A majority of modern research-grade optical microscopes have binocular or trinocular observation tube systems. The eye tubes are usually adjustable for a range of interocular distances to accommodate the interpupillary separation of the microscopist (usually between 55 and 75 millimeters).

This tutorial examines how the eyepieces are adjusted for the microscopist's optimum interpupillary distance and how the diopter adjustment on each eyepiece can be made independently. To operate the tutorial, use the Interpupillary Distance slider to adjust this value (the size, in millimeters, is given just below the slider). Once the proper interpupillary distance has been set, use the Microscope Focus slider to bring the sample very close to focus while observing the image formed in the left eyepiece. When the specimen approaches focus, utilize the Left Eyepiece Diopter slider to bring the sample into sharp focus. Next, use the Right Eyepiece Diopter slider to bring this eyepiece into focus, matching the left eyepiece.

Many polarized light microscopes are equipped with an eyepiece diopter adjustment, which should be made to each of the eyepieces individually. Some microscopes have a graded scale on each eyepiece that indicates the position of the eye lens with respect to main body of the eyepiece. Other models hold the body of the eyepiece in a fixed position securely in the eye tube with a pin and slot. The first step in diopter adjustment is to either line up the graded markings (Figure 10) on eyepieces equipped with such markings or turn the eye lenses clockwise to the shortest focal length position. Next, focus the specimen with the 10x objective and then rotate the nosepiece until a lower magnification objective (usually the 5x) is above the specimen. At this point, refocus each eye lens individually (do not use the microscope coarse or fine focus mechanisms) until the specimen is in sharp focus. Rotate the 20x objective into the optical path and refocus the microscope with the fine focus knob. Repeat the diopter eye lens adjustments with the 5x objective (again not disturbing the microscope fine focus mechanism), and the microscope should be adjusted to the correct diopter settings. These settings will vary from user to user, so record the position of the eye lenses if the eyepiece has a graded scale for quick return to the proper adjustment.

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