Use of the Microscope Oil Immersion Objective
(1) First, observe the general outline of the specimen with a dry objective lens.
(2) Switch to a high‑power dry objective lens and move the target part of the specimen to the center of the field of view.
(3) Raise the body tube by about 1.5 cm, then rotate the oil immersion objective into the working position.
(4) Place a small drop of cedar oil (or paraffin oil) on the area of the coverslip to be observed. For immersion objectives with N.A. > 1.0 or under darkfield illumination, oil should also be added on the condenser (this step is generally omitted).
(5) Carefully turn the coarse focusing screw to slowly lower the body tube. Watch the clearance between the front tip of the objective and the specimen closely. Let the front tip of the objective make contact with the oil drop first, then keep lowering the tube slowly until the objective tip approaches the coverslip (do not touch it). Take extra care in this step to avoid crushing the specimen or damaging the oil immersion objective (the working distance of the oil objective is only about 0.2 mm).
(6) Look through the eyepiece and turn the fine focusing screw to slowly raise the body tube until the specimen is clearly focused. Be careful not to rotate the fine focusing screw in the wrong direction and crush the specimen.
(7) After obtaining a clear image, remove the eyepiece and look directly down the tube. Close the iris diaphragm of the condenser and then open it slowly until its aperture matches the size of the field of view, so that the aperture of the condenser matches that of the objective. If the condenser aperture is smaller than the objective aperture, the full performance of the objective cannot be utilized.
(8) After observation, raise the body tube by 1 cm, swing the oil immersion objective away from the optical axis, and clean it promptly. Wipe off most of the oil with lens paper once or twice first. Then wipe twice with lens paper moistened with xylene, followed by a dry piece of lens paper. Wipe straight along the diameter; do not wipe in circles around the lens. Use gentle, light movements and do not press hard. Clean the condenser in the same way if oil gets on it. For oil on the coverslip, use the "pull‑off method": place a small piece of lens paper over the oil drop on the coverslip, add a drop of xylene onto the paper, and pull the paper outward. Repeat 3–4 times to clean it. (If paraffin oil is used, clean with dry lens paper without xylene first; use xylene only after the experiment.)
(9) Lower the condenser by about 1 cm and move the specimen slide on the stage to a suitable position.
