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Shine a light into the woods at night, and you might see the glow of eyes staring back at you.
夜晚，用灯照亮一片树林，你或许会看到闪着光的眼睛正在凝视着你。
But the eyes aren't actually glowing—
但是这些眼睛并不是真的在发光—
they're reflecting light off a special layer of tissue found in some animals called a tapetum lucidum.
它们只是在反射组织中一种特殊膜的光，一些动物眼睛里就有这种特殊膜—反光膜。
The color and shape of this 'eye shine' can tell you what's peering at you from the darkness.
这种‘眼睛闪烁’的颜色和形状能够让你辨别出在黑暗中盯着你的是什么动物。
But things are more complicated if you're in the arctic regions of Norway.
但是如果你是在挪威的北极区的话，事情就要复杂的很多。
Because there, reindeer change the color of that part of their eye seasonally.
因为那里的驯鹿会随着季节改变眼睛反光膜的颜色。
That's right—the animals most famous for pulling Santa's sleigh
没错—这种以帮圣诞老人拉雪橇而出名的动物
have eyes that change color to help them see in the darkness of winter.
它们的眼睛能够改变颜色帮助它们看清冬季的黑夜。
Animals that need to see well in low light, like cats, raccoons and even reindeer, all have tapeta lucida.
在需要低光看见的动物，比如猫、浣熊，甚至驯鹿，它们都有反光膜。
They sit in back of the eyes right in between the outer layers of the eye and the retina—the part that actually "sees".
反光膜位于眼睛后部，正在眼睛和视网膜（这个部位可以“看见”）外层之间。
These thin tissues act as reflectors, giving retina a second chance at absorbing light.
这些薄层组织充当反光片作用，给视网膜第二次吸收光的机会。
Most animals have a particular color of eye shine which depends on the composition and structure of their tapetum lucida.
多数动物的眼睛都闪烁着一种特定的颜色，取决于它们这反光膜的构成和结构。
But the Eurasian mountain reindeer is different—it's the only one we know of that switches colors,
但是欧亚山脉的驯鹿则不同—这是我们知道的唯一一种变换颜色的动物，
changing from a summery golden yellow to a deep blue in winter.
从夏季的金黄变成冬季的深蓝。
And scientists think that's a side effect of how their eyes have adapted to long periods of darkness.
科学家们认为这是它们眼睛已经适应长期黑暗的副作用。
The arctic summers where they live have 24 hour days, and the winters include long stretches of night.
北极区的夏季24小时都是白昼，冬季夜晚时间延长。
The animals compensate for this in part by making their retinas more responsive to low light in winter.
为了补偿这种昼夜差别，它们的视网膜在冬季里会更加适应低光。
And it's believed they can see some wavelengths of UV light. But that's not the most obvious adaptation.
人们认为它们可以看到紫外线的一些波长，但是这并不是最明显的适应。
To see better when it's so dang dark, a reindeer's pupils open wide to let more light in—kind of like yours do if you go into a dark room.
为了在黑夜中看得更清楚，驯鹿的瞳孔扩张以吸入更多的光—有点像是人类进入一间黑屋子时的眼睛一样。
But when a pupil stretches wider it also flattens the front of the eye ever so slightly, which increases the pressure inside the eye.
但是当瞳孔扩张时，还会轻微压到眼睛的前部，这就增加了眼睛内部的压力。
Since their pupils are constantly wide open during winter, that pressure builds up,
由于它们的瞳孔在冬季时被放大，其所形成的的压力
squeezing the tiny collagen fibers of the deer's tapetum lucidum closer together.
使驯鹿反光膜微小的胶原纤维几乎挤压在一起。
And this compression causes the color change.
而这种压缩引起了颜色的改变。
The tightly-packed fibers strongly reflect shorter, blue wavelengths of light,
紧紧包裹的纤维强力反射更短的蓝色光波长，
while the more spaced out ones reflect longer, yellow ones instead -- a phenomenon known as Bragg's Law.
而更加松散的纤维则反射更长的黄色光波长—这种现象被称为布拉格定律。
Researchers can actually compress a dissected piece of the reindeer's eye with a tiny weight
研究人员可以用微重压缩驯鹿眼睛的切割片
and induce this same yellow to blue color change.
并引起同样的黄色向蓝色的转变。
But the color isn't what's really important to the animal.
但是颜色对动物而言并非真的重要。
The compressed fibers also scatter light sideways through the retina instead of reflecting it out of the eye,
压缩纤维也通过视网膜侧面散射光线，而非反射眼中的光线，
increasing the amount of light the retina can absorb.
增加视网膜可吸收光线的数量。
While summer eyes reflect more than 95% of the light shone into them, winter eyes reflect only 40%.
夏季眼睛反射超过95%的摄入光线，而冬季则仅反射40%。
And that likely means the reindeer can see in what seems like total blackness.
这可能意味着驯鹿在完全黑暗的空间中也能看见。
The only downside to this amazing feat is that, because light rays inside the eye are more scattered,
这种特点唯一的缺点就是，由于眼睛里边的光线更加分散，
everything also looks a little fuzzier. But that seems a small price to pay.
所以所有东西看起来都有点模糊。但是似乎这个代价很小。
I wish I could trade a little blurriness for night vision! Thanks for watching this episode of SciShow!
我希望可以用一点点小的模糊为代价交换夜视功能！感谢收看本期《科学秀》！
If you liked learning about reindeers eyes, you might also like our episode where we explain what eyeballs are actually made of.
如果你喜欢学习驯鹿眼睛的知识，你也许会喜欢我们关于眼球组成的视频。
