Assumptions about properties of congruences of curves in a spacetime have powerful implications for the Weyl tensor. A well-known example is the conclusion that follows from the propagation equations of kinematical tensors in relativistic hydrodynamics [1]: If, in a perfect fluid spacetime, there exists a family of timelike curves with zero shear, rotation and acceleration, then the spacetime must be conformally flat. The theorem proven in the paper reprinted here is another example, where limitations imposed on the Weyl tensor by properties of congruences of null curves are discussed. Namely, a geodesic and shearfree null congruence exists in a vacuum spacetime if and only if its Weyl tensor is algebraically special. The congruence is then the degenerate principal null congruence of the Weyl tensor. (The theorem is further extended to include electromagnetic field with geodesic rays and a null field.)The republication of the original paper can be found via