What did Rainsford see that astonished him?
I know you put this in the Fahrenheit 451 group, but there is no character named Rainsford in that book and Rainsford is the main character in this story.
After Rainsford falls off his yacht, he manages to swim to the island. He starts exploring to see what he can find. It is, of course, night time.
Rainsford eventually sees a bunch of lights. At first, he thinks that he has found a village. However, he is soon astonished to see that all the lights are coming from just one huge building.
He thinks he must be imagining it, but he is not -- it is Zaroff's home.