This has got to be one of the the greatest historical photos of all time. That is indeed Nikola Tesla, nearly wraith-like in the background, while Mark Twain commands control of a phenomenon Tesla discovered — the human body’s ability to conduct electricity.
The picture is accidentally a perfect portrait of both. We see the innocent, curious Tesla bathed in shadow in the background, along with Twain well lit, front and center. The well-meaning Tesla led a haunted existence, and was consigned to relative obscurity after rival Thomas Edison stole credit for his advances. The commanding Twain, on the other hand, benefitted from his fame.
The two were good friends. See History Buff for more.