It is the electric light in Roch's laboratory as seen through a narrow window in the front.
Except in that particular spot, the southern shore of the lake is in darkness, whereas, in the opposite direction, the Beehive is lit up to its extremity at the northern wall.

Through the opening in the dome, over the lake I can see the stars shining.

The sky is clear, the tempest has abated, and the squalls no longer penetrate to the interior of Back Cup.
When near the laboratory, I creep along the wall and peep in at the window.
Thomas Roch is there alone.

The light shines full on his face.

If it is somewhat drawn, and the lines on the forehead are more pronounced, his physiognomy, at least, denotes perfect calmness and self-possession.