real refraction of a ray of light. Do you see the flower painted at the
bottom of the inside of this tea-cup? (Fig. 3.)
_Emily._ Yes.--But now you have moved it just out of sight; the rim of
the cup hides it.
_Mrs. B._ Do not stir. I will fill the cup with water, and you will see
the flower again.
_Emily._ I do, indeed! Let me try to explain this: when you drew the cup
from me, so as to conceal the flower, the rays reflected by it, no
longer met my eyes, but were directed above them; but now that you have
filled the cup with water, they are refracted, and bent downwards when
passing out of the water, into the air, so as again to enter my eyes.
_Mrs. B._ You have explained it perfectly: fig. 3. will help to imprint
it on your memory. You must observe that when the flower becomes visible
by the refraction of the ray, you do not see it in the situation which
it really occupies, but the image of the flower appears higher in the
cup; for as objects always appear to be situated in the direction of the
rays which enter the eye, the flower will be seen at B, in the direction
of the refracted ray.
_Emily._ Then, when we see the bottom of a clear stream of water, the
rays which it reflects, being refracted in their passage from the water
into the air, will make the bottom appear higher than it really is.
_Mrs. B._ And the water will consequently appear more shallow. Accidents
have frequently been occasioned by this circumstance; and boys, who are
in the habit of bathing, should be cautioned not to trust to the
apparent shallowness of water, as it will always prove deeper than it
appears.
The refraction of light prevents our seeing the heavenly bodies in their
real situation: the light they send to us being refracted in passing
into the atmosphere, we see the sun and stars in the direction of the
refracted ray; as described in fig. 4, plate 19., the dotted line
represents the extent of the atmosphere, above a portion of the earth, E
B E: a ray of light coming from the sun S, falls obliquely on it, at A,
and is refracted to B; then, since we see the object in the direction of
the refracted ray, a spectator at B, will see an image of the sun at C,
instead of its real situation, at S.
_Emily._ But if the sun were immediately over our heads, its rays,
falling perpendicularly on the atmosphere, would not be refracted, and
we should then see the real sun, in its true situation.
_Mrs. B._ You must recollect that the sun, is vertical only to the
inhabitants of the torrid zone; its rays, therefore, are always
refracted, in this latitude. There is also another obstacle to our
seeing the heavenly bodies in their real situations: light, though it
moves with extreme velocity, is about eight minutes and a quarter, in
its passage from the sun to the earth; therefore, when the rays reach
us, the sun must have quitted the spot he occupied on their departure;
yet we see him in the direction of those rays, and consequently in a
situation which he had abandoned eight minutes and a quarter, before.
_Emily._ When you speak of the sun's motion, you mean, I suppose, his
apparent motion, produced by the diurnal motion of the earth?
_Mrs. B._ Certainly; the effect being the same, whether it is our earth,
or the heavenly bodies, which move: it is more easy to represent things
as they appear to be, than as they really are.
_Caroline._ During the morning, then, when the sun is rising towards the
meridian, we must (from the length of time the light is in reaching us)
see an image of the sun below that spot which it really occupies.
_Emily._ But the refraction of the atmosphere, counteracting this
effect, we may, perhaps, between the two, see the sun in its real
situation.
_Caroline._ And in the afternoon, when the sun is sinking in the west,
refraction, and the length of time which the light is in reaching the
earth, will conspire to render the image of the sun, higher than it
really is.
_Mrs. B._ The refraction of the sun's rays, by the atmosphere, prolongs
our days, as it occasions our seeing an image of the sun, both before he
rises, and after he sets; when below our horizon, he still shines upon
the atmosphere, and his rays are thence refracted to the earth: so
likewise we see an image of the sun, previously to his rising, the rays
that fall upon the atmosphere being refracted to the earth.
_Caroline._ On the other hand, we must recollect that light is eight
minutes and a quarter on its journey; so that, by the time it reaches
the earth, the sun may, perhaps, have risen above the horizon.
_Emily._ Pray, do not glass windows, refract the light?
_Mrs. B._ They do; but this refraction would not be perceptible, were
the surfaces of the glass, perfectly flat and parallel, because, in
passing through a pane of glass, the rays suffer two refractions, which,
being in contrary directions, produce nearly the same effect as if no
refraction had taken place.
_Emily._ I do not understand that.
_Mrs. B:_ Fig. 5, plate 19, will make it clear to you: A A represents a
thick pane of glass, seen edgeways. When the ray B approaches the glass,
at C, it is refracted by it; and instead of continuing its course in the
same direction, as the dotted line describes, it passes through the
pane, to D; at that point returning into the air, it is again refracted
by the glass, but in a contrary direction to the first refraction, and
in consequence proceeds to E. Now you must observe that the ray B C and
the ray D E being parallel, the light does not appear to have suffered
any refraction: the apparent, differing so little from the true place of
any object, when seen through glass of ordinary thickness.
_Emily._ So that the effect which takes place on the ray entering the
glass, is undone on its quitting it. Or, to express myself more
scientifically, when a ray of light passes from one medium into another,
and through that into the first again, the two refractions being equal,
and in opposite directions, no sensible effect is produced.
_Caroline._ I think the effect is very sensible, for, in looking through
the glass of the window, I see objects very much distorted; articles
which I know to be straight, appear bent and broken, and sometimes the
parts seem to be separated to a distance from each other.
_Mrs. B._ That is because common window glass is not flat, its whole
surface being uneven. Rays from any object, falling upon it under
different angles, are, consequently, refracted in various ways, and thus
produce the distortion you have observed.
_Emily._ Is it not in consequence of refraction, that the glasses in
common spectacles, magnify objects seen through them?
_Mrs. B._ Yes. Glasses of this description are called _lenses_; of
these, there are several kinds, the names of which it will be necessary
for you to learn. Every lens is formed of glass, ground so as to form a
segment of a sphere, on one, or both sides. They are all represented at
fig. 1, plate 20. The most common, is the _double convex_ lens, D. This
is thick in the middle, and thin at the edges, like common spectacles,
or reading glasses. A B, is a _plano-convex_ lens, being flat on one
side, and convex on the other. E is a _double concave_, being, in all
respects, the reverse of D. C is a _plano-concave_, flat on one side,
and concave on the other. F is called a _meniscus_, or _concavo-convex_,
being concave on one, and convex on the other side. A line passing
through the centre of a lens, is called its _axis_.
_Caroline._ I should like to understand how the rays of light are
refracted, by means of a lens.
_Mrs. B._ When parallel rays (fig. 6) fall on a double convex _lens_,
that only, which falls in the direction of the axis of the lens, is
perpendicular to the surface; the other rays, falling obliquely, are
refracted towards the axis, and will meet at a point beyond the lens,
called its _focus_.
Of the three rays, A B C, which fall on the lens D E, the rays A and C
are refracted in their passage through it, to _a_, and _c_; and on
quitting the lens, they undergo a second refraction in the same
direction, which unites them with the ray B, at the focus F.
_Emily._ And what is the distance of the focus, from the surface of the
lens?
_Mrs. B._ The focal distance depends both upon the form of the lens, and
on the refracting power of the substance of which it is made: in a glass
lens, both sides of which are equally convex, the focus is situated
nearly at the centre of the sphere, of which the surface of the lens
forms a portion; it is at the distance, therefore, of the radius of the
sphere.
The property of those lenses which have a convex surface, is to collect
the rays of light to a focus; and of those which have a concave surface,
on the contrary, to disperse them. For the rays A and C, falling on the
concave lens X Y, (fig. 7, plate 19.) instead of converging towards the
ray B, in the axis of the lens, will each be attracted towards the thick
edges of the lens, both on entering and quitting it, and will,
therefore, by the first refraction, be made to diverge to _a_, _c_, and
by the seconds, to _d_, _e_.
[Illustration: PLATE XX.]
_Caroline._ And lenses which have one side flat, and the other
convex, or concave, as A and B, (fig. 1, plate 20.) are, I suppose,
less powerful in their refractions?
_Mrs. B._ Yes; the focus of the plano-convex, is at the distance of the
diameter of a sphere, of which the convex surface of the lens, forms a
portion; as represented in figure 2, plate 20. The three parallel rays,
A B C, are brought to a focus by the plano-convex lens, X Y, at F.
_Emily._ You have not explained to us, Mrs. B., how the lens serves to
magnify objects.
_Mrs. B._ By turning again to fig. 6, plate 19. you will readily
understand this. Let A C, be an object placed before the lens, and
suppose it to be seen by an eye at F; the ray from the point A, will be
seen in the direction F G, that from C, in the direction F H; the visual
angle, therefore, will be greatly increased, and the object must appear
larger, in proportion.
I must now explain to you the refraction of a ray of light, by a
triangular piece of glass, called a prism. (Fig. 3.)
_Emily._ The three sides of this glass are flat; it cannot, therefore,
bring the rays to a focus; nor do I suppose that its refraction will be
similar to that of a flat pane of glass, because it has not two sides
parallel; I cannot, therefore, conjecture what effect the refraction by
a prism, can produce.
_Mrs. B._ The refractions of the ray, both on entering and on quitting
the prism, are in the same direction, (Fig. 3.) On entering the prism P,
the ray A is refracted from B to C, and on quitting it from C to D. In
the first instance it is refracted towards, and in the last, from the
perpendicular; each causing it to deviate in the same way, from its
original course, A B.
I will show you this by experiment; but for this purpose it will be
advisable to close the window-shutters, and admit, through the small
aperture, a ray of light, which I shall refract, by means of this prism.
_Caroline._ Oh, what beautiful colours are represented on the opposite
wall! There are all the colours of the rainbow, and with a brightness, I
never saw equalled. (Fig. 4, plate 20.)
_Emily._ I have seen an effect, in some respects similar to this,
produced by the rays of the sun shining upon glass lustres; but how is
it possible that a piece of white glass can produce such a variety of
brilliant colours?
_Mrs. B._ The colours are not formed by the prism, but existed in the
ray previously to its refraction.
_Caroline._ Yet, before its refraction, it appeared perfectly white.
_Mrs. B._ The white rays of the sun, are composed of rays, which, when
separated, produce all these colours, although when blended together,
they appear colourless or white.
Sir Isaac Newton, to whom we are indebted for the most important
discoveries respecting light and colours, was the first who divided a
white ray of light, and found it to consist of an assemblage of coloured
rays, which formed an image upon the wall, such as you now see
exhibited, (fig. 4.) in which are displayed the following series of
colours: red, orange, yellow, green, blue, indigo, and violet.
_Emily._ But how does a prism separate these coloured rays?
_Mrs. B._ By refraction. It appears that the coloured rays have
different degrees of refrangibility; in passing through the prism,
therefore, they take different directions according to their
susceptibility of refraction. The violet rays deviate most from their
original course; they appear at one of the ends of the spectrum, A B:
contiguous to the violet, are the blue rays, being those which have
somewhat less refrangibility; then follow, in succession, the green,
yellow, orange, and lastly, the red, which are the least refrangible of
the coloured rays.
_Caroline._ I cannot conceive how these colours, mixed together, can
become white?
_Mrs. B._ That I cannot pretend to explain: but it is a fact that the
union of these colours, in the proportions in which they appear in the
spectrum, produce in us the idea of whiteness. If you paint a circular
piece of card, in compartments, with these seven colours, as nearly as
possible in the proportion, and of the shade exhibited in the spectrum,
and whirl it rapidly on a pin, it will appear white; as the velocity of
the motion, will have the effect of blending the colours, in the
impression which they make upon the eye.
But a more decisive proof of the composition of a white ray is afforded,
by reuniting these coloured rays, and forming with them, a ray of white
light.
_Caroline._ If you can take a ray of white light to pieces, and put it
together again, I shall be quite satisfied.
_Mrs. B._ This can be done by letting the coloured rays, which have been
separated by a prism, fall upon a lens, which will converge them to a
focus; and if, when thus reunited, we find that they appear white as
they did before refraction, I hope you will be convinced that the white
rays, are a compound of the several coloured rays. The prism P, you
see, (fig. 5.) separates a ray of white light, into seven coloured rays,
and the lens L L brings them to a focus at F, where they again appear
white.
_Caroline._ You succeed to perfection: this is indeed a most interesting
and conclusive experiment.
_Emily._ Yet, Mrs. B., I cannot help thinking, that there may, perhaps,
be but three distinct colours in the spectrum, red, yellow, and blue;
and that the four others may consist of two of these colours blended
together; for, in painting, we find, that by mixing red and yellow, we
produce orange; with different proportions of red and blue, we make
violet or any shade of purple; and yellow, and blue, form green. Now, it
is very natural to suppose, that the refraction of a prism, may not be
so perfect as to separate the coloured rays of light completely, and
that those which are contiguous, in order of refrangibility, may
encroach on each other, and by mixing, produce the intermediate colours,
orange, green, violet, and indigo.
_Mrs. B._ Your observation is, I believe, neither quite wrong, nor quite
right. Dr. Wollaston, who has performed many experiments on the
refraction of light, in a more accurate manner than had been previously
done, by receiving a very narrow line of light on a prism, found that it
formed a spectrum, consisting of rays of four colours only; but they
were not exactly those you have named as primitive colours, for they
consisted of red, green, blue, and violet. A very narrow line of yellow
was visible, at the limit of the red and green, which Dr. Wollaston
attributed to the overlapping of the edges of the red and green light.
_Caroline._ But red and green mixed together, do not produce yellow?
_Mrs. B._ Not in painting; but it may be so in the primitive rays of the
spectrum. Dr. Wollaston observed, that, by increasing the breadth of the
aperture, by which the line of light was admitted, the space occupied by
each coloured ray in the spectrum, was augmented, in proportion as each
portion encroached on the neighbouring colour, and mixed with it; so
that the intervention of orange and yellow, between the red and green,
is owing, he supposes, to the mixture of these two colours; and the blue
is blended on the one side with the green, and on the other with the
violet, forming the spectrum, as it was originally observed by Sir Isaac
Newton, and which I have just shown you.
The rainbow, which exhibits a series of colours, so analogous to those
of the spectrum, is formed by the refraction of the sun's rays, in their
passage through a shower of rain; every drop of which acts as a prism,
in separating the coloured rays as they pass through it; the combined
effect of innumerable drops, produces the bow, which you know can be
seen, only when there are both rain, and sunshine.
_Emily._ Pray, Mrs. B., cannot the sun's rays be collected to a focus by
a lens, in the same manner as they are by a concave mirror?
_Mrs. B._ The same effect in concentrating the rays, is produced by the
refraction with a lens, as by the reflection from a concave mirror: in
the first, the rays pass through the glass and converge to a focus,
behind it, in the latter, they are reflected from the mirror, and
brought to a focus, before it. A lens, when used for the purpose of
collecting the sun's rays, is called a burning glass. I have before
explained to you, the manner in which a convex lens, refracts the rays,
and brings them to a focus; (fig. 6, plate 19.) as these rays contain
both light and heat, the latter, as well as the former, is refracted;
and intense heat, as well as light, will be found in the focal point.
The sun now shines very bright; if we let the rays fall on this lens,
you will perceive the focus.
_Emily._ Oh yes: the point of union of the rays, is very luminous. I
will hold a piece of paper in the focus, and see if it will take fire.
The spot of light is extremely brilliant, but the paper does not burn?
_Mrs. B._ Try a piece of brown paper;--that, you see, takes fire almost
immediately.
_Caroline._ This is surprising; for the light appeared to shine more
intensely, on the white, than on the brown paper.
_Mrs. B._ The lens collects an equal number of rays to a focus, whether
you hold the white or the brown paper, there; but the white paper
appears more luminous in the focus, because most of the rays, instead of
entering into the paper, are reflected by it; and this is the reason
that the paper does not readily take fire: whilst, on the contrary, the
brown paper, which absorbs more light and heat than it reflects, soon
becomes heated and takes fire.
_Caroline._ This is extremely curious; but why should brown paper,
absorb more rays, than white paper?
_Mrs. B._ I am far from being able to give a satisfactory answer to that
question. We can form but mere conjecture on this point; it is supposed
that the tendency to absorb, or reflect rays, depends on the
arrangement of the minute particles of the body, and that this diversity
of arrangement renders some bodies susceptible of reflecting one
coloured ray, and absorbing the others; whilst other bodies, have a
tendency to reflect all the colours, and others again, to absorb them
all.
_Emily._ And how do you know which colours bodies have a tendency to
reflect, or which to absorb?
_Mrs. B._ Because a body always appears to be of the colour which it
reflects; for, as we see only by reflected rays, it can appear of the
colour of those rays, only.
_Caroline._ But we see all bodies of their own natural colour, Mrs. B.;
the grass and trees, green; the sky, blue; the flowers of various hues.
_Mrs. B._ True; but why is the grass green?--because it absorbs all,
except the green rays; it is, therefore, these only which the grass and
trees reflect to our eyes, and this makes them appear green. The
flowers, in the same manner, reflect the various colours of which they
appear to us; the rose, the red rays; the violet, the blue; the jonquil,
the yellow, &c.
_Caroline._ But these are the permanent colours of the grass and
flowers, whether the sun's rays shine on them or not.
_Mrs. B._ Whenever you see those colours, the flowers must be illumined
by some light; and light, from whatever source it proceeds, is of the
same nature; composed of the various coloured rays which paint the
grass, the flowers, and every coloured object in nature.
_Caroline._ But, Mrs. B., the grass is green, and the flowers are
coloured, whether in the dark, or exposed to the light?
_Mrs. B._ Why should you think so?
_Caroline._ It cannot be otherwise.
_Mrs. B._ A most philosophical reason indeed! But, as I never saw them
in the dark, you will allow me to dissent from your opinion.
_Caroline._ What colour do you suppose them to be, then, in the dark?
_Mrs. B._ None at all; or black, which is the same thing. You can never
see objects, without light. White light is compounded of rays, from
which all the colours in nature are produced; there, therefore, can be
no colour without light; and though a substance is black, or without
colour, in the dark, it may become coloured, as soon as it becomes
visible. It is visible, indeed, only by the coloured rays which it
reflects; therefore, we can see it only when coloured.
_Caroline._ All you say seems very true, and I know not what to object
to it; yet it appears at the same time incredible! What, Mrs. B., are we
all as black as negroes in the dark? you make me shudder at the thought.
_Mrs. B._ Your vanity need not be alarmed at the idea, as you are
certain of never being seen, in that state.
_Caroline._ That is some consolation, undoubtedly; but what a melancholy
reflection it is, that all nature which appears so beautifully
diversified with colours, is really one uniform mass of blackness!
_Mrs. B._ Is nature less pleasing for being coloured, as well as
illumined, by the rays of light? and are colours less beautiful, for
being accidental, rather than essential properties of bodies?
Providence seems to have decorated nature with the enchanting diversity
of colours, which we so much admire, for the sole purpose of beautifying
the scene, and rendering it a source of sensible gratification: it is an
ornament which embellishes nature, whenever we behold her. What reason
is there to regret, that she does not wear it when she is invisible?
_Emily._ I confess, Mrs. B., that I have had my doubts, as well as
Caroline, though she has spared me the pains of expressing them: but I
have just thought of an experiment, which, if it succeed, will, I am
sure, satisfy us both. It is certain, that we cannot see bodies in the
dark, to know whether they have then any colour. But we may place a
coloured body in a ray of light, which has been refracted by a prism;
and if your theory is true, the body, of whatever colour it naturally
is, must appear of the colour of the ray in which it is placed; for
since it receives no other coloured rays, it can reflect no others.
_Caroline._ Oh! that is an excellent thought, Emily; will you stand the
test, Mrs. B.?
_Mrs. B._ I consent: but we must darken the room, and admit only the ray
which is to be refracted; otherwise, the white rays will be reflected on
the body under trial, from various parts of the room. With what do you
choose to make the experiment?
_Caroline._ This rose: look at it, Mrs. B., and tell me whether it is
possible to deprive it of its beautiful colour?
_Mrs. B._ We shall see.--I expose it first to the red rays, and the
flower appears of a more brilliant hue; but observe the green leaves----
_Caroline._ They appear neither red nor green; but of a dingy brown with
a reddish glow?