An idea suddenly occurred to him, an idea so reasonable, so simple that he did not doubt its correctness for a second. Were not that D and that F the initials of the two most important words in the document, the words that represented—together with the Needle—the essential stations on the road to be followed: the Chambre des _Demoiselles_ and Fort _Fréfossé:_ D for Demoiselles, F for Fréfossé: the connection was too remarkable to be a mere accidental fact. In that case, the problem stood thus: the two letters D F represent the relation that exists between the Chambre des Demoiselles and Fort Fréfossé, the single letter D, which begins the line, represents the Demoiselles, that is to say, the cave in which you have to begin by taking up your position, and the single letter F, placed in the middle of the line, represents Fréfossé, that is to say, the probable entrance to the underground passage. Between these various signs, are two more: first, a sort of irregular rectangle, marked with a stripe in the left bottom corner, and, next, the figure 19, signs which obviously indicate to those inside the cave the means of penetrating beneath the fort. The shape of this rectangle puzzled Isidore. Was there around him, on the walls of the cave, or at any rate within reach of his eyes, an inscription, anything whatever, affecting a rectangular shape? He looked for a long time and was on the point of abandoning that particular scent when his eyes fell upon the little opening, pierced in the rock, that acted as a window to the chamber. Now the edges of this opening just formed a rectangle: corrugated, uneven, clumsy, but still a rectangle; and Beautrelet at once saw that, by placing his two feet on the D and the F carved in the stone floor—and this explained the stroke that surmounted the two letters in the document—he found himself at the exact height of the window! He took up his position in this place and gazed out. The window looking landward, as we know, he saw, first, the path that connected the cave with the land, a path hung between two precipices; and, next, he caught sight of the foot of the hillock on which the fort stood. To try and see the fort, Beautrelet leaned over to the left and it was then that he understood the meaning of the curved stripe, the comma that marked the left bottom corner in the document: at the bottom on the left-hand side of the window, a piece of flint projected and the end of it was curved like a claw. It suggested a regular shooter’s mark. And, when a man applied his eye to this mark, he saw cut out, on the slope of the mound facing him, a restricted surface of land occupied almost entirely by an old brick wall, a remnant of the original Fort Fréfossé or of the old Roman _oppidum_ built on this spot. Beautrelet ran to this piece of wall, which was, perhaps, ten yards long. It was covered with grass and plants. There was no indication of any kind visible. And yet that figure 19?