translation.en: Oh, before I even turn that on, one of the things that we did about three weeks ago in my class -- this is through the lens, and one of the things we used a lens for was to measure the speed of light.
translation.zh: 噢，在我把机器打开前， 我要向你们解释大约三周前我给课堂里讲的东西─ 这是镜片的许多应用之一，就是通过镜片 来测量光速。