In the middle third, there's a mix of grass species that grow to an intermediate height.

在中间的三分之一处，有混合的草种，可以长到中等高度。

And now it's ready, I'm going to divide it roughly into three — just little marks with my fingers — and in the middle third spread the butter.

擀好了，我要把它大致分成三份——用我的手指划出一点痕迹——在中间三分之一处涂上黄油。

The rays passing through the middle third and bottom third of the slit will be half a wavelength apart, so they'll cancel each other out.

通过狭缝中间三分之一和底部三分之一的光线将相隔半个波长，因此它们会相互抵消。