  We show that the accumulated light of galaxies in the ultraviolet can be
evaluated from their luminosity density as a function of the evolution of the
cosmic star formation rate and dust extinction properties. Constraints on the
evolution rate are expected in future. Data available at the moment are
consistent with an evolution rate at low z steeper than (1+z)^(3.5). A
shallower rate remains possible if the luminosity-weighted dust extinction at
2000 A, as suggested by some data, is lower than about 1.2.
