A traveller puts his head under the edge of the firmament
The image depicts a man crawling under the edge of the sky, depicted as if it were a solid hemisphere, to look at the mysterious Empyrean beyond. The caption underneath the engraving (not shown here) translates to "A medieval missionary tells that he has found the point where Heaven and Earth meet..."
The Flammarion engraving is a wood engraving by an unknown artist, so named because its first documented appearance is in Camille Flammarion's 1888 book L'atmosphère : météorologie populaire ("The Atmosphere: Popular Meteorology"). It has been used as a metaphorical illustration of either the scientific or the mystical quests for knowledge.
The print depicts a man, clothed in a long robe and carrying a staff, who is at the edge of the Earth, where it meets the sky. He kneels down and passes his head, shoulders, and right arm through the star-studded sky, discovering a marvellous realm of circling clouds, fires and suns beyond the heavens. One of the elements of the cosmic machinery bears a strong resemblance to traditional pictorial representations of the "wheel in the middle of a wheel" described in the visions of the Hebrew prophet Ezekiel. The caption that accompanies the engraving in Flammarion's book reads: A missionary of the Middle Ages tells that he had found the point where the Sky and the Earth touch...
In Flammarion's L'atmosphère: météorologie populaire, the image refers to the text on the facing page (p. 162), which also clarifies the author's intent in using it as an illustration:
Whether the sky be clear or cloudy, it always seems to us to have the shape of an elliptic arch; far from having the form of a circular arch, it always seems flattened and depressed above our heads, and gradually to become farther removed toward the horizon. Our ancestors imagined that this blue vault was really what the eye would lead them to believe it to be; but, as Voltaire remarks, this is about as reasonable as if a silk-worm took his web for the limits of the universe. The Greek astronomers represented it as formed of a solid crystal substance; and so recently as Copernicus, a large number of astronomers thought it was as solid as plate-glass. The Latin poets placed the divinities of Olympus and the stately mythological court upon this vault, above the planets and the fixed stars. Previous to the knowledge that the earth was moving in space, and that space is everywhere, theologians had installed the Trinity in the empyrean, the glorified body of Jesus, that of the Virgin Mary, the angelic hierarchy, the saints, and all the heavenly host.... A naïve missionary of the Middle Ages even tells us that, in one of his voyages in search of the terrestrial paradise, he reached the horizon where the earth and the heavens met, and that he discovered a certain point where they were not joined together, and where, by stooping his shoulders, he passed under the roof of the heavens...