阶梯透镜

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.