||We review the theory of visual thresholds and applications to the limiting magnitude of a telescope and of the eyes, based on Schaefers's model with minor improvements. We apply out formulation to the Yerkes Observatory refractor and to naked eye observations at Mount Wilson Observatory. We reanalyze Bowen's telescopic observations at Mount Wilson by his approximate method and by our more elaborate theory. An extension of his method leads to a determination of the night sky brightness of the visual activity of the observer is assumed to be average. Our more elaborate method allows a determination of the sky brightness, the visual acuity of the observer, and the average seeing during the observations.