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An instrument has been developed to automatically scan a microscopic area for the quantitative evaluation of the light transmitting properties of biological materials on the cellular and subcellular levels. The specimen may be viewed through an aperture of selected size in the microscope both visually and electronically, while the slide is electronically manipulated to scan a rectangular area whose length and width are independently adjustable from 5 to 100Â¿. The slide is swept back and forth horizontally at a uniform rate which is adjustable from 0.4 to 4 Â¿/sec. At the end of each horizontal sweep it is moved vertically by an adjustable step of 0.2 to 10 Â¿. The scanning process proceeds entirely automatically until the selected area has been scanned. The stage motion is produced by thermal expansion and contraction in tungsten wires through which electronically controlled heating currents are passed. Precise control of the velocity and position of the stage is achieved by the use of feedback systems to control the heating currents in the wires so that their resistances, and therefore their temperatures, are accurately related to the values of analog control voltages. The relation between the horizontal and vertical stage displacements and the controlling voltages is linear within the precision of an occular micrometer.