Microscope Objective Parameters and Their Effects on Microscopic Imaging Quality
Ⅰ. Correct Installation
Before using the microscope, the eyepiece and objective lenses should be installed first. Installing the eyepiece is simple. The main difficulty lies in mounting objectives. Objective lenses are expensive. If students fail to align the threads properly during installation, the lens may drop to the ground and get damaged. For safety reasons, students are required to support the objective with the index and middle fingers of the left hand while screwing it in with the right hand. In this way, the objective will not fall even if it is not installed correctly.
Ⅱ. Correct Illumination Alignment
Illumination alignment is a critical step for microscope operation. Some students randomly select an objective facing the condenser aperture instead of starting with the low-power objective as required. They also tend to rotate the mirror with one hand, which may pull the mirror off. Teachers should emphasize that illumination alignment must be performed with the low-power objective. Use a small aperture and plane mirror under bright light conditions; use a large aperture and concave mirror under dim light conditions. Rotate the mirror with both hands until an evenly lit circular field of view is obtained. Do not move the microscope after alignment, otherwise light cannot enter the condenser aperture correctly via the mirror.
Ⅲ. Proper Use of Focusing Knobs
Focusing to locate the specimen image is the most important and often the most difficult step for students. Students frequently make the following mistakes: focusing directly under high magnification; keeping eyes at the eyepiece continuously whether the tube rises or falls; being unaware of the working distance limit and continuing to adjust rapidly when the working distance reaches 2–3 cm. The first two mistakes may cause the objective to strike the slide and damage the slide or lens. The third mistake is very common among students. Teachers must remind students that focusing should start under low magnification. Turn the coarse focusing knob to lower the tube slowly and bring the objective close to the slide, watching the objective from the side to avoid collision with the slide. Then look through the eyepiece with the left eye and slowly turn the coarse focusing knob in the opposite direction to raise the tube until the image appears.
Students should also know that the typical working distance of a microscope is about 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 focusing speed may be too fast. Reposition the slide and repeat the focusing steps. When a blurry image appears in the field of view, switch to the fine focusing knob. This narrows the searching range and speeds up image acquisition.
