  We present a novel modular magnetic system that can introduce a large and
continuously-variable path length difference without simultaneous variation of
the longitudinal dispersion. This is achieved by using a combination of an
electrically-adjustable magnetic chicane and a mechanically-adjustable focus-
ing chicane. We describe how such a system may be made either isochronous or
with a given longitudinal dispersion, and show that the nonlinear terms in such
a system are relatively small.
