  We demonstrate that, in monolayers (MLs) of semiconducting transition metal
dichalcogenides, the $s$-type Rydberg series of excitonic states follows a
simple energy ladder: $\epsilon_n=-Ry^*/(n+\delta)^2$, $n$=1,2,\ldots, in which
$Ry^*$ is very close to the Rydberg energy scaled by the dielectric constant of
the medium surrounding the ML and by the reduced effective electron-hole mass,
whereas the ML polarizability is only accounted for by $\delta$. This is
justified by the analysis of experimental data on excitonic resonances, as
extracted from magneto-optical measurements of a high-quality WSe$_2$ ML
encapsulated in hexagonal boron nitride (hBN), and well reproduced with an
analytically solvable Schr\"odinger equation when approximating the
electron-hole potential in the form of a modified Kratzer potential. Applying
our convention to other, MoSe$_2$, WS$_2$, MoS$_2$ MLs encapsulated in hBN, we
estimate an apparent magnitude of $\delta$ for each of the studied structures.
Intriguingly, $\delta$ is found to be close to zero for WSe$_2$ as well as for
MoS$_2$ monolayers, what implies that the energy ladder of excitonic states in
these two-dimensional structures resembles that of Rydberg states of a
three-dimensional hydrogen atom.
