Having two eyes is certainly better than having just one because two eyes provide us with stereo vision and depth perception -- two things that just eye could not give us.

With 2 1/2  inches separating our two eyes, each eye views an object from a slightly different angle. For instance, if you hold up a flower and look at it with just your right eye, the image is different from the image of when you look at it with just your left eye. The right eye sees more of the right side of the flower while the left eye sees more of the left side of the flower. If you placed the two different images on top of one another, they would not match and our vision would be out of focus. However, our brains sort out these varying visual messages from our two eyes, combines the images, and recreates one three-dimensional image. 

This is referred to as binocular vision. Just like your look through two lenses in binoculars, humans view the world through two lenses. The eyes of many other animals are placed differently than ours. Many birds have an eye on each side of their head. Each eye sees a completely separate area stretching out on the left or the right.

Viewing the world through two eyes provides us with depth perception. When you look at the flower through just one eye, it looks a lot flatter.

stereo vision: 立体效果

depth perception: 深度知觉

binocular: 双目的

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