The sky seems to be like the solid base of a vessel. The fire-fly appears like the actual spark of fire. In sooth, however, the sky has no base and there is no fire in the fire-fly.
तलवद् दृश्यते व्योम खद्योतो हव्यवाडिव। न चैवास्ति तलं व्योम्नि खद्योते न हुताशनः॥