Normalization of congruence of bitangents to a hypersurface in $mathbb P^3$

A congruence is a surface in the Grassmannian ${rm Gr}(2, 4)$. In this paper, we consider the normalization of congruence of bitangents to a hypersurface in $mathbb P^3$. We call it the Fano congruence of bitangents. We give a criterion for smoothness of the Fano congruence of bitangents and describe explicitly their degenerations in a general Lefschetz pencil in the space of hypersurfaces in $mathbb P^3$.