100 Turn the microscope on, and adjust the interpupillary distance between the two ocular lenses so that you see a single illuminated field, even if you move your head very slightly from side to side. If side-to-side movements of your head cause one field to black out, the interpupillary distance is set incorrectly. You must set this distance for your own eyes each time you use a microscope. The iris diaphragm in the substage condenser serves primarily to vary the contrast of the specimen being viewed. Generally, this iris diaphragm should be closed as far as possible consistent with good brightness (obtained by adjustment of the rheostat at the side of the microscope). If the image is muddy, lacks contrast after such adjustment, or is overlaid by blotches that do not move when the slide is moved back and forth with the mechanical stage , then some cleaning is indicated (see below). Make sure the ocular focusing rings located on one or both of the ocular tubes are set at middle positions on their numerical scales. Examine the field of view through the scanning lens and then through each of the other objectives, noting
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