This paper presents the theoretical basis of a technique for observing reflecting surfaces which have been perturbed from a completely plane configuration. Surface distortions that are uniform in one direction are considered, a case relevant to the observation of long-crested water waves. The technique employs the direct visual or photographic observation of the reflections of straight-line sources in the surface, and is a commonly used and extremely simple technique for the examination of water-surface phenomena. Expressions are found for the observed image shape in terms of the shape of the surface and the relative dispositions of the light source and the observer, for the case in which the source is held parallel with the undisturbed surface at the same height as the observer. Experiments are described, using a reflecting solid surface of known profile, and the observed images are shown to be closely in agreement with the predictions of the theory. A qualitative explanation of the relation of the image to the surface shape is given and the conditions are examined for the occurrence of gaps in the image as the surface distortions become steep.