Part 17

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?

Now I will add certain other lines to the same figure. Are the original lines still straight and the same distance apart everywhere?

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.

Then turn them up side down and see what they really are.

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.

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.

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!