  We show how to correctly extract from the ellipsometric data the surface
susceptibility and the surface conductivity that describe the optical
properties of monolayer $\rm MoS_2$. Theoretically, these parameters stem from
modelling a single-layer two-dimensional crystal as a surface current, a truly
two-dimensional model. Currently experimental practice is to consider this
model equivalent to a homogeneous slab with an effective thickness given by the
interlayer spacing of the exfoliating bulk material. We prove that the error in
the evaluation of the surface susceptibility of monolayer $\rm MoS_2$, owing to
the use of the slab model, is at least 10% or greater, a significant
discrepancy in the determination of the optical properties of this material.
