  In this paper we deal with a general type of integral formulas of the visual
angle, among them those of Crofton, Hurwitz and Masotti, from the point of view
of Integral Geometry. The purpose is twofold: to provide an interpretation of
these formulas in terms of integrals of densities with respect to the canonical
measure in the space of pairs of lines and to give new simpler proofs of them.
