If only I had known, like Captain Nemo and his companion, how to exchange thoughts by means of prearranged signals!

我怎样才能跟尼摩船长和他的同伴一样，利用一种约定的记号来传达我的思想呢！

At a prearranged signal the lights went out.

【signal】预定的信号一发出，灯就全灭了。