Issues on Proper Use of the Focusing Knobs of Optical Microscope
Finding the specimen by adjusting the focusing knobs is one of the most important steps in microscope operation, and it is also the most difficult part for students. Students tend to make three common mistakes in operation: first, adjusting the coarse focus knob when using the high-power objective; second, keeping eyes at the eyepiece while lowering or raising the objective; third, not knowing the limit of the working distance and continuing to turn the focusing knob rapidly when the working distance is only 2–3 cm. The first two mistakes often cause the objective lens to crash into the slide, damaging the lens or specimen. The third error is frequently seen among students using microscopes. Teachers should emphasize to students that focusing adjustment must be carried out under low magnification. First rotate the coarse focus knob to slowly lower the objective lens close to the slide, taking care not to let the lens touch the slide. During this process, watch the objective from the side. Then look into the eyepiece with the left eye and slowly rotate the coarse focus knob to raise the objective, while checking continuously until the specimen comes into view. Meanwhile, explain to students that the working distance of most microscopes is about 1 cm. If the working distance is far more than 1 cm and the specimen is still invisible, the target is outside the field of view or the focusing knob is turned too fast. In this case, reposition the slide and repeat the above steps. When the specimen outline appears in the field of view, switch to the fine focus knob. Only in this way can the resolution be improved and the specimen be observed clearly.
