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.