  A large set of high precision full-Stokes spectropolarimetric observations of
prominences in He-D3, Halpha and Hbeta lines has been recorded with the ZIMPOL
polarimeter at the Gregory-Coude Telescope in Locarno. The observational
technique allows us to obtain measurements free from seeing induced spurious
effects. The instrumental polarization is well under control and taken into
account in the data analysis. We present our observational results for each of
the above-mentioned lines. Of particular interest is that most of our Halpha
measurements show anti-symmetric V profiles that are a characteristic signature
of the Zeeman effect in the prominence plasma. A Stokes inversion technique
based on the quantum theory of the Hanle and Zeeman effects is being applied on
observed Stokes profiles in the He-D3 line in order to obtain information on
the magnetic field vector that confines the prominence plasma.
