MAGIC GLASSES, AND HOW TO USE THEM
[Illustration]
The sun shone brightly into the science class-room at mid-day. No gaunt
shadows nor ghostly moonlight now threw a spell on the magic chamber
above. The instruments looked bright and business-like, and the
Principal, moving amongst them, heard the subdued hum of fifty or more
voices rising from below. It was the lecture hour, and the subject for
the day was, "Magic glasses, and how to use them." As the large clock in
the hall sounded twelve, the Principal gathered up a few stray lenses
and prisms he had selected, and passed down the turret stair to his
platform. Behind him were arranged his diagrams, before him on the table
stood various instruments, and the rows of bright faces beyond looked up
with one consent as the hum quieted down and he began his lecture.
"I have often told you, boys, have I not? that I am a Magician. In my
chamber near the sky I work spells as did the magicians of old, and by
the help of my magic glasses I peer into the secrets of nature. Thus I
read the secrets of the distant stars; I catch the light of wandering
comets, and make it reveal its origin; I penetrate into the whirlpools
of the sun; I map out the craters of the moon. Nor can the tiniest being
on earth hide itself from me. Where others see only a drop of muddy
water, that water brought into my magic chamber teems with thousands of
active bodies, darting here and whirling there amid a meadow of tiny
green plants floating in the water. Nay, my inquisitive glass sees even
farther than this, for with it I can watch the eddies of water and green
atoms going on in each of these tiny beings as they feed and grow.
Again, if I want to break into the secrets of the rock at my feet, I
have only to put a thin slice of it under my microscope to trace every
crystal and grain; or, if I wish to learn still more, I subject it to
fiery heat, and through the magic prisms of my spectroscope I read the
history of the very substances of which it is composed. If I wish to
study the treasures of the wide ocean, the slime from a rock-pool teems
with fairy forms darting about in the live box imprisoned in a crystal
home. If some distant stars are invisible even in the giant glasses of
my telescope, I set another power to work, and make them print their own
image on a photographic plate and so reveal their presence.
"All these things you have seen through my magic glasses, and I
promised you that one day I would explain to you how they work and do my
bidding. But I must warn you that you must give all your attention;
there is no royal road to my magician's power. Every one can attain to
it, but only by taking trouble. You must open your eyes and ears, and
use your intelligence to test carefully what your senses show you.
"We have only to consider a little to see that we depend entirely upon
our senses for our knowledge of the outside world. All kinds of things
are going on around us, about which we know nothing, because our eyes
are not keen enough to see, and our ears not sharp enough to hear them.
Most of all we enjoy and study nature through our eyes, those windows
which let in to us the light of heaven, and with it the lovely sights
and scenes of earth; and which are no ordinary windows, but most
wonderful structures adapted for conveying images to the brain. They are
of very different power in different people, so that a long-sighted
person sees a lovely landscape where a short-sighted one sees only a
confused mist; while a short-sighted person can see minute things close
to the eye better than a long-sighted one."
[Illustration: Fig. 10.
Eye-ball seen from the front. (After Le Gros Clark.)
_w_, White of eye. _i_, Iris. _p_, Pupil.]
"Let us try to understand this before we go on to artificial glasses,
for it will help us to explain how these glasses show us many things we
could never see without them. Here are two pictures of the human eyeball
(Figs. 10 and 11), one as it appears from the front, and the other as we
should see the parts if we cut an eyeball across from the front to the
back. From these drawings we see that the eyeball is round; it only
looks oval, because it is seen through the oval slit of the eyelids.
It is really a hard, shining, white ball with a thick nerve cord
(_on_, Fig. 11) passing out at the back, and a dark glassy mound
_c_, _c_ in the centre of the white in front. In this mound we can
easily distinguish two parts--first, the coloured _iris_ or elastic
curtain (_i_, Fig. 10); and secondly, the dark spot or pupil _p_ in the
centre. The iris is the part which gives the eye its colour; it is
composed of a number of fibres, the outer ones radiating towards the
centre, the inner ones forming a ring round the pupil; and behind these
fibres is a coat of dark pigment or colouring matter, blue in some
people, grey, brown, or black in others. When the light is very strong,
and would pain the nerves inside if too much entered the pupil or window
of the eye, then the ring of the iris contracts so as partly to close
the opening. When there is very little light, and it is necessary to let
in as much as possible, the ring expands and the pupil grows large. The
best way to observe this is to look at a cat's eyes in the dusk, and
then bring her near to a bright light; for the iris of a cat's eye
contracts and expands much more than ours does."
[Illustration: Fig. 11.
Section of an eye looking at a pencil. (Adapted from Kirke.)
_c_, _c_, Cornea. _w_, White of eye. _cm_, Ciliary muscle. _a_, _a_,
Aqueous humour. _i_, _i_, Iris. _l_, _l_, Lens. _r_, _r_, Retina. _on_,
Optic nerve. 1, 2, Pencil. 1', 2', Image of pencil on the retina.]
"Now look at the second diagram (Fig. 11) and notice the chief points
necessary in seeing. First you will observe that the pupil is not a mere
hole; it is protected by a curved covering _c_. This is the cornea, a
hard, perfectly transparent membrane, looking much like a curved
watch-glass. Behind this is a small chamber filled with a watery fluid
_a_, called the aqueous humour, and near the back of this chamber is the
dark ring or iris _i_, which you saw from the front through the cornea
and fluid. Close behind the iris again is the natural 'magic glass' of
our eye, the crystalline lens _l_, which is composed of perfectly
transparent fibres and has two rounded or convex surfaces like an
ordinary magnifying glass. This lens rests on a cushion of a soft
jelly-like substance _v_, called the vitreous humour, which fills the
dark chamber or cavity of the eyeball and keeps it in shape, so that the
retina _r_, which lines the chamber, is kept at a proper distance from
the lens. This retina is a transparent film of very sensitive nerves; it
forms a screen at the back of the chamber, and has a coating of very
dark pigment or colouring matter behind it. Lastly, the nerves of the
retina all meet in a bundle, called the optic nerve, and passing out of
the eyeball at a point _on_, go to the brain. These are the chief parts
we use in seeing; now how do we use them?
"Suppose that a pencil is held in front of the eye at the distance at
which we see small objects comfortably. Light is reflected from all
parts of the surface of the pencil, and as the rays spread, a certain
number enter the pupil of the eye. We will follow only two cones of
light coming from the points 1 and 2 on the diagram Fig. 11. These you
see enter the eye, each widely spread over the cornea _c_. They are bent
in a little by this curved covering, and by the liquid behind it, while
the iris cuts off the rays near the edges of the lens, which would be
too much bent to form a clear image. The rest of the rays fall upon the
lens _l_. In passing through this lens they are very much bent (or
_refracted_) towards each other, so much so that by the time they reach
the end of the dark chamber _v_, each cone of light has come to a point
or focus 1', 2', and as rays of this kind have come from every point all
over the pencil, exactly similar points are formed on the retina, and a
real picture of the pencil is formed there between 1' and 2'."
[Illustration: Fig. 12.
Image of a candle-flame thrown on paper by a lens.]
"We will make a very simple and pretty experiment to illustrate this.
Darkening the room I light a candle, take a square of white paper in my
hand, and hold a simple magnifying glass between the two (see Fig. 12)
about three inches away from the candle. Then I shift the paper nearer
and farther behind the lens, till we get a clear image of the
candle-flame upon it. This is exactly what happens in our eye. I have
drawn a dotted line _c_ round the lens and the paper on the diagram to
represent the eyeball in which the image of the candle-flame would be on
the retina instead of on the piece of paper. The first point you will
notice is that the candle-flame is upside down on the paper, and if you
turn back to Fig. 11 you will see why, for it is plain that the cones of
light _cross_ in the lens _l_, 1 going to 1' and 2 to 2'. Every picture
made on our retina is upside down.
"But it is not there that we see it. As soon as the points of light from
the pencil strike upon the retina, the thrill passes on along the optic
nerve _on_, through the back of the eye to the brain; and our mind,
following back the rays exactly as they have come through the lens, sees
a pencil, outside the eye, right way upwards.
"This is how we see with our eyes, which adjust themselves most
beautifully to our needs. For example, not only is the iris always ready
to expand or contract according as we need more or less light, but there
is a special muscle, called the ciliary muscle (_cm_, Fig. 11), which
alters the lens for us to see things far or near. In all, or nearly all,
perfect eyes the lens is flatter in front than behind, and this enables
us to see things far off by bringing the rays from them exactly to a
focus on the retina. But when we look at nearer things the rays require
to be more bent or refracted, so without any conscious effort on our
part this ciliary muscle contracts and allows the lens to bulge out
slightly in front. Instantly we have a stronger magnifier, and the rays
are brought to the right focus on the retina, so that a clear and
full-size image of the near object is formed. How little we think, as we
turn our eyes from one thing to another, and observe, now the distant
hills, now the sheep feeding close by; or, as night draws on, gaze into
limitless space and see the stars millions upon millions of miles away,
that at every moment the focus of our eye is altering, the iris is
contracting or expanding, and myriads of images are being formed one
after the other in that little dark chamber, through which pass all the
scenes of the outer world!
"Yet even this wonderful eye cannot show us everything. Some see farther
than others, some see more minutely than others, according as the lens
of the eye is flatter in one person and more rounded in another. But the
most long-sighted person could never have discovered the planet Neptune,
more than 2700 millions of miles distant from us, nor could the
keenest-sighted have known of the existence of those minute and
beautiful little plants, called diatoms, which live around us wherever
water is found, and form delicate flint skeletons so infinitesimally
small that thousands of millions go to form one cubic inch of the stone
called tripoli, found at Bilin in Bohemia."
[Illustration: Fig. 13.
Arrow magnified by a convex lens.
_a_, _b_, Real arrow. C, D, Magnifying-glass. A, B, Enlarged image of
the arrow.]
"It is here that our 'magic glasses' come to our assistance, and reveal
to us what was before invisible. We learnt just now that we see near
things by the lens of our eye becoming more rounded in front; but there
comes a point beyond which the lens cannot bulge any more, so that when
a thing is very tiny, and would have to be held very close to the eye
for us to see it, the lens can no longer collect the rays to a focus, so
we see nothing but a blur. More than 800 years ago an Arabian, named
Alhazen, explained why rounded or convex glasses make things appear
larger when placed before the eye. This glass which I hold in my hand
is a simple magnifying-glass, such as we used for focusing the
candle-flame. It bends the rays inwards from any small object (see the
arrow _a_, _b_, Fig. 13) so that the lens of our eye can use them, and
then, as we follow out the rays in straight lines to the place where we
see clearly (at A, B), every point of the object is magnified, and we
not only see it much larger, but every mark upon it is much more
distinct. You all know how the little shilling magnifying-glasses you
carry show the most lovely and delicate structures in flowers, on the
wings of butterflies, on the head of a bee or fly, and, in fact, in all
minute living things."
[Illustration: Fig. 14.
Student's microscope. _ep_, Eye-piece. _o_, _g_, Object-glass.]
[Illustration: Fig. 15.
Skeleton of a microscope, showing how an object is magnified.
_o_, _l_, Object-lens. _e_, _g_, Eye-glass. _s_, _s_, Spicule.
_s'_, _s'_, Magnified image of same in the tube. S, S, Image again
enlarged by the lens of the eye-piece.]
"But this is only our first step. Those diatoms we spoke of just now
will only look like minute specks under even the strongest
magnifying-glass. So we pass on to use two extra lenses to assist our
eyes, and come to this compound microscope (Fig. 14) through which I
have before now shown you the delicate markings on shells which were
themselves so minute that you could not see them with the naked eye. Now
we have to discover how the microscope performs this feat. Going back
again for a minute to our candle and magnifying-glass (Fig. 12), you
will find that the nearer you put the lens to the candle the farther
away you will have to put the paper to get a clear image. When in a
microscope we put a powerful lens _o_, _l_ close down to a very minute
object, say a spicule of a flint sponge _s_, _s_, quite invisible to the
unaided eye, the rays from this spicule are brought to a focus a long
way behind it at _s'_, _s'_, making an enlarged image because the lines
of light have been diverging ever since they crossed in the lens. If you
could put a piece of paper at _s'_ _s'_, as you did in the candle
experiment, you would see the actual image of the magnified spicule upon
it. But as these points of light are only in an empty tube, they pass
on, spreading out again from the image, as they did before from the
spicule. Then another convex lens or eye-glass _e_, _g_ is put at the
top of the microscope at the proper distance to bend these rays so that
they enter our eye in nearly parallel lines, exactly as we saw in the
ordinary magnifying-glass (Fig. 13), and our crystalline lens can then
bring them to a focus on our retina.
"By this time the spicule has been twice magnified; or, in other words,
the rays of light coming from it have been twice bent towards each
other, so that when our eye follows them out in straight lines they are
widely spread, and we see every point of light so clearly that all the
spots and markings on this minute spicule are as clear as if it were
really as large as it looks to us.
"This is simply the principle of the microscope. When you come to look
at your own instruments, though they are very ordinary ones, you will
find that the object-glass _o_, _l_ is made of three lenses, flat on the
side nearest the tube, and each lens is composed of two kinds of glass
in order to correct the unequal refraction of the rays, and prevent
fringes of colour appearing at the edge of the lens. Then again the
eye-piece will be a short tube with a lens at each end, and halfway
between them a black ledge will be seen inside the tube which acts like
the iris of our eye (_i_, Fig. 11) and cuts off the rays passing through
the edges of the lens. All these are devices to correct faults in the
microscope which our eye corrects for itself, and they have enabled
opticians to make very powerful lenses.
"Look now at the diagram (Fig. 16) showing a group of diatoms which you
can see under the microscope after the lecture. Notice the lovely
patterns, the delicate tracery, and the fine lines on the diatoms shown
there. Yet each of these minute flint skeletons, if laid on a piece of
glass by itself, would be quite invisible to the naked eye, while
hundreds of them together only look like a faint mist on the slide on
which they lie. Nor are they even here shown as much magnified as they
might be; under a stronger power we should see those delicate lines on
the diatoms broken up into minute round cups."
[Illustration: Fig. 16.
Fossil diatoms seen under the microscope. The largest of these is an
almost imperceptible speck to the naked eye.]
"Is it not wonderful and delightful to think that we are able to add in
this way to the power of our eyes, till it seems as if there were no
limit to the hidden beauties of the minute forms of our earth, if only
we can discover them?
"But our globe does not stand alone in the universe, and we want not
only to learn all about everything we find upon it, but also to look out
into the vast space around us and discover as much as we can about the
myriads of suns and planets, comets and meteorites, star-mists and
nebulae, which are to be found there. Even with the naked eye we can
admire the grand planet Saturn, which is more than 800 millions of miles
away, and this in itself is very marvellous. Who would have thought that
our tiny crystalline lens would be able to catch and focus rays, sent
all this enormous distance, so as actually to make a picture on our
retina of a planet, which, like the moon, is only sending back to us the
light of the sun? For, remember, the rays which come to us from Saturn
must have travelled twice 800 millions of miles--884 millions from the
sun to the planet, and less or more from the planet back to us,
according to our position at the time. But this is as nothing when
compared to the enormous distances over which light travels from the
stars to us. Even the nearest star we know of, is at least twenty
_millions_ of _millions_ of miles away, and the light from it, though
travelling at the rate of 186,300 miles in a second, takes four years
and four months to reach us, while the light from others, which we can
see without a telescope, is between twenty and thirty years on its road.
Does not the thought fill us with awe, that our little eye should be
able to span such vast distances?
"But we are not yet nearly at the end of our wonder, for the same power
which devised our eye gave us also the mind capable of inventing an
instrument which increases the strength of that eye till we can actually
see stars so far off that their light takes _two thousand years_ coming
to our globe. If the microscope delights us in helping us to see things
invisible without it, because they are so small, surely the telescope is
fascinating beyond all other magic glasses when we think that it brings
heavenly bodies, thousands of billions of miles away, so close to us
that we can examine them."
[Illustration: Fig. 17.
An astronomical telescope.
_ep_, Eye-piece. _og_, Object-glass. _f_, Finder.]
"A Telescope (Fig. 17) can, like the microscope, be made of only two
glasses: an object-glass to form an image in the tube and a magnifying
eye-piece to enlarge it. But there is this difference, that the object
lens of a microscope is put close down to a minute object, so that the
rays fall upon it at a wide angle, and the image formed in the tube is
very much larger than the object outside. In the telescope, on the
contrary, the thing we look at is far off, so that the rays fall on the
object-glass at such a very narrow angle as to be practically parallel,
and the image in the tube is of course _very, very_ much smaller than
the house, or church, or planet it pictures. What the object-glass of
the telescope does for us, is to bring a small _real image_ of an object
very far off close to us in the tube of the telescope so that we can
examine it.
"Think for a moment what this means. Imagine that star we spoke of (p.
41), whose light, travelling 186,300 miles in one second, still takes
2000 years to reach us. Picture the tiny waves of light crossing the
countless billions of miles of space during those two thousand years,
and reaching us so widely spread out that the few faint rays which
strike our eye are quite useless, and for us that star has no existence;
we cannot see it. Then go and ask the giant telescope, by turning the
object-glass in the direction where that star lies in infinite space.
The widespread rays are collected and come to a minute bright image in
the dark tube. You put the eye-piece to this image, and there, under
your eye, is a shining point: this is the image of the star, which
otherwise would be lost to you in the mighty distance.
"Can any magic tale be more marvellous, or any thought grander, or more
sublime than this? From my little chamber, by making use of the laws of
light, which are the same wherever we turn, we can penetrate into depths
so vast that we are not able even to measure them, and bring back unseen
stars to tell us the secrets of the mighty universe. As far as the stars
are concerned, whether we see them or not depends entirely upon the
number of rays collected by the object-glass; for at such enormous
distances the rays have no angle that we can measure, and magnify as
you will, the brightest star only remains a point of light. It is in
order to collect enough rays that astronomers have tried to have larger
and larger object-glasses; so that while a small good hand telescope,
such as you use, may have an object-glass measuring only an inch and a
quarter across, some of the giant telescopes have lenses of two and a
half feet, or thirty inches, diameter. These enormous lenses are very
difficult to make and manage, and have many faults, therefore
astronomical telescopes are often made with curved mirrors to _reflect_
the rays, and bring them to a focus instead of _refracting_ them as
curved lenses do.
"We see, then, that one very important use of the telescope is to bring
objects into view which otherwise we would never see; for, as I have
already said, though we bring the stars into sight, we cannot magnify
them. But whenever an object is near enough for the rays to fall even at
a very small perceptible angle on the object-glass, then we can magnify
them; and the longer the telescope, and the stronger the eye-piece, the
more the object is magnified.
[Illustration: Fig. 18.
Skeletons of telescopes.
A, A one-foot telescope with a three-inch eye-piece. B, A two-foot
telescope with a three-inch eye-piece. _e_, _p_, Eye-piece. _o_, _g_,
Object-glass. _r_, _r_, Rays which enter the telescopes and crossing at
_x_ form an image at _i_, _i_, which is magnified by the lens _e_, _p_.
The angles _r_, _x_, _r_ and _i_, _x_, _i_ are the same. In A the angle
_i_, _o_, _i_ is four times greater than that of _i_, _x_, _i_. In B it
is eight times greater.]
"I want you to understand the meaning of this, for it is really very
simple, only it requires a little thought. Here are skeleton drawings of
two telescopes (Fig. 18), one double the length of the other. Let us
suppose that two people are using them to look at an arrow on a
weathercock a long distance off. The rays of light _r_, _r_ from the
two ends of the arrow will enter both telescopes at the same angle
_r_, _x_, _r_, cross in the lens, and pass on at _exactly the same
angle_ into the tubes. So far all is alike, but now comes the
difference. In the short telescope A the object-glass must be of such a
curve as to bring the cones of light in each ray to a focus at a
distance of _one foot_ behind it,[1] and there a small image _i_, _i_ of
the arrow is formed. But B being twice the length, allows the lens
to be less curved, and the image to be formed _two feet_ behind the
object-glass; and as the rays _r_, _r_ have been _diverging_ ever since
they crossed at _x_, the real image of the arrow formed at _i_, _i_ is
twice the size of the same image in A. Nevertheless, if you could put a
piece of paper at _i_, _i_ in both telescopes, and look through the
_object-glass_ (which you cannot actually do, because your head would
block out the rays), the arrow would appear the same size in both
telescopes, because one would be twice as far off from you as the other,
and the angle _i_, _x_, _i_ is the same in both."
[1] In our Fig. 18 the distances are inches instead of feet, but the
proportions are the same.
"But by going to the proper end of the telescope you can get quite near
the image, and can see and magnify it, if you put a strong lens to
collect the rays from it to a focus. This is the use of the eye-piece,
which in our diagram is placed at a quarter of a foot or three inches
from the image in both telescopes. Now that we are close to the images,
the divergence of the points _i_, _i_ makes a great difference. In the
small telescope, in which the image is only _one foot_ behind the
object-glass, the eye-piece being a quarter of a foot from it, is four
times nearer, so the angle _i_, _o_, _i_ is four times the angle
_i_, _x_, _i_, and the man looking through it sees the image magnified
_four times_. But in the longer telescope the image is _two feet_ behind
the lens, while the eye-piece is, as before, a quarter of a foot from
it. Thus the eyepiece is now eight times nearer, so the angle
_i_, _o_, _i_ is eight times the angle _i_, _x_, _i_, and the observer
sees the image magnified _eight times_.
"In real telescopes, where the difference between the focal length of
the object-glass and that of the eye-glass can be made enormously
greater, the magnifying power is quite startling, only the object-glass
must be large, so as to collect enough rays to bear spreading widely.
Even in your small telescopes, with a focus of eighteen inches, and an
object-glass measuring one and a quarter inch across, we can put on a
quarter of an inch eye-piece, and so magnify seventy-two times; while in
my observatory telescope, eight feet or ninety-six inches long, an
eye-piece of half an inch magnifies 192 times, and I can put on a
1/8-inch eye-piece and magnify 768 times! And so we can go on
lengthening the focus of the object-glass and shortening the focus of
the eye-piece, till in Lord Rosse's gigantic fifty-six-foot telescope,
in which the image is fifty-four feet (648 inches) behind the
object-glass, an eye-piece one-eighth of an inch from the image
magnifies 5184 times! These giant telescopes, however, require an
enormous object-glass or mirror, for the points of light are so spread
out in making the large image that it is very faint unless an enormous
number of rays are collected. Lord Rosse's telescope has a reflecting
mirror measuring six feet across, and a man can walk upright in the
telescope tube. The most powerful telescope yet made is that at the Lick
Observatory, on Mount Hamilton, in California. It is fifty-six and a
half feet long, the object-lens measures thirty-six inches across. A
star seen through this telescope appears 2000 times as bright as when
seen with the naked eye.
"You need not, however, wait for an opportunity to look through giant
telescopes, for my small student's telescope, only four feet long, which
we carry out on to the lawn, will show you endless unseen wonders; while
your hand telescopes, and even a common opera-glass, will show many
features on the face of the moon, and enable you to see the crescent of
Venus, Jupiter's moons, and Saturn's rings, besides hundreds of stars
unseen by the naked eye.
"Of course you will understand that Fig. 18 only shows the _principle_
of the telescope. In all good instruments the lenses and other parts are
more complicated; and in a terrestrial telescope, for looking at
objects on the earth, another lens has to be put in to turn them right
way up again. In looking at the sky it does not matter which way up we
see a planet or a star, so the second glass is not needed, and we lose
light by using it.
"We have now three magic glasses to work for us--the magnifying-glass,
the microscope, and the telescope. Besides these, however, we have two
other helpers, if possible even more wonderful. These are the
Photographic camera and the Spectroscope."
[Illustration: Fig. 19.
Photographic camera.
_l_, _l_, Lenses. _s_, _s_, Screen cutting off diverging rays. _c_, _c_,
Sliding box. _p_, _p_, Picture formed.]
"Now that we thoroughly understand the use of lenses, I need scarcely
explain this photographic camera (Fig. 19), for it is clearly an
artificial eye. In place of the _crystalline lens_ (compare with Fig.
11) the photographer uses one, or generally two lenses _l_, _l_, with a
black ledge or stop _s_ between them, which acts like the iris in
cutting off the rays too near the edge of the lens. The dark camera _c_
answers to the _dark chamber_ of the eyeball, and the plate _p_, _p_ at
the back of the chamber, which is made sensitive by chemicals, answers
our _retina_. The box is formed of two parts, sliding one within the
other at _c_, so as to place the plate at a proper distance from the
lens, and then a screw adjusts the focus more exactly by bringing the
front lens back or forward, instead of altering the curve as the
_ciliary muscle_ does in our eye. The difference between the two
instruments is that in our eye the message goes to the brain, and the
image disappears when we turn our eyes away from the object; but in the
camera the waves of light work upon the chemicals, and the image can be
fixed and remain for ever.
"But the camera has at least one weak point. The screen at the back is
not curved like our retina, but must be flat because of printing off the
pictures, and therefore the parts of the photograph near the edge are a
little out of proportion.
"In many ways, however, this photographic eye is a more faithful
observer than our own, and helps us to make more accurate pictures. For
instance, instantaneous photographs have been taken of a galloping
horse, and we find that the movements are very different from what we
thought we saw with our eye, because our retina does not throw off one
impression after another quickly enough to be quite certain we see each
curve truly in succession. Again, the photograph of a face gives minute
curves and lines, lights and shadows, far more perfectly than even the
best artist can see them, and when the picture is magnified we see more
and more details which escaped us before.
"But it is especially when attached to the microscope or the telescope
that the photographic apparatus tells us such marvellous secrets;
giving us, for instance, an accurate picture of the most minute
water-animal quite invisible to the naked eye, so that when we enlarge
the photograph any one can see the beautiful markings, the finest fibre,
or the tiniest granule; or affording us accurate pictures, such as the
one at p. 19 of the face of the moon, and bringing stars into view which
we cannot otherwise see even with the strongest telescope.
"Our own eye has many weaknesses. For example, when we look through the
telescope at the sky we can only fix our attention on one part at once,
and afterwards on another; and the picture which we see in this way, bit
by bit, we must draw as best we can. But if we put a sensitive
photographic plate into the telescope just at the point (_i_, _i_, Fig.
18), where the _image_ of the sky is focused, this plate gives
attention, so to speak, to the whole picture at once, and registers
every point exactly as it is; and this picture can be kept and enlarged
so that every detail can be seen.
"Then, again, if we look at faint stars, they do not grow any brighter
as we look. Each ray sends its message to the brain, and that is all; we
cannot heap them up in our eye, and, indeed, after a time we see less,
because our nerves grow tired. But on a photographic plate in a
telescope, each ray in its turn does a little work upon the chemicals,
and the longer the plate remains, the stronger the picture becomes. When
wet plates were used they could not be left long, but since dry plates
have been invented, with a film of chemically prepared gelatine, they
can be left for hours in the telescope, which is kept by clockwork
accurately opposite to the same objects. In this way thousands of faint
stars, which we cannot see with the strongest telescope, creep into view
as their feeble rays work over and over again on the same spot; and, as
the brighter stars as well as the faint ones are all the time making
their impression stronger, when the plate comes out each one appears in
its proper strength. On the other hand, very bright objects often become
blurred by a long exposure, so that we have sometimes to sacrifice the
clearness of a bright object in order to print faint objects clearly.
"We now come to our last magic glass--the Spectroscope; and the hour has
slipped by so fast that I have very little time left to speak of it. But
this matters less as we have studied it before.[1] I need now only
remind you of some of the facts. You will remember that when we passed
sunlight through a three-sided piece of glass called a prism, we broke
up a ray of white light into a line of beautiful colours gradually
passing from red, through orange, yellow, green, blue, and indigo, to
violet, and that these follow in the same order as we see them in the
rainbow or in the thin film of a soap-bubble. By various experiments we
proved that these colours are separated from each other because the many
waves which make up white light are of different sizes, so that because
the waves, of red light are slow and heavy, they lag behind when bent in
the three-sided glass, while the rapid violet waves are bent more out of
their road and run to the farther end of the line, the other colours
ranging themselves between."
[1] _Fairyland of Science_, Lecture II.; and _Short History of Natural
Science_, chapter xxxiv.
"Now when the light falls through the open window, or through a round
hole or _large_ slit, the images of the hole made by each coloured wave
overlap each other very much, and the colours in the spectrum or
coloured band are crowded together. But when in the spectroscope we pass
the ray of light through a very narrow slit, each coloured image of the
upright slit overlaps the next upright image only very little. By using
several prisms one after the other (see Fig. 21), these upright coloured
lines are separated more and more till we get a very long band or
spectrum. Yet, as you know from our experiments with the light of a
glowing wire or of molten iron, however much you spread out the light
given by a solid or liquid, you can never separate these coloured lines
from each other. It is only when you throw the light of a glowing gas or
vapour into the slit that you get a few bright lines standing out alone.
This is because _all_ the rays of white light are present in glowing
solids and liquids, and they follow each other too closely to be
separated. But a gas, such as glowing hydrogen for example, gives out
only a few separate rays, which, pouring through the slit, throw red,
greenish-blue, and dark blue lines on the screen. Thus you have seen the
double, orange-yellow sodium line (3, Plate I.) which starts out at once
when salt is held in a flame and its light thrown into the spectroscope,
and the red line of potassium vapour under the same treatment; and we
shall observe these again when we study the coloured lights of the sun
and stars."
[Illustration: Fig. 20.
Kirchhoff's spectroscope.
A, The telescope which receives the ray of light through the slit in O.]
[Illustration: Fig. 21.
Passage of rays through the spectroscope.
S, S', Slit through which the light falls on the prisms. 1, 2, 3, 4,
Prisms in which the rays are dispersed more and more. _a_, _b_, Screen
receiving the spectrum, of which the seven principal colours are
marked.]
"We see, then, that the work of our magic glass, the spectroscope, is
simply to sift the waves of light, and that these waves, from their
colour and their position in the long spectrum, actually tell us what
glowing gases have started them on their road. Is not this like magic?
I take a substance made of I know not what; I break it up, and, melting
it in the intense heat of an electric spark, throw its light into the
spectroscope. Then, as I examine this light after it has been spread out
by the prisms, I can actually read by unmistakable lines what metals or
non-metals it contains. Nay, more; when I catch the light of a star, or
even of a faint nebula, in my telescope, and pass it through these
prisms, there, written up on the magic-coloured band, I read off the
gases which are glowing in that star-sun or star-dust billions of miles
away.
"Now, boys, I have let you into the secrets of my five magic
glasses--the magnifying-glass, the microscope, the telescope, the
photographic camera, and the spectroscope. With these and the help of
chemistry you can learn to work all my spells. You can peep into the
mysteries of the life of the tiniest being which moves unseen under your
feet; you can peer into that vast universe, which we can never visit so
long as our bodies hold us down to our little earth; you can make the
unseen stars print their spots of light on the paper you hold in your
hand, by means of light-waves, which left them hundreds of years ago; or
you can sift this light in your spectroscope, and make it tell you what
substances were glowing in that star when they were started on their
road. All this you can do on one condition, namely, that you seek
patiently to know the truth.
"Stories of days long gone by tell us of true magicians and false
magicians, and the good or evil they wrought. Of these I know nothing,
but I do know this, that the value of the spells you can work with my
magic glasses depends entirely upon whether you work patiently,
accurately, and honestly. If you make careless, inaccurate experiments,
and draw hasty conclusions, you will only do bad work, which it may take
others years to undo; but if you question your instruments honestly and
carefully, they will answer truly and faithfully. You may make many
mistakes, but one experiment will correct the other; and while you are
storing up in your own mind knowledge which lifts you far above this
little world, or enables you to look deep below the outward surface of
life, you may add your little group of facts to the general store, and
help to pave the way to such grand discoveries as those of Newton in
astronomy, Bunsen and Kirchhoff in spectrum analysis, and Darwin in the
world of life."