You are getting sleepy.

你開始覺得睏倦。

No, just kidding.

開玩笑的。

I'm not going to hypnotize you.

我沒有要催眠你。

But are you starting to see colors in the rings?

不過你開始看到圓圈中的色彩了嗎？

If so, your eyes are playing tricks on you.

如果看得到， 表示你的眼睛正在愚弄你。

The disk was only ever black and white.

轉盤從頭到尾都是黑白的。

You see, your eyes don't always capture the world as a video camera would.

人的眼睛並不總如攝影機般捕捉世界。

In fact, there are quite a few differences, owing to the anatomy of your eye and the processing that takes place in your brain and its outgrowth, the retina.

事實上，兩者有許多不同。這都歸因於眼睛的構造以及大腦的處理過程，和其發展結果——視網膜。

Let's start with some similarities.

先來看看一些相似之處。

Both have lenses to focus light and sensors to capture it, but even those things behave differently.

兩者都有鏡頭可以聚焦光線，還有感光元件捕捉光線，但它們的作用方式很不一樣。

The lens in a camera moves to stay focused on an object hurtling towards it, while the one in your eye responds by changing shape.

相機鏡頭透過移動聚焦向他飛來的物體，而眼睛則是改變形狀。

Most camera lenses are also achromatic, meaning they focus both red and blue light to the same point.

大部分相機鏡頭都是「消色差」的，意即相機會將紅光與藍光聚焦在同一個點上。

Your eye is different.

眼睛則不同。

When red light from an object is in focus, the blue light is out of focus.

當眼睛對焦物體的紅光，藍光就會失焦。

So why don't things look partially out of focus all the time?

那為何我們看東西不會一直覺得有部分失焦？

To answer that question, we first need to look at how your eye and the camera capture light:

想回答這個問題，首先要來看眼睛和相機如何捕捉光線：

photoreceptors.

光感受器。

The light-sensitive surface in a camera only has one kind of photoreceptor that is evenly distributed throughout the focusing surface.

相機的感光表面只有一種光感受器平均分布在整個對焦表面。

An array of red, green, and blue filters on top of these photoreceptors causes them to respond selectively to long, medium, and short-wavelength light.

而光感受器上有排列的紅、綠、藍濾鏡，分別對長、中、短波光線產生反應。

Your eye's retinas, on the other hand, have several types of photoreceptors, usually three for normal light conditions, and only one type for lowlight,

視網膜則有幾種不同的光感受器，正常光線下有三種，而低光源下只有一種，

which is why we're color blind in the dark.

這也是為什麼我們在黑暗中會無法分辨顏色。

In normal light, unlike the camera, we have no need for a color filter because our photoreceptors already respond selectively to different wavelengths of light.

正常光線下，我們不需要像相機一樣的色彩濾鏡，因為光感受器會分別對不同波長產生反應。

Also in contrast to a camera, your photoreceptors are unevenly distributed, with no receptors for dim light in the very center.

另一個和相機不同之處在於眼睛的光感受器並不平均分布。眼睛最中間的地方，沒有可接收微弱光線的接收器。

This is why faint stars seem to disappear when you look directly at them.

這就是為什麼當我們直視微弱的星光時，星星會消失不見。

The center also has very few receptors that can detect blue light, which is why you don't notice the blurred blue image from earlier.

中間位置也只有極少數接收器可以偵測藍光，所以沒辦法注意到模糊的藍色影像。

However, you still perceive blue there because your brain fills it in from context.

不過你仍然可以察覺到那裡有藍色，因為大腦會依據周圍狀況將其補充進去。

Also, the edges of our retinas have relatively few receptors for any wavelength light.

而且視網膜邊緣任何波長光線的接收器都比較少。

So our visual acuity and ability to see color falls off rapidly from the center of our vision.

所以我們的視覺敏銳度和察覺色彩的能力會隨著視線中心，急速往兩邊下降。

There is also an area in our eyes called the blind spot where there are no photoreceptors of any kind.

眼睛裡也有個區域叫盲點，那裡沒有任何種類的光感受器。

We don't notice a lack of vision there because once again, our brain fills in the gaps.

但我們不會察覺到盲點，因為大腦再次幫我們填補了空白。

In a very real sense, we see with our brains, not our eyes.

事實上，我們是用大腦而非眼睛看東西。

And because our brains, including the retinas, are so involved in the process, we are susceptible to visual illusions.

而且因為我們的大腦，包括視網膜在這個過程中涉入極深，導致我們很容易產生視覺上的錯覺。

Here's another illusion caused by the eye itself.

這裡有另一個錯覺的例子，是眼睛本身造成的。

Does the center of this image look like it's jittering around?

這個畫面的中心看起來一直跳動嗎？

That's because your eye actually jiggles most of the time.

這是因為你的眼球大多是時間都一直在移動。

If it didn't, your vision would eventually shut down because the nerves on the retina stop responding to a stationary image of constant intensity.

如果不移動， 你的視覺最終將會停止，因為視網膜上的神經對強度不變的靜止畫面會停止產生反應。

And unlike a camera, you briefly stop seeing whenever you make a larger movement with your eyes.

與相機不同的是，當眼睛大幅移動時，視力會短暫停止。

That's why you can't see your own eyes shift as you look from one to the other in a mirror.

這就是為什麼在鏡子中看不到自己的眼睛從一處移到另一處。

Video cameras can capture details our eyes miss, magnify distant objects and accurately record what they see.

攝影機可以捕捉眼睛錯過的細節、放大遠處物體，並正確記錄看到的東西。

But our eyes are remarkably efficient adaptations, the result of hundreds of millions of years of coevolution with our brains.

但我們的眼睛是幾億年來，與大腦共同演化後高效進化的產物。

And so what if we don't always see the world exactly as it is.

就算我們沒有每次都看到世界確切的樣子又如何。

There's a certain joy to be found watching stationary leaves waving on an illusive breeze, and maybe even an evolutionary advantage.

觀察靜止的樹葉在錯覺中隨風搖曳，也別有樂趣，甚至有可能是演化的優勢。

But that's a lesson for another day.

不過這堂課改天再說吧。