The chief problem: They tend to have a very narrow viewing sweet spot-you can't shift too far to either side without being left with an image that's blurry at best and nauseating at worst.
2
另一方面,西非内陆的降雨量增加,采采蝇活动区域的北部边界向南移动——一种对牛致命的昆虫。
On the other hand, rainfall increased in the interior of western Africa, and the northern limit of the tsetse fly, an insect fatal to cattle, moved south.
A controlled number of these dots are purposely moving uniformly in the same direction, and the test subject simply has to say whether he believes those dots are moving to the left or right.