It appears therefore that there are no bands at all unless [omega]
lies between 0 and +4h1, and that within these limits the best bands
are formed at the middle of the range when [omega] = 2h1. The
formation of bands thus requires that the retarding plate be held upon
the side already specified, so that [omega] be positive; and that the
thickness of the plate (to which [omega] is proportional) do not
exceed a certain limit, which we may call 2T0. At the best thickness
T0 the bands are black, and not otherwise.
The linear width of the band (e) is the increment of [xi] which alters
[rho] by 2[pi], so that
e = 2[pi]/[=omega] (13).