Since the delivery of my lecture to the Royal Institution, on June
1st, Herr Zehnder has published[41] a mode of getting elliptically and
circularly polarised electric radiation. He takes a couple of plane
polarising grids, such as are depicted in Fig. 21, page 37, and places
them parallel to each other at a little distance apart with their wires
crossed.
If the two grids are close together they will act like wire-gauze,
reflecting any kind of polarised radiation equally; but if the warp
and woof are an eighth-wave length apart, and the plane of the
incident radiation is at 45° to the wires, the reflected radiation
will be circularly polarised. A change in the circumstances will, of
course, make it elliptical. Such a pair of grids acts, in fact, like a
Babinet’s Compensator.
[41] _Berichte der Naturforschenden Gesellschaft zu Freiburg i. B._,
Bd. IX. Heft 2, June 21, 1894.