  We investigate the optical conductivity and far-infrared magneto-optical
response of BaNiS$_2$, a simple square-lattice semimetal characterized by Dirac
nodal lines that disperse exclusively along the out-of-plane direction. With
the magnetic field aligned along the nodal line the in-plane Landau level
spectra show a nearly $\sqrt{B}$ behavior, the hallmark of a conical-band
dispersion with a small spin-orbit coupling gap. The optical conductivity
exhibits an unusual temperature-independent isosbestic line, ending at a Van
Hove singularity. First-principles calculations unambiguously assign the
isosbestic line to transitions across Dirac nodal states. Our work suggests a
universal topology of the electronic structure of Dirac nodal lines.
