Emerson is telling you that people don't like to commit crimes when other people can see them as they commit those crimes.
Gaslight lit up the night. It did a better job of protecting people's property than police did. Criminals hate well-lit areas because other people can see what they are doing.
The universe protects itself from harm by "pitiless publicity": Journalists report the wrongdoings of others. People who want to do something evil must face the very real interest of the public in reading about their crimes. I've never seen the quote before, but I imagine that Emerson was using "the universe" as some sort of metaphor for the world of people and their activities.
Both gaslight and "pitiless publicity" ensure that witnesses will know when bad people do evil. Whether a witness sees a burgler breaking into a house by gaslight or the entire world reads about some banking scandal in a newspaper, people know that something is wrong.