Several Essentials for Correct Operation of Optical Microscopes
Ⅰ. Proper Installation
Before using the microscope, the eyepiece and objective lenses must first be installed. Installing the eyepiece is relatively straightforward. The main concern lies with the objective lens. Objective lenses are expensive. If the threads are not aligned properly during installation, the lens may fall to the ground and get damaged. For safety, users are advised to 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.
Ⅱ. Correct Illumination Alignment
Light alignment is a critical step for microscope operation. Some students randomly place an objective lens over the aperture instead of following the requirement to use the low-power objective for light alignment. When rotating the mirror, many use only one hand, which often causes the mirror to fall off. Therefore, instructors must emphasize to students that the low-power objective should be used for light alignment. Use the small diaphragm and plane mirror for bright ambient light, and the large diaphragm and concave mirror for dim light. Rotate the mirror with both hands until an evenly illuminated circular field of view is obtained. Do not move the microscope arbitrarily after alignment, otherwise light may fail to enter the aperture through the mirror properly.
Ⅲ. Proper Use of Focusing Knobs
Adjusting the focus with the focusing knobs to locate the specimen image is one of the most important and difficult steps for students. Students commonly make the following mistakes: focusing directly under high magnification; keeping their eyes looking into the eyepiece while the lens barrel moves up or down; failing to recognize the critical working distance and continuing to raise the lens barrel rapidly when it is already 2–3 cm above the slide. The first two mistakes often cause the objective lens to collide with the slide, damaging the slide or the lens. The third error is also very common among students.
To avoid these issues, instructors should stress that focusing must start under low magnification. First turn the coarse focusing knob to slowly lower the lens barrel and bring the objective close to the slide, taking care not to let the objective touch the coverslip. Watch the objective from the side during this process. Then look into the eyepiece with the left eye and slowly turn the coarse focusing knob in the reverse direction to raise the lens barrel gradually until the image comes into view. Students should also know that the typical working distance of a microscope is about 1 cm. If the distance greatly exceeds 1 cm with no image visible, the specimen may be outside the field of view or the knob was turned too quickly. Reposition the slide and repeat the procedure. Once a blurry image appears in the field of view, switch to the fine focusing knob. This narrows the search range and speeds up focusing.
