Seeing Is Believing When It Comes To Blind Spots

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A while back I was talking about microscopes and telescopes, and I happened to mention that all my life it “seemed that there was something I wanted to get a look at that human eyes just weren’t quite good enough for.”

I ran across that column this morning and happened to catch sight of that sentence. Something about what I had said bothered me, but I didn’t know what it was. Then it dawned on me: That comment makes it sound like I don’t think much of the human eye.

Talk about something that isn’t true!

I can’t think of anything we are blessed with that is more important than sight, or the pair of organs that enable it. Sight is the most wonderful thing we have. Compared to the other four senses — taste, touch, smell and hearing — it stands alone in its ability to put us in contact with the world around us.

I would hate to lose any of the other four senses. The world would be surely a lesser place without them. But sight? Think how great sight is. Of all the senses it is the only one that lets us come in touch with something at a distance. Our whole concept of the universe depends on vision. What a marvelous thing it is!

And so, since it works so well and does so much for us, it’s easy to assume that the human eye is not only a great marvel, but one of the best-engineered parts of our bodies.

Wel-l-l-l, not quite.

I’m going to tell you something about our eyes, your’s and mine, which is not only fun to know, and fun to fool around with, but is so atrociously unlikely it will knock your socks clean off.

Suppose you were designing an eye. How would you make it? Suppose someone gave you the parts and said, “Here, Johnny. Make an eye out of this stuff.”

What would you do?

I’ll guarantee you wouldn’t come up with something that works the way our eyes do. It just wouldn’t happen.

Want to know why I’m so sure?

Easy. Basically, an eye is like a camera. A lens up front to focus the light. A retina in back to receive it. The retina has light receptors. You know, the rods and cones. The rods and cones are connected to nerve endings.

Light strikes a rod or cone and a signal speeds down a nerve on its way to the brain.

Pretty straightforward, yes?

No.

There’s a slight glitch, you see.

If you were making an eye, where would you run the nerves? You’d hook them to the tail ends of the rods and cones, right? At the back of the retina. And run from there to the brain.

But that isn’t what happens.

This is truly incredible. Next time you go to the eye doctor ask him to show you your optic nerve. He can do it, and you’ll enjoy the sight. He just has to illuminate your eye in a special way so you can see a reflected image of your optic nerve.

But how can you see a reflection of something that’s in back of your eye, behind your retina?

You can’t. And here’s the incredible thing about all this: The nerves in your eyes are not connected to the back of your retina, they run right across the surface of it.

That’s right. Right across the surface of the retina.

Hey! If some engineer were making a television tube or a camera, and he ran the wires across the light receptors he’d be out on the street looking for a new job the next day. On the other hand, I don’t suggest you start casting aspersions on the engineer who designed your eyeballs. He might not like it.

A word to the wise ...

“OK,” you may be saying, “so the nerves in my eye run right across my retina. So what?”

So what is this: How does the optic nerve get through the retina? By induction? By a radio signal? By some kind of magic?

Uh-uh. There’s a hole back there that it passes through.

I know what you’re thinking. “Nuts! If there’s a hole in the back of my eye, a place where the optic nerve passes through the retina I ought to be blind in the place where it goes through.”

I hope you’re thinking that. Because you’re right!

That’s right. You have a blind spot in each of your eyes.

So how come you don’t see it? Three reasons.

One is that the blind spot is in a different place in each eye, and the images from your eyes are processed in your brain and overlapped. The blind spot’s there, but you just don’t notice it.

The second reason is that even if you only have one eye open it moves around all the time to keep the blind spot moving so you don’t notice it.

The third reason is more fun: You don’t see the blind spot because you never tried to see it.

So-o-o-o ...

What do you say? Want to try it?

Do this: Take any old piece of blank paper. Draw a pair of spots on it.

Make them about a quarter of an inch in diameter. Draw them about six inches apart.

Sit down at a desk or table. Put the paper on the table. Close your left eye. Look straight at the left hand dot with your right eye. Lower your head toward the table, being sure you keep staring at the left hand dot. When you get about a foot or 15 inches from the table the right hand dot will disappear. Why? It’s right below the blind spot in your right eye. If this doesn’t work for you it just means that you have to tilt the dots a tiny bit because the blind spot is a little higher or lower in your eye.

Try it now.

Ain’t that a gas!

If you try it with your left eye, just close your right eye, look at the right hand dot, and the left dot will disappear.

You can get very good at this, doing it with two fingers in midair, or by staring at a dot and moving the tip of a pencil in and out of the blind spot.

Isn’t that amazing? There it is. Something that has been right in front of your eyes all your life, and yet you’ve never seen it before. How unlikely is that?

Now next week ...

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