buds out from the side of the brain, and then doubles in to form a
cup; and how this cup becomes at length the nervous portion of the
eye, the retina, which therefore, tho it lines the eyeball, is
really part of the brain; and how this retina somehow or other, in a
way that nobody understands, picks up the image of the things we
see, and sends it along the optic nerves to the part of the brain
which lies above the ear and round toward the back of the head. I
think you know also how these optic nerves cross over, just as most
of the other nerves do, so that the left eye reports to the right
side of the brain, and the right eye reports to the left side. You
know also how, in the end, both these reports get turned over to the
left side of the brain, and remembered there; so that while we see
with both sides of the brain, we remember what we have seen with one
side only.
Aside from these matters, there are various little points about the
eye which one can make out pretty well for himself. One of these is
the reflections from the front of the eye. You know, if you look
into a window in the day time, or try to look out of a window after
dark, or look into a glass tumbler, or at the face of a watch, or in
general, look at a glassy surface or at water, when it is lighter on
your side than it is on the other, instead of seeing thru quite
clearly, you see reflections from your side.
It is, naturally, the same with the glassy front of the eye. Look
into another person’s eye, or into your own with a mirror, and you
see reflections of windows, lamps, your own head, or any bright
objects. You ought to be able to find three reflections of each
bright spot. The largest, which is always right side up, is the
reflection from the clear glassy front of the eye which covers the
entire colored part from which we call our eyes blue or brown or
gray or what not. If you look carefully, a little sideways, you will
be able to get a still smaller picture, coming from the front
surface of the lens of the eye, which lies just behind the round
black hole in the center of the colored curtain. This also is always
right side up. But there is still another, always up side down,
which is the reflection from the back side of this same eye lens.
These last two, you can get also by looking at a common spectacle
lens, or by looking into the front of a camera.
For of course, the eye is really a little living camera. It takes a
little picture like that in a camera, always upside down, at the
back where the plate holder or the spool of films goes in a kodac.
We can actually see this picture at the back of an animal’s eye; and
what is more, people have sometimes taken out the lense of an ox’s
eye, and taken a photograph with it as if it were a lens of glass.
Indeed it is possible, tho the process is decidedly difficult, to
take an ox’s eye from the butcher’s shop, keep it in the dark, let
it look quickly at something bright, and then by treating it with
the proper chemicals, actually to fix on the retina, as on a camera
plate or film, the last object which the eye saw. There is a dark
pigment in the retina, called the visual purple, which changes color
in the light, and so forms the image.
But how this image or picture gets to the mind is another question;
a question, I am sorry to say, which nobody can altogether answer.
We do know, however, that there are nerve endings in the retina,
something like hot spots, cold spots, touch spots, and pain spots in
the skin, only of course very much nearer together. Probably there
are three kinds of these—red spots, green spots, and blue spots.
Each spot sees one color; and by combining these colors in all sorts
of ways, we build up the complicated pictures which we see. Still it
is by no means impossible that there may be, not three, but six
elements in our eye-pictures—white, black, red, yellow, green, and
blue. Nobody really knows; and it all shows how little, after all,
we have succeeded in finding out about ourselves, in spite of whole
lifetimes of study of many hundreds of scientific men. Who knows but
that some of you who read these pages may be the ones to discover
some of these things which all the world thus far has not been able
to learn.
There are still other curious facts about our sight which anybody
can make out for himself. If you take any colored object, this book
for example, put it behind your head; and then slowly bring it round
in front, while you keep your eyes looking out steadily straight
forward, you will notice certain very peculiar facts. In the first
place, you will discover that you can see surprisingly far toward
the back of your head. A horse can see all the way round, and if he
did not wear blinders, could watch the people in the carriage behind
and the road which stretches out in front, along with everything in
between, all at once and about equally well. Many animals, in short,
can see clear round their heads. We can’t; we can see only about
half way round.
Then you will notice that you can see that something is there and
moving, while it is still so far round to the side that you cannot
at all make out either its shape or its color. Furthermore, you can
see the color perfectly well, long before you can make out the exact
shape. Indeed, you can make out the shape of ordinary letters well
enough to read them only when you hold them exactly in front of the
eye. The least little movement out of that one small spot mixes a
whole page to a gray blur; curiously too, you can make out blue and
green decidedly farther round toward the corner of the eye than you
can tell red.
In short, then, we can see movement considerably farther round
toward the backs of our heads than we can see color. We can see blue
farther round than we can see green and green farther round than we
can see red. But we cannot see shapes accurately, except right in
front of our noses.
Now curiously enough, all animals that can see at all, can see
something moving; tho they cannot see colors at all perfectly, nor
make out the shape of anything. Many lowly sea creatures have eyes
of this sort. A better kind of eye, like those of many insects, can
see colors, but not make out much about shapes; while certain ants
can see blue and green but are blind to red. Few indeed are the
creatures that can see anything like as clearly as we see, looking
hard at an object straight in front. Even a dog cannot do it, nor a
horse.
XXVII
Seeing and Believing
Even we ourselves, we human beings, by no means always see so truly
as we think we do. Take a look at this figure and say which of the
two lines in the south-west corner continues the single line in the
north-east corner. Then lay on a ruler or a strip of paper, and see
which line really does run clear across the figure.
[Illustration]
Or look at the figure below, and say which way the curved lines
bend. Then take a straight-edge and test them.
[Illustration]
Where, in this next figure, are the white bands widest, in the
middle or at the ends?
[Illustration]
Lay two strips of paper along the sides of a white band, one on each
side, so as to cover up the slanting lines. Where now is the band
widest?
Here is a square with some lines drawn across it. Are the lines
straight? Are they the same distance apart everywhere?
[Illustration]
Now I will add certain other lines to the same figure. Are the
original lines still straight and the same distance apart
everywhere?
[Illustration]
Or look at the B’s, H’s, S’s, 3’s, and 8’s on this page, that look
about the same size at top and bottom.
[Illustration]
Then turn them up side down and see what they really are.
[Illustration]
Or to take one more curious illusion, the lines of the figure on the
next page are really flat on the paper, where the printer put them.
But there is a point near the bottom of the page, about as far from
the line nearest the bottom as that is from the ones nearest the
top, where if you cover one eye and look at the lines with the
other, each line will appear to stand straight up from the paper
like a little post.
[Illustration]
Or possibly you think your eyes always report correctly concerning
colors? Then try looking at a landscape, with your head up side
down, so that the view appears under your arm or between your knees.
Are the colors the same as before. If not which is right?
Or try this experiment, take some brightly colored object—paper,
cloth, or almost anything—in size anywhere between one and four
inches across, lay it on a sheet of white paper, put the two in a
strong light, and getting arm’s length or more away, stare steadily
at the colored object for a half minute or so, until the eyes begin
to tire. Then whisk away the colored object, continue looking at the
same place, and notice what you see on the white paper, where
nothing is. Or you can do what is really much the same thing, by
looking at a window up against the bright sky, and after a moment
turning away and shutting your eyes.
In all such experiments, one sees the outline of something that
isn’t there, but in a contrasting color. We have, as you will
recall, at least three sorts of color spots in the retina, red
spots, blue spots, and green spots. By looking at a bright red we
tire the red-seeing spots, so that everything looks blue-green. If
we look hard at bright green, we tire the green-seeing nerves, and
things look red-blue, which is purple. An eye tired of blue, sees
yellow.
The curious thing about this is that about one man in thirty and one
woman in three hundred is “born tired” to red. Such persons are said
to be red blind. Otherwise they can see as well as anybody; but red
things do not look colored at all. None of us can see red far round
to the side out of the corner of the eye, as well as we can see
green and blue. Color-blind persons have the corner of the eye all
the way across, and cannot see red anywhere. They can see red
things; but they cannot see them red. Railway train men and masters
and pilots of vessels have to depend on red and green lights for
signals. Such persons, now-a-days, are carefully tested for
color-blindness; and all who cannot see red as the rest of us see it
have to find some other occupation.
Why do we have two eyes? We can see outlines exactly as well with
one; in fact, all the more difficult sorts of seeing, sighting a
gun, using a microscope or telescope are done entirely with one eye.
We can see colors exactly as well with one eye as with two. The only
thing that we can’t see well with one eye is distance.
Try with both eyes open to put your finger rapidly on various spots
arm’s length or so away. You can hit the mark every time. Now cover
one eye—always when you want to use one eye, don’t shut the other;
cover it, but keep it open. Also, by the way, if you are to use a
microscope or a gun, don’t shut either eye; learn to keep both open,
but to look with only one. With one eye only, then, try to put your
finger rapidly on various points which you did not look at until
after you had shut off the sight of the other eye. You can do it,
but much less quickly and certainly than with both eyes. The nearer
anything is, the more do the two eyes have to turn in, when both
look at it at once. After eight or ten years of practice, as most of
us have had, we learn to judge distances pretty accurately, just by
the feel of this turning in.
All these peculiarities of our eyes, the judgment of distance, the
different portions of the retina which see most clearly and which
see colors, the various ways in which the eye is deceived, and the
like, all these you can make out pretty easily for yourselves. There
remains, however, one especially curious matter which you will
hardly be able to discover, unless you take some little pains and
follow directions pretty closely.
This is the so-called blind spot. We have, as you have seen, in each
eye, a small spot in the center of the field of vision where the
sight is especially sharp. This is, in fact, the only part of the
eye that we can see to read with. Near this, between it and the
nose, is another spot, about the same size, with which we cannot see
at all.
We can prove this from the figure below.
[Illustration]
Hold the page about a foot in front of the face, as if you were
reading. Cover the right eye and look at the cross on the right, or
cover the left eye and look at the dot on the left. Keep looking
steadily but without too much effort, while you move the book back
and forth, bringing it nearer to the face, or trying it farther
away.
You should be able to find a distance at which the other mark, the
one at which you are not looking, entirely disappears. You can see
all round it, but the place itself is on the blind spot and is gone.
With some practice, one can make anything, not too large, disappear
in the blind spot. Boys in college, when they are studying about the
eye, sometimes amuse themselves in church by getting the clergyman
on the blind spot, and so blotting him out. It really is queer
enough. You cover one eye, and look with the other at the wall
behind the preacher a little toward the side on which your own nose
is. When you get just the right point to look at, the man simply
disappears. You see the wall and the pulpit and the chairs or what
not, on both sides. You hear the preacher’s voice. But the preacher
is gone. You don’t even see a black spot where he was. Or if you are
clever, you can cut off his head and leave his body; or cut off his
body and leave his head hanging in the air.
All this, however, requires more control over the body and more
steadiness of attention than boys and girls usually possess. I
should not have told you anything about the wicked students if I
were not sure that you will have forgotten all about the matter long
before you get old enough to try it.
Meanwhile, don’t forget that as there is a blind spot in each of
your eyes with which you simply cannot see what you know is there
all the while, so there are many other things in heaven and earth
which you cannot see, though they are there. Then don’t be too
certain, when you happen to be blind to what other people see, that
the people who do see are mistaken.
XXVIII
Some Other Senses
We really know far less about hearing than about sight. The eye is
where we can get at it, to look inside, and to see how it works. But
the ear, the inner ear that is, where the hearing is done, is set
deep inside the head, in the midst of a solid bone, the hardest
piece of bone in the whole body except the teeth. Nobody, therefore,
really understands how we hear.
We do know, however, that the ear is two different things. One part
of it is the organ of hearing—I am speaking always of the inner
ear—while another part, as I have already explained, is the
organ of the sense of equilibrium, the feeling of direction and
right-side-up-ness. But just how much of the ear goes for hearing,
and how much for right-side-up-ness, and exactly how either part
works, and especially just how we tell one sound from another, are
things that are still left for somebody to find out.
Nor do we know much more about smell. We know that the smelling is
done in the upper part of the nose, that the nerves of smell do not
cross over and report to opposite sides of the brain, as so many
other nerves do. But how we tell one odor from another, nobody
understands; and we are even farther away from understanding than we
are in the case of the eye.
We do, however, understand taste. At least we understand as much
about it as we do about sight; for the two senses are much alike.
There are four kinds of taste spots, scattered over the tongue and
the inside of the mouth, mostly on the tongue. Each of these gets
one kind of taste—salt, bitter, sour, or sweet. Oddly enough,
different people have these four sorts bunched in different parts of
the tongue, so that not all people taste the same thing in quite the
same place.
But you will say at once, we taste many things that are neither
sweet, sour, salt, nor bitter; there must be many more than four
tastes. There are not. What we commonly call tastes are really
smells. We smell things that are in the mouth, and think we taste
them.
If you don’t believe this, simply hold your nose. It is an old trick
to get somebody to close his nose tightly, and then while all sense
of smell is thus cut off, to bring into the room a piece of raw
onion, put it in the victim’s mouth, and ask him to guess what it
is. If the onion is not brought into the room until after his nose
is shut off, he cannot tell what it is that he is eating. For the
onion has almost no taste. But the moment one lets go his nose—then
he knows! There is no doubt that the onion has smell—enough and to
spare!
Almost any of the senses can be fooled. Put your finger on you
forehead; then move your head slowly from side to side so that the
finger, held motionless, slides over the skin. Your muscular sense
and your sense of equilibrium both testify that the head is moving
and the finger is still. Yet you can’t make yourself believe it. It
insists on feeling as if the head were still, and the finger moving.
Or try the senses of heat and cold. Take three dishes, one of hot
water, one of cold, and one of a mixture of the two that shall feel
neither warm nor cold but tepid. Put the fingers of one hand in the
hot water, and the fingers of the other hand in the cold water. Keep
them there a minute; then put them both in the tepid water. The
tepid water will feel hot to one hand and cold to the other. Really
it isn’t either. Perhaps, too, you have noticed, when you go in
bathing, that as you wade in, you feel the cold only at the surface
of the water where the skin was last wet. Hence the wisdom of going
in all over at once with a header.
Even the sense of touch, in general the most reliable of the senses,
can be deceived. When you are fishing and get a bite, where do you
feel it? Most fisherman feel the bite at the end of the line, as if
their nerves actually ran the length of the rod and down the string
to the hook! And when a ball player cracks out a long hit, I leave
it to all boys, if he doesn’t feel the place in the bat where it
hits the ball. Or to take a commoner example, when you touch your
hair, where do you feel the touch? In the hair itself where there is
no feeling at all, or in the scalp where it really is? Or once more,
if you hold the point of your nose between two fingers you feel one
nose; but if you cross the fingers, and then touch your nose between
the crossed parts, then you feel two noses.
Still on the whole, our senses are pretty reliable. The eleven
different sorts of feeling spots in eye, tongue, and skin, that tell
us about red, green, blue, heat, pressure, cold, pain, sweet, sour,
bitter, or salt, and the ears and nose which we don’t know so much
about, all these tell us, on the whole, the truth. Yet we never can
be quite sure; so that wise people, and especially wise boys and
girls, will beware of contradicting other people who chance to see,
hear, taste, smell, feel or believe, a little differently from
themselves.
XXIX
The Sight and Hearing Of Ants
So much then for our own senses, our sight, hearing, taste, pain,
and the rest of the ten, or as many more as one thinks it worth
while to count. The animals also have their senses, never
apparently, more than ours, oftentimes fewer, sometimes very few
indeed. So far as they have senses, these are like our own. But
since some animals haven’t any eyes, yet can see—a little; and some
haven’t any noses, yet can smell; and most of them haven’t any
skins, yet can feel; one may easily guess that their seeing and
smelling and feeling is not done quite in the same way that ours is.
I begin, then, with an animal that has eyes and can see, has no nose
and can smell, and does its hearing with its legs. This animal is
the ant. Of course, there are a great many different kinds of ants,
as there are a great many different kinds of human beings, and these
are by no means all alike. Some are black, some white, some yellow.
Some are, for size, like the smallest letters on this page; some are
more than an inch in length—and you can imagine their bite!
Naturally also, sight and hearing, taste and smell, are not quite
the same in them all.
Time would fail me to tell one half the strange ways of these
interesting creatures, the most interesting creatures, probably, in
all the world of little animals. Just as soon as you can, you must
get hold of the books of Fabre, M’Cook, Sir John Lubbock, or
Professor Wheeler, and read these strange things for yourselves—how
the ants live in cities underground, have workmen and soldiers,
carry on wars against their neighbors, raid their enemies’ nests and
make slaves of the captives, have plant-lice for cows, and milk them
of their sweet juice, and in return for this, feed and care for the
plant-lice and their young, pasturing them on the roots of plants,
and making no end of trouble for the farmers whose plants they are.
All this, I say, and many times more, no less fascinating, you can
read for yourselves in the proper books, not only about ants, but
about their cousins the wasps and bees as well. Just now, however,
we are concerned with how much the ant knows, and how he manages to
find it out.
Ants, in general, you must remember, live for the most part in total
darkness under ground. The workers, to be sure, leave the nest in
search of food, but the industries of the ant city, the storage of
food, the care of eggs and young, and the building of the city
itself, go on as if at the bottom of a mine. The queen ants, which
lay all the eggs for the colony, and the male ants, who like the
drone bees are gentlemen of leisure and don’t do much but loaf, are
for most of their lives like the vine tendrils which I have already
told you about. Whenever the light falls on them, they turn their
heads down stream to the ray; and so if they move at all, they have
to go toward the dark.
This, of course, holds them prisoners in the nest. But when at
certain times of the year, a new brood of males and females appears,
these ants, which, unlike the workers, have wings, suddenly become
like the leaves and stems of plants; they have to head toward the
light, and when they crawl or fly, they have to fly toward it. So
when the rays of the sun happen to strike the nest, and light up the
interior, out comes the swarm of winged males and females, leaving
the wingless workers behind. Away they fly toward the sunlight; and
those who are fortunate enough to find a suitable spot unhook their
wings, settle down to found a new colony and a new nest. Thereupon,
for the remainder of their lives, they turn their backs on the light
like a tree root. The rest, however, die, after they have lost their
wings, so that one sometimes finds great quantities of these
scattered about after the swarming.
The workers, on the other hand, who have to be in and out of the
nest about their business, do not have this tropism. They can take
the light sidewise, or end on, or any other way, just as we can. The
object of the tropism is to keep the males and females in the nest
until swarming time, and then to get them out. Really, could there
be invented a simpler or more effective way?
The worker ants can see. What is more, they can see colors.
Nevertheless, they do not usually see quite the same colors that we
see. For the most part they are red-blind, just as one man in thirty
is. But unlike the color-blind human being, many ants make up for
this red-blindness by seeing one or two other colors to which we are
blind.
Of course, you know the colors of the rainbow, beginning with red at
the bottom and running up thru orange, yellow, and green to blue at
the end. You see the same colors also in a dew drop, or in the light
which has come thru the corner of a square ink-well or the beveled
corner of a mirror. These are the so-called primary colors, by
mixing which all other colors can be made.
Now we, ourselves, do not all see the same rainbow colors. The great
Sir Isaac Newton, who made a special study of rainbow colors and
gave them their names, claimed to see seven—red, orange, yellow,
green, blue, indigo, and violet. I myself can see only six; that is
to say, I see only two colors beyond the green. More persons,
apparently, see six than seven. Try it for yourselves and see how
many you see.