  Applied optics was one of the major topics Hans Walter was engaged in during
his scientific life. He contributed to the understanding of optical properties
of thin films, which could be used to design coating layers to improve the
properties of optical and other surfaces. He developed the theoretical
description of the basic principles of phase-contrast, schlieren and
interference optics applied to enhance low contrast details and to increase the
resolution in studies of biological samples. And last, but not least, Hans
Wolter proposed an optical system of two grazing--incidence mirrors for use in
an X--ray imaging microscope. A microscope using such an optics never was put
into practice, but the optical design turned out to be well suited for
telescopes.
