The structure of a liquid crystal (L.C) thin film on water has been studied optically and the molecular orientation at its interfaces has been deduced. The surface tension shiftΔγ corresponding to the orientations of the molecules either normal or parallel to the free surface has been deduced. From the study of the formation and of the stability of holes in the thin film we deduce the order of magnitude of the line tensionx. This tension originates in the elastic deformation of LC in the neighbourhood of the hole.