
than two feet high, yet we are in no danger of mistaking him for a dwarf. A
distant tree seems to be no higher than a bush ; but, if we see a horse feed-
ing beneath it, we intuitively compare the two, and arrive at a correct idea
of the tree's size.

A white object can be distinguished at a greater distance than one of any
other color, and is visible twice as far when the sun shines directly on it as
Avhen simply illumined by ordinary light. An object is brought out most
distinctly by a back-ground which contrasts strikingly with it in color.
Dark-colored eyes, for the most part, see farther than light ones ; and those
who are in the habit of looking at remote objects, like sailors, can discern



Online LibraryG. P. (George Payn) QuackenbosA natural philosphy: embracing the most recent discoveries in the various branches of physics .. → online text (page 25 of 42)