  BaNiS$_2$ is a system dominated by a fourfold Dirac-cone network. We measured
the optical conductivity and Landau level spectra of the Dirac nodal lines and
quantitatively modeled the response through ab initio calculations. The optical
conductivity shows a highly unusual temperature-independent isosbestic line.
Magneto-optical spectra show a nearly $\sqrt{B}$ behavior, the hallmark of a
conical dispersion. BaNiS$_2$ is a simple prototype of a Dirac nodal line
semimetal: its Dirac nodal lines are only slightly gapped, and they disperse
exclusively along the out-of-plane direction. Our first-principles calculations
account for the observed isosbestic conductivity, which terminates in a Van
Hove singularity. We argue that such an optical response is universal for Dirac
cones, which must connect with each other within the Brillouin zone by breaking
their conical shape.
