Almost 2000 years ago, the Roman philosopher Seneca peered at his book through a glass of water.
差不多二千年前，罗马哲学家塞内卡透过一杯水来看他的书。
Suddenly, the text below was transformed. The words magically became clear.
突然，杯下的文字改变了。文字神奇地变得清晰。
But it wasn't until a millennium later that that same principle would be used to create the earliest glasses.
但直至一千年后，这一原理才被用以制作最早的眼镜。
Today, glasses can help millions of people with poor vision due to uncorrected refractive errors.
今天，眼镜帮助数百万人因未矫正“屈光不正”所致的视力不良。
The key to understanding what that means lies with the term refraction,
要理解背后意义，关键在于折射，
the ability of a transparent medium, like glass, water, or the eye to change the direction of light passing through it.
光通过一种透明介质，例如玻璃、水或眼睛时，介质改变光行进方向的能力。
The eye has two main refractive surfaces: the cornea and the lens.
眼睛有两个主要的折射面：角膜和晶状体。
Ideally, these surfaces work together to refract light in a way that accurately focuses light onto the retina,
理想情况下，这两个折射面会共同运作，把光线折射聚焦至视网膜，
the layer of light-sensitive tissue at the back of the eye that works with the brain to give rise to vision.
即眼球后方的一层感光组织，它会与大脑合作，从而产生影像。
But many people develop refractive errors,
但很多人发生屈光不正，
either during childhood as their eyes are growing, or in later life as their eyes age.
是在眼睛正成长的孩童时期，或是在眼睛衰退的老年时。
Imperfections in the cornea and lens cause refracted light to be focused in front of or behind the retina, making images appear blurry.
角膜和晶状体的缺陷，使得折射光线聚焦在视网膜前面或后面，令影像模糊。
People with refractive errors can still see color, movement, and light,
患有屈光不正的人，仍能看见颜色、动作和光，
but the details of what they're looking at are out of focus.
但他们所见细节是模楜的。
People experience refractive errors in different ways, owing to differences in their eyes.
人们由于眼睛的差异，有不同种类的屈光不正。
In some, light refracts too much, and in others, too little.
一部分人的眼睛，光线过度折射，另一部分人则太少。
Eyes with a focal point in front of the retina are called myopic, or short-sighted.
焦点落在视网膜前的眼睛称为近视。
They can see close objects clearly, but those far away are out of focus.
他们能清楚看见近处的东西，但远处的则模楜不清。
But when the focus point is behind the retina, people are hyperopic, or long-sighted.
但若焦点落在视网膜后的，这些人则有远视。
For them, objects close up are unfocused, but distant objects are crystal clear.
对于他们而言，近处物体模楜不清，但远方事物却清晰可见。

Finally, some people have a cornea with a non-spherical shape that causes astigmatism,
最后，有些人的角膜并非呈球面状，造成散光，
a form of out-of-focus vision that makes all objects seem blurred, whether close or far.
这是一种不能对焦的视力，致使无论远近事物皆模糊不清。
As we age, our eyes face new challenges.
当我们年岁渐长，眼睛会面对新挑战。
When we're young, the lens of the eye is flexible
我们年幼时，眼睛的晶状体柔韧有弹性，
and can change shape to bring images into focus, something called accommodation.
能够改变形状来把影像聚焦，这称为眼调节。
This keeps objects in focus when we shift our gaze from far to near.
当我们由远至近转移目光时，这使物体影像维持清楚。
But as we get older, the lens becomes less flexible, and can't change shape when we want to look at near objects.
但当我们渐渐年长，晶状体的弹性渐减，我们想看近物时，晶状体未能改变形状配合。
This is called presbyopia, and it affects adults starting around the age of 40 years.
这称为老花，人从约四十岁开始受它影响。
Myopia, hyperopia, astigmatism, and presbyopia. Each of these is a refractive error.
近视、远视、散光和老花，每一种都是屈光不正。
Nowadays we can fix them all with glasses or contact lenses,
现今我们可以透过眼镜或隐形眼镜来矫正它们，
which work by refocusing light so it strikes the retina precisely.
原理在于重新聚焦光线，使其准确落在视网膜上。
It's even possible to correct vision with surgery
甚至我们可以透过手术改正视力，
using lasers that change the shape of the cornea and alter its refractive properties.
利用雷射改变角膜的形状，从而改变它的屈旋光性。
But glasses remain the most popular.
但眼镜仍是最普及的方法。
By using carefully crafted lenses to steer light to exactly the right spot on the retina,
透过仔细打磨的镜片，引导光线至视网膜的正确点上，
a person's clear vision can be restored.
从而恢复一个人的清晰视力。
We've come a long way since Seneca's discovery and the crude glasses of yesteryear.
自塞内卡的发现和旧时简陋的眼镜以来，我们已有了很大的进展。
In 1727, a British optician named Edward Scarlett developed the modern style of glasses
1727年，英国光学仪器制造者爱德华·斯佳丽研究出现代款式的眼镜，
which are kept in place with arms which hook over each ear.
以两托架挂在两耳上，以固定位置。
Today's glasses take their inspiration from that design, but they're also much more precise and personal.
现今的眼镜都取材自这一设计，但它们同时更精细和个人化。
Each pair is tailored for an individual to bring out their unique powers of sight.
每副眼镜都为个人度身订造，带出他们独特的视力。
So if you're one of the 500 million people with a problem with close or far vision, or both,
所以如果你是这五亿人之一：有近视或远视，或是两者皆有，
there's a pair of glasses out there waiting to reveal a whole new world that's hiding in plain view.
总有一副眼镜等待你，以展现一个全新的世界--一个隐藏在平日视线的世界。