The narrator of The Invisible Man is clearly "invisible", seeing as he states so himself at the very start of the book by saying, "I am an invisible man". And he clearly doesn't mean it in the sense he is unable to be seen because he clarifies by saying, "No, I am not a spook like those who haunted Edgar Allan Poe; nor am I one of your Hollywood-movie ectoplasms". So the question is why is he invisible? As he puts it, "they see only my surroundings, themselves, or figments of their imagination -- indeed, everything and anything except me." What this means is that when people look at him they see a generalization of someone who is African-American, instead of an actual individual person. So what he means by being invisible is actually just hiding behind the fact that people see him as a generalization and therefore don't identify him in their minds as a person. This is shown when the narrator is taking to Norton and Norton refers to the narrator as a cog, showing him he thinks of him as no different from the rest of the students.
Bigger is invisible in the same way. With the exception of Max and Jan, most white people in the book fail to see him as his own person. For example. Dalton sees Bigger as just another African-American he's helped just like how Norton views the narrator. Another example is how during the search for Bigger the mob rounds up lots of other people like him even though they have a picture of him. This is similar to when the narrator beats the man up and the man claims he was mugged just because it fit convention. Another thing to consider about Bigger is that he is written to be invisible. An important part of Native Son was how Bigger was defined by environment more than he was by his actions. So in a way Bigger is meant to be invisible to the reader so that we can look past his character and more at the environment which Wright is trying to criticize.
