Natural Wonders

by Edwin Tenney Brewster

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.