SOME OPTICAL ILLUSIONS
WHEN SEEING EYES ARE BLIND
“BUT, I tell you, I saw it; surely I can trust my own eyes!”
How often have we heard this uttered as a conclusive proof of some
friend’s statement!
And really at first it would seem to be an assertion admitting of no
further question, were it not for the fact that we know our eyes are no
more infallible than anything else in this world, and are quite as
liable to make mistakes as are our memories.
It is true that eyes are good and faithful servants, fit to be trusted
in ninety-nine cases out of a hundred, but like all good and faithful
servants there is that hundredth case when their judgment goes
wandering, and when they leap to rash conclusions, carried away by
deceptive appearances.
Strange as it may seem, upon certain occasions, the best eyes are
actually blind! If you shut one eye and hold the page with Fig. 1 at
arm’s length, you will be able to see both the spots A and B. Now look
steadily at A, and you will still see B quite plainly, but if you
gradually draw the book nearer to your eye, a certain point will be
reached when B becomes invisible, although if you continue to make the
book approach your face B will spring into view once more. In other
words, at the moment when you could no longer see B your blind spot had
been directed towards it, and of course saw nothing.
[Illustration: FIG. 1.--When two are one.]
[Illustration: FIG. 2.--Section of the eye.]
No doubt you would like to know where this blind spot is, and why our
eyes should possess such a thing. Fig. 2 shows the section of an eye
which can be explained in very simple terms. The thick black line A is a
sheet of nerves which entirely envelops three-quarters of the eye, and
meeting in a point at E passes upwards into the brain, where it records
what the eye has seen. The light enters between the points C C, the
iris, and striking through the lens B throws all objects within the
scope of vision upon what is called the retina or screen, D. Now this
screen is furnished with millions of little nerves, each one of which
records on the large nerve A whatever is thrown upon it, and all these
records are gathered together by A and passed up to the brain.
But at the spot E, where these big nerves are collected together, the
retina, as you notice, is pointed, and gives no record of what is thrown
upon it. So, you see, when any object happens to come into such a
position with the eye that its image is cast upon the point E of the
retina, we have no record sent to the brain--in other words, we cannot
see it.
But the eye is not only blind in one point; it is very apt to be
deceived by appearances, and to make all kinds of mistakes in
consequence. Take Fig. 3 for instance. Would you not say that B D is
shorter than A C? Yet if you measure them you will find they are the
same length. Or in Fig. 4, A B is surely longer than C D. They are
identical. Or take Fig. 5, A is clearly farther from B than C is from B,
and yet A B and B C are of the same length.
[Illustration: FIG. 3.--Is A C longer than B D?]
The truth is that your eye is so confused by these different lines that
it is wholly unable to form any clear estimate of how great the
distances really are. This is shown even more clearly in Fig. 6
(technically known as Zollner’s lines), where you see A B and C D, which
have every appearance of being about to meet shortly in the direction
of A C. Now if you will measure the distances between B D and A C you
will find that the lines are exactly parallel, but the eye has been so
deceived by the little cross lines running in different directions, that
it seems incredible the two thick lines are not inclined towards one
another at quite a considerable angle.
[Illustration: FIG. 4.--Which is longer--A B or C D?]
[Illustration: FIG. 5.--The distance from A to B is the same as B to C.]
[Illustration: FIG. 6.--Zollner’s lines.]
HILLS THAT DON’T RISE
Should it ever happen that you go cycling in France, you will find this
deception practiced upon your eyes all day long. The roads in that
country are very straight, and are bordered upon either side by tall
trees, so that from wherever you stand a long avenue stretches before
you to a point where the trees seem to merge into one another, as
parallel lines invariably appear to do. But flat as the country may be,
you will always find yourself confronted with a gentle incline, as it
seems, very slight but none the less perceptible. You brace for a long
and steady climb, yet somehow, as you cover the ground, the hill seems
always before you and yet there is no noticeable ascent. The reason is
simple. There _is_ no ascent. The borders of trees, like the little
lines in Fig. 6, deceive the eyes in a similar way until it is almost
impossible to believe that the hill is merely an optical illusion, and
that the road is flat as the proverbial pancake.
There is another trick the eye is very fond of playing us. A straight
line, held on a level with the eye appears very much shorter than it
really is. Look at Fig. 7, which appears to represent a number of pins
lying with their points towards you. Now lift the book to the level of
the eyes, close the right one, and they will appear to be sticking
upright in the page.
What a jumble of lines there is in Fig. 8, something like a spider’s
web, and one can make nothing out of it. But lift the book up, as in the
last example, and close one eye--the letters are plain enough, are they
not? You have played a trick on your own eye, and made its habit of
shortening lines serve to interpret a message that would otherwise be
unintelligible.
[Illustration: FIG. 7.--The standing pins.]
[Illustration: FIG. 8.--“Yes or no?”]
THE STARS DON’T TWINKLE
Every cloudless night the eyes make a mistake that we can easily
discover, but which we are totally unable to remedy.
Of course you have looked up to the sky thousands of times and seen the
stars twinkling. Not only that, but if the night is clear you can see
they are stellate, or star-shaped, like the starfish which is named
after them. You can see both of these things, and yet the strange fact
is that neither of them is true!
The stars do not twinkle at all, and they are not stellate. The
twinkling is the result of the intervening atmosphere, and not the fault
of our eyes; but the second error can be easily brought home to our
untrustworthy organs of vision by the following experiment.
[Illustration: FIG. 9.--The illusion of the stars.]
Take a piece of tinfoil and prick a small hole with the point of a pin.
Now when it is dark put a candle behind the tinfoil in such a way that
the light comes through the tiny hole. Hold the tinfoil about ten inches
from your face, and the hole will appear irregular. If you bring it
nearer, it will lose even the least resemblance to a hole and appear as
a star! Of course you know perfectly well that it is round, but your
eyes have deceived you once more in the same way that they deceive you
every starlight night, and the little hole looks something like Fig.
9--varying slightly with each individual observer. This deception, or to
put it charitably, this mistake of the eyes, is given the very
high-sounding name of “irregular astigmatism,” but for all that it is an
illusion pure and simple.
Like many well-trained servants, the eyes are quite at a loss if
anything contrary to the usual routine is presented to them. They know
perfectly well the laws of perspective,--how in the ordinary course of
nature these laws are never broken by a hairbreadth. They are therefore
accustomed to judge in the fraction of an instant the size of an object
by its apparent distance away. That this is the result of practice can
be easily seen from the fact that very young creatures--human and
otherwise--have no idea of the relative distances of objects, and strain
to touch a distant gas-light, or, like a young calf, rush headlong into
a neighboring wall which their green young fancy deludes them into
thinking is really some distance away. But as we grow older we learn
many things, and perspective amongst others.
THE DWARF, THE MAN, AND THE GIANT
Now if we make a drawing such as Fig. 10, which represents three men
walking down a passage, our eyes know quite well that if all these men
were of the same size, Mr. Jones in front would appear smaller than Mr.
Smith behind him. And Mr. Smith in his turn would appear smaller than
Brown who closes the procession.
Yet in our illustration Jones appears a veritable giant, towering above
Smith and making Brown appear a mere pigmy. If you measure them, you
will find they are all three the same size.
The reason of the deception is this. The lines showing the passage
disappearing into the far distance immediately suggest to the eye the
correct perspective, and, knowing the laws of that perspective, the eye
is perfectly convinced that if all three were the same size, Brown in
the rear would appear proportionately bigger than Jones. As he does not
do so, the eye immediately leaps to the conclusion that he must be very
much smaller. It therefore telegraphs to the brain that Brown is a
dwarf, following in the tracks of an ordinary man and a giant!
[Illustration: FIG. 10.--The dwarf, the man, and the giant.]
COLOR ILLUSIONS
Most of us know the result of turning a series of circles (as in Fig.
11) horizontally with the eye. The circles appear to revolve rapidly
round their center, and in different directions. This is solely because
the eyes become confused, giving one more proof, were it needed, that
they are no more infallible than anything else on this wide earth.
Some very interesting experiments in color illusions can be made. So
cunning is the deception played upon us by our eyes, it is extremely
difficult to believe that some of the tints we see in the experiments
are but imaginary.
[Illustration: FIG. 11.--Revolving circles.]
[Illustration: FIG. 12.--The color top.]
In Fig. 12 you see a top which can be constructed of cardboard in this
way. Take a postcard and cut a circle, upon which you draw a diameter as
A B. Black the part A C B with India ink, and divide the other half into
four equal portions by lightly penciling the radii G E, G D, and G F.
Now, still using your India ink, make arcs in these four divisions in
the same way as is shown in the figure.
[Illustration: FIG. 13.--To make black print red.]
Having done this carefully and rubbed out the pencil radii when the ink
has dried, put a pin through the center G from the back, so that the
card can easily revolve whilst the pin-head prevents it from falling
off. Your color top is now ready. Make it turn rapidly upon the pin;
look closely at the card and what do you see? The inner circles become
red and the outer ones blue! And yet you know perfectly well that the
only colors really upon the card are black and white!
You can make another top, after a similar fashion. Cut your postcard as
before, making one half of it black. Now, out of the white side, cut a
segment with an angle of 45 degrees, leaving a little piece near the
center as shown in Fig. 13. This piece you have left has nothing to do
with the effect, but is simply to make the top revolve better.
Take an ordinary book, of which the printing is presumably black, and
revolve the top upon the pin at the rate of about five turns a second (a
sharp twitch with the finger will do this perfectly well). If you look
at the printing now you will find the letters are colored red, as though
the book had been printed in red ink!
In both these experiments the alternation of black and white has not
only confused the eye, but has deceived it into seeing colors which do
not really exist.
So we have shown very conclusively that the old proverb, “all is not
gold that glitters,” can be applied to even the plainest of black and
white; and, as the poet remarks, “things are not what they seem.”
Therefore, when people wish to impress you with the evidence of their
own eyes and clinch an argument by stating that they saw such and such a
thing and cannot be wrong, show some of these optical illusions and
demand a better proof of what they affirm, very courteously assuring the
dogmatist that the best of eyes are liable to make mistakes.