<p>A new principle for appraising and correcting the focus of a high-power microscope is described. Besides affording greater precision than is achieved visually, the principle lends itself readily to fully automatic operation, including “tracking” a moving slide. A servo system employing this principle has been built and been operated routinely for over a year. It maintains correct focus within about 0.1 µ and does so overaperiod of several days if necessary. The equipment operates irrespective of changes in thickness and index of cover slip, index of immersion fluid, thickness and flatness of specimen slide, temperature effects in the various optical and mechanical components. It operates even when the specimen slide area in question is blank (i.e., unoccupied).</p><p>The principle is well suited to microscope systems that employ photoelectric or photographic detection and that call for the greatest possible accuracy of focus. It may be used with UV, visible, or IR radiation, and hence it is applicable to UV and IR microscopy as well as visual range microscopy.</p><p>Besides assisting focusing, the new principle can be used to explore the surface topography of microscopically “stepped” surfaces; unlike various other methods it should be applicable even if the steps have an area as small as one square micron. It has been found effective also in evaluating longitudinal chromatic aberration in high quality objectives.</p>

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