What is the narrative voice in Oliver Optic's Rich and Humble?

The narrative voice in Rich and Humble: Or The Mission of Bertha Grant by William Taylor Adams writing as Oliver Optic is an all-knowing narrator who observes Bertha's life and details it to the reader.
Narrative voice is simply the voice of the narrator. It's not necessarily the point-of-view in which a story is told—although that does have something to do with it. Rich and Humble is told in the third-person perspective, which means that the narrator is describing the action but doesn't take part in it themselves.
The narrator of Rich and Humble may be assumed to be Adams himself, but that's not always the case. Sometimes the narrator is simply a character who isn't part of the narrative. The narrator is just the narrator—not the author or a person in the story. This is the case with Rich and Humble.
In short, Rich and Humble is written in a third-person perspective and the narrative voice is that of an unnamed narrator. Since the narrator knows people's thoughts and feelings of the characters, he is an omniscient third-person narrator.