The words "Second Coming" seem to have made the speaker think of something else, so that he repeats the phrase as an exclamation. It’s like, "Eureka!" It makes him think of a "vast image out of Spiritus Mundi."To know what this means, you have to know that Yeats was very interested in the occult and believed that people have a supernatural connection to one another. It’s in the same ballpark as telepathy or a psychic connection, but not quite as kooky as those other things. It’s more like we’re all connected to a big database of communal memories going back all the way through human history, which we can get in contact with when we’re feeling truly inspired.Literally, Spiritus Mundi means "spirit of the world."
