Words, like glass, obscure when they do not aid vision.

[Les mots, comme les verres, obscurcissent tout ce qu’ils n’aident pas à mieux voir.]

Joseph Joubert (1754-1824) French moralist, philosopher, essayist, poet

Pensées [Thoughts], ch. 22 “Du Style [On Style],” ¶ 25 (1850 ed.) [tr. Lyttelton (1899), ch. 21, ¶ 15]
    (Source)

(Source (French)). Alternate translations:

Words, like glass, darken whatever they do not help us to see.
[tr. Attwell (1896), ¶ 304]

Words, like eyeglasses, obscure everything they do not make clear.
[Source]