This paper is devoted to review the development and the results of the
program "solar prominences" that has been aimed to observe the Hanle
effect at the Pic-du-Midi during the ascending phase of Cycle XXI
(1974-1982). This aim had been defined and the observations have been
performed by Jean-Louis Leroy. The Hanle effect is the effect of a
weak magnetic field on the scattering linear polarization: its main
features are, for some field orientations, a depolarization and
eventually a rotation of the polarization direction. The magnetic
field diagnostic from polarization measurements requires a modelling
of the polarized line formation, that has been achieved in Meudon in
the well-adapted formalism of the atomic density matrix. It is shown
how the program has been developed to determine the 3 components of
the field vector and the electron density, by setting multi-line
polarimetric observations. Particular attention has been devoted on
the solution of the 180 degrees ambiguity, which has been solved by 3
independent methods. By using this solution, one unique average
magnetic field vector has been determined in each of 296 quiescent
prominences, leading to results on the field strength, direction,
vertical gradient, cyclic variations. The future perspective opened
by the low scattered light level of THEMIS and other
spectropolarimeters is to increase the spatial resolution of the
measurements.