Lumen consist of a series of dialogues between a man and a disembodied spirit which is free to roam the Universe at will. The novel includes observations about the implications of the finite velocity of light, and many images of otherworldly life adapted to Alien circumstances Considered "early science fiction", by author Camille Flammarion, one of the most interesting characters produced by the 19th century, and a definitive influence on authors such as Edgar Rice Burroughs and many others.

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