“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.

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.

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.

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.

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.

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!

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.

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.

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.