国家地理-2008-07-31 不凡的沙滩救生员

Imagine a summer weekend on a California beach dense¹ with bodies. But for one onlooker², this seemingly calm scene may be a series of accidents waiting to happen. How does a lifeguard know when a raised arm means "I need help", not "hey,this is fun"? The guard's skill at spotting that one desperate person among thousands is phenomenal, truly testing his sight and understanding.

 We see the way we do, so we can spot danger to ourselves. But nothing is directing the lifeguard. In fact the eye, observing a harmless pattern across its view, normally relaxes. Motion-sensing rod cells switch off when they detect action that's consistent and constant. So the lifeguard has to trick his eyes. He does this by scanning, forcing his eyes to lock onto small details.

 Our frontline defence are the tower guards, and their job is to scan the water. So their eyes are moving across the water and letting their brains filter out that information that they see, looking for something wrong, looking for that odd one out that truly is in danger.

 Taking in all this information is hard work. Human sight has only two degrees of detail vision at the center. To check the whole beach, the lifeguard sweeps, jumping from point to point for detail. Each jump is called a saccade.

 A saccade is the movement that the eyes make together when they're looking directly at one thing, and all of a sudden, they look at something else. We have mechanisms³ that wire the muscles that move our eyes to the image, and we can quickly lock onto a new image all at once.

 The saccade function lets him jump visually from each potential risk to the next. He repeatedly scans his field of vision, updating his visual memory every few seconds.

1 dense   

a.密集的，稠密的，浓密的；密度大的

参考例句：

• The general ambushed his troops in the dense woods. 将军把部队埋伏在浓密的树林里。
• The path was completely covered by the dense foliage. 小路被树叶厚厚地盖了一层。

2 onlooker   

n.旁观者，观众

参考例句：

• A handful of onlookers stand in the field watching.少数几个旁观者站在现场观看。
• One onlooker had to be restrained by police.一个旁观者遭到了警察的制止。

3 mechanisms   

n.机械( mechanism的名词复数 )；机械装置；[生物学] 机制；机械作用

参考例句：

• The research will provide direct insight into molecular mechanisms. 这项研究将使人能够直接地了解分子的机理。 来自《简明英汉词典》
• He explained how the two mechanisms worked. 他解释这两台机械装置是如何工作的。 来自《简明英汉词典》