Today, though, around half of the world's kids have  eyeballs that are too long.

然而，世界上大約有一半的兒童眼球太長，是以，他們的眼睛也太小、

And as a result, they have blurry vision.

他們視力模糊。歡迎來到 MinuteEarth。

Welcome to MinuteEarth. 

Most babies are born with short eyeballs.

大多數嬰兒出生時眼球較短。但隨著嬰兒的成長，他們的眼球會變長--晶狀體

But as  a baby grows, their eyeballs get longer - the lens  and the retina get farther and farther apart.

By  the time the kid is about 6, their eyeballs are  just the right length for the lens to be able  to focus incoming light and form a crisp image  right on the retina, rather than focusing well  behind it.

和視網膜的距離越來越遠。到孩子 6 歲左右，他們的眼球已經恰到好處的長度，使鏡頭能夠聚焦入射光，形成清晰的影像

At that point, the brain sends a signal  to the eyeballs, telling them to stop growing.

就在視網膜上，而不是聚焦在視網膜後面；此時，大腦會發出信號

But, starting a few decades ago, many kids like,  more than 90% of kids in some countries, these kids' eyeballs continued to lengthen  well past that spot.

告訴他們不要再長了。但是，從幾十年前開始，許多孩子--比如：在一些國家，超過 90% 的兒童......這些兒童的眼球不斷變長

As a result, instead  of focusing light right onto the retina,  the lenses in these kids' longer eyeballs  focus light on a point in front of the retina.

遠超過該點。是以，光線無法直接聚焦到視網膜上、這些孩子較長眼球中的鏡片將光線聚焦在視網膜前方的一個點上；

From there, the light spreads back out, causing  them to see a fuzzy image rather than a crisp one. 

在這種情況下，光線會向後擴散，導致他們看到的影像模糊而不清晰。

For years, most scientists thought this  was happening because of screens.

多年來，大多數科學家認為出現這種情況的原因是螢幕--或者更多的原因是......

Or more   specifically, because kids were spending most of  their time looking at things only a short distance  away.

特別是，因為孩子們把大部分時間都花在了看不遠處的東西上

You see, our eyes focus most easily on stuff  in the middle distance.

遠處。你看，我們的眼睛最容易聚焦在中距離的東西上。為了看清楚

In order to clearly see   stuff far away, the muscles have to work to  stretch the lens to bring those images back  he sweet spot on the retina,

遠處的東西，肌肉必須努力拉伸鏡頭，才能將這些影像帶回來

and in order  to clearly see things close up, the muscles   have to work to smoosh the lens to bring those  images forward to the sweet spot on the retina.  

視網膜上的甜點，為了清楚地看到近處的事物，肌肉必須努力晃動鏡頭，把這些影像帶到視網膜上的最佳位置。

Scientists wondered whether kids' eyeballs were  growing extra long to shift this entire range  farther back, allowing them to see close up stuff  in focus without having to use their muscles,  

科學家們想知道，孩子們的眼球是否長得特別長，以轉移整個範圍這樣，他們就能在不使用肌肉的情況下看清近處的東西、

but leaving their eyes unable to focus on things  far away at all, no matter how much they strained. 

但卻使他們的眼睛無論如何用力，也根本無法聚焦於遠處的事物。

But recently, we've found that kids who  spend a lot of time parked in front of  a screen don't necessarily have longer  eyeballs than those who don't.

但最近我們發現，孩子們如果把大量時間花在電腦前螢幕上的人並不一定比不螢幕上的人眼球更長。相反、

Instead,   it seems that the likely culprit is the hormone  that carries the stop-growing signal from the  brain to the eyeballs – or, really, a lack of  this hormone.

看來，罪魁禍首很可能是一種荷爾蒙，這種荷爾蒙能從細胞中傳遞停止生長的信號

We still don't totally understand   how the entire signaling-process works, but  we do know that our eyes need to be exposed   to a certain level of light in order for  the hormone to form in the first place.  

從大腦到眼球--或者說，缺乏這種激素。我們仍然不完全瞭解整個信號傳遞過程是如何運作的，但我們確實知道，我們的眼睛需要接觸到 在一定程度的光照下，荷爾蒙才能首先形成。

Kids today - who only spend about half as much  time outside as their parents did – simply aren't   getting the light their eyes need to create enough  of that hormone and give the stop-growing signal.  

現在的孩子戶外活動的時間只有父輩們的一半左右，他們根本沒有它們的眼睛需要光線來產生足夠的荷爾蒙，併發出停止生長的信號。

As a result, their eyeballs keep lengthening  - past the sweet spot – creating an epidemic  of blurry vision the likes of which the  world has never .. uhh .. seen.   

是以，他們的眼球不斷變長--超過了甜蜜點--造成了一種流行病世界上從未有過的模糊視力。很幸運

Luckily, there's an easy solution for future generations  - go outside to watch your YouTube videos.

為子孫後代提供了一個簡單的解決方案--到戶外觀看 YouTube 視頻。