The Stieltjes integrals in the theory of harmonic and analytic functions

We study various Stieltjes integrals as Poisson-Stieltjes, conjugate Poisson-Stieltjes, Schwartz-Stieltjes and Cauchy-Stieltjes and prove theorems on the existence of their finite angular limits a.e. in terms of the singular Hilbert-Stieltjes integral in the sense of the principal value. These results hold for arbitrary $2\pi -$periodic bounded integrands that are differentiable a.e. and, in particular, for integrands of the class $\mathcal{C}\mathcal{B}\mathcal{V}$ (countably bounded variation).