The narrator is describing how Marius is learning about Napoleon Bonaparte: "every day he saw better and he began to climb slowly, one by one, almost reluctantly at first, then with intoxication and as though drawn by an irresistible fascination, steps that started off dark then gradually became dimly illuminated, only to end in the luminous and splendid blaze of enthusiasm."
I liked this description so much that I lookt it up in the original French: "chaque jour il voyait mieux; et il se mit à gravir lentement, pas à pas, au commencement presque à regret, ensuite avec enivrement et comme attiré par une fascination irrésistible, d'abord les degrés sombres, puis les degrés vaguement éclairés, enfin les degrés lumineux et splendides de l'enthousiasme."
I think C.E. Wilbur's translation, which I previously read (in an abridged form), hews closer to Hugo's original: "each day he saw more clearly; and he began to mount slowly, step by step, in the beginning almost with regret, afterwards with rapture, and as if drawn by an irresistible fascination, at first the somber stages, then the dimly lighted stages, finally the luminous and splendid stages of enthusiasm."
Each stage has more description: first just an adjective ("the somber stages" "les degrés sombres"), then an adjective (technically a participle) modified by an adverb ("the dimly lighted stages" "les degrés vaguement éclairés"), and finally a pair of adjectives ("the luminous and splendid stages" "les degrés lumineux et splendides"). Even in the language, then, where each "step" is more advanced than the last, the narrator traces Marius's progression.
