  We report an observation of a commensurate-incommensurate phase transition in
the Dzyaloshinskii-Moriya spiral antiferromagnet Ba_2 Cu Ge_2 O_7. The
transition is induced by an external magnetic field applied along the c-axis of
the tetragonal structure, i. e., in the plane of spin rotation. Bulk magnetic
measurements and neutron diffraction experiments show that the transition
occurs in a critical field Hc=2.1T. Experimental results for the period of the
magnetic structure and magnetization as functions of magnetic field are in
quantitative agreement with our exact analytical solution for Dzyaloshinskii's
model of commensurate-incommensurate transitions in spiral magnets.
