Key Points for Correct Use of an Optical Microscope
① Proper Installation Before using the microscope, install the eyepiece and objective lens first. Installing the eyepiece is straightforward. The main risk lies with the objective lens. Objective lenses are expensive. If the threads are not properly engaged during installation, the lens may fall and be damaged. For safety, users should support the objective lens with the index and middle fingers of the left hand while screwing it in with the right hand. This prevents the lens from dropping even if the fitting is not successful.
② Proper Alignment for Illumination Illumination alignment is a critical step. Some students randomly select an objective facing the aperture instead of starting with the low-power objective as required. When adjusting the mirror, they often use only one hand, which may knock the mirror off. Instructors must emphasize that alignment should always be done with the low‑power objective. Use a small diaphragm and plane mirror for bright ambient light; use a large diaphragm and concave mirror for dim light. Rotate the mirror with both hands until an evenly illuminated circular field of view appears. Once aligned, do not move the microscope arbitrarily, otherwise light cannot pass through the mirror into the aperture properly.
③ Proper Use of Focus Knobs Adjusting the focus knobs to locate the specimen image is one of the most important and difficult steps for students. Common mistakes include focusing directly under high magnification, constantly looking through the eyepiece while moving the barrel up or down, and ignoring the working distance limit. Some students keep turning the knob rapidly even when the working distance reaches 2–3 cm. The first two errors may cause the objective to hit the slide and damage either the slide or the lens. The third mistake is very common.
Teachers should stress that focusing must start on low power. First turn the coarse focus knob to lower the barrel slowly so the objective approaches the slide, but avoid contact. Watch the objective from the side during this process. Then look through the eyepiece with the left eye and slowly turn the coarse knob in the reverse direction to raise the barrel until the image appears. Explain that the typical working distance is around 1 cm. If the distance greatly exceeds 1 cm and no image is seen, the specimen may be outside the field of view or the knob was turned too fast. Reposition the slide and repeat the procedure. When a blurry image appears in the field, switch to the fine focus knob. This narrows the search range and speeds up image acquisition.
