Nikola Tesla's ghostly face isn't the most significant part of this photograph. It's the light—emitted by a fluorescent light bulb of Tesla's own invention. It was taken in 1894, decades before fluorescent lights came into popular use.
This image isn't as famous as the photograph of Tesla with his wireless lightbulb or of Mark Twain acting as the conduit between an electrical source and a lamp, but it is the first photograph ever exposed by phosphorescent light. Both Thomas Edison and Tesla experimented with fluorescent lights, expanding on the research of Alexandre Edmond Becquerel and others, but neither ended up making their lamps commercially available. It was physicist and GE consultant Arthur Compton who would convince GE to produce fluorescent lamps for the public in the 1930s.
Tesla's lamps glowed a bright green, so even in person, this might have been a slightly eerie sight.
First photograph ever taken by phosphorescent light. [Tesla Universe via It's Okay To Be Smart]