What is "echelon lens"

Echelon \Ech"e*lon\ ([e^]sh"e*l[o^]n), n. [F., fr. ['e]chelle ladder, fr. L. scala.]

1. (Mil.) An arrangement of a body of troops when its divisions are drawn up in parallel lines each to the right or the left of the one in advance of it, like the steps of a ladder in position for climbing. Also used adjectively; as, echelon distance.
   --Upton (Tactics).

2. (Naval) An arrangement of a fleet in a wedge or V form.
   --Encyc. Dict.

   Echelon lens (Optics), a large lens constructed in several parts or layers, extending in a succession of annular rings beyond the central lens; -- used in lighthouses.

n. A large lens constructed in several parts or layers, extending in a series of annular rings beyond the central lens, and used in lighthouses.