with Chloe and the Nurb.

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Nurb, just because eyeballs are the size

of ping pong balls doesn't mean they make good ping pong balls.

A nurb cannot be blamed for his love of scientific exploration,

my dear Chloe.

It is what makes him a nurb.

Then can we use the eyeballs to explore

how an eyeball works instead?

We could, but these are kind of small and--

Squished.

One might say that.

How about we take a look at him?

Most excellent idea.

Let's do.

The eyeball is a beautiful machine

with lots of different parts working together

to let you see.

Poets say the eyes are the window to the soul.

Well, the window to the eyeball is the cornea,

a dome of clear tissue up in front of the eye

that focuses light as it passes through.

Look at that beautiful green eye.

And brown eye.

And blue eye.

The colorful part is called the iris, right?

Yup.

It's right behind the cornea.

In the middle of the iris is a black circle called

the pupil, an opening that lets light into the eye.

The iris has muscles attached to it

that change its size, making the pupil bigger and smaller

to control how much light gets through.

So the pupil gets smaller when there's a lot of light

and bigger when it's dimmer.

Don't look now, but I think we're being watched.

Hmm.

Ha!

He blinked first.

Which is a good thing.

Blinking protects and moistens the eye.

Good point.

So what happens after the light has

passed through the cornea and the pupil?

The light passes through the lens.

Like the lens in a camera?

Precisely.

The lens focuses the light onto the back of the eye, where

seeing really starts to happen.

Can the lens in the eye focus on stuff

that's close and stuff that's far, like a camera lens would?

It sure can.

Let's head inside to see how.

Last one through the pupil's a rotten egg!

The lens is held in place by a bunch

of fibers, which are attached to this ciliary muscles.

(SINGING) Ciliary!

Ciliary!

Ciliary!

The ciliary muscles change the shape

of the lens to let the eye change its focus from something

close by to something far away.

What are you waiting for?

Let's get focusing!

To see something near, the ciliary muscle

makes the lens the thicker.

To see something far, the ciliary muscles

makes the lens thinner.

From the lens, we travel to the retina,

the back wall of the eyeball.

Right, because the lens focuses the light onto the retina.

The retina has millions of light sensitive cells called

rods and cones-- about 120 million rods and 7

million cones in each eye.

Whoa.

That's a lot of rods and cones.

What's the difference between them?

It's the difference between black and white and color!

The rods see in black, white, and shades of gray,

and help us see the shape and form of a thing.

Rods also help us see in the dark.

And the cones see color?

Cones are sensitive to one of three colors-- red, green,

or blue.

Together, they let us see millions of colors,

but cones need more light than rods to work with.

Hey, what's this thing behind the retina?

Hey, no bouncing on the optic nerve.

It carries messages to the brain about what you're seeing.

The rods and cones change the colors

and shapes you see into millions of nerve messages.

Then those messages are carried along the optic nerve

to the brain.

It's like your eye is sending the brain

a report on what you're seeing.

Then your brain translates the report

into cat, apple, or bicycle.

Or-- a ping pong ball!

Hm?

Keep your eye on the ball there, Nurb.

Oh!

Ha ha, it's on.

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