Several precautions for using an optical microscope
As the most commonly used instrument in middle school biology experiments, the correct use of a microscope directly affects the success or failure of many biology experiments. Pointing out some errors in students' use of microscopes and guiding them to use them correctly should be an extremely important content in middle school biology experimental teaching. In the practice of biology teaching, the author believes that it is necessary for teachers to emphasize the following points in their lectures, based on the observation of students' use of microscopes during operation and some issues reflected during experimental examinations.
1、 Correct installation issues
Before using a microscope, first install the eyepiece and objective of the microscope. The installation of eyepieces is relatively simple, and the main problem lies in the installation of the objective lens. Due to the high value of the objective lens, if the thread is not properly closed during installation, it is easy for students to fall to the ground and cause lens damage. Therefore, for safety reasons, it is emphasized that students should use their left index and middle fingers to hold the objective lens when installing it, and then use their right hand to install it, so that even if it is not properly installed, it will not fall to the ground.
2、 Correct alignment of light issues
Focusing on light is an important step when using a microscope. Some students casually turn an objective lens towards the aperture when focusing on light, rather than using a low-power lens as required. When rotating the reflector, I like to use one hand and often pull it off. So when guiding students, teachers must emphasize using low-power mirrors to aim at light. When the light is strong, small aperture and flat mirrors should be used, while when the light is weak, large aperture and concave mirrors should be used. Reflectors should be rotated with both hands until a uniformly bright circular field of view is seen. After the light is aligned, do not move the microscope casually to prevent light from accurately entering the aperture through the reflector.
The problem of correctly using the quasi focal screw
Using a quasi focal screw to adjust the focal length and find the object image can be said to be the most important step in the use of a microscope, and it is also the most difficult step for students. Students are prone to the following errors during the operation process: firstly, directly focusing under a high-power mirror; The second is that regardless of whether the lens barrel rises or falls, the eyes always look into the eyeglass to see the field of view; The third reason is that the critical value of the object distance is not understood. When the object distance is adjusted to 2-3 centimeters, it is still upward, and the speed of rotating the quasi focal screw is very fast. The first two types of errors often result in the objective lens colliding with the mount, damaging the mount or lens, while the third type of error is the most common phenomenon for students when using a microscope. In response to the above errors, the teacher must emphasize to the students that adjusting the focal length must be lowered at a low magnification. First, rotate the coarse focusing screw to slowly lower the lens barrel, and the objective lens should be close to the glass slide. However, be careful not to let the objective lens touch the glass slide. During this process, the eye should look at the objective lens from the side, then use the left eye to gaze into the objective lens, and slowly reverse adjust the coarse focusing screw to slowly raise the lens barrel until the object image is seen, At the same time, explain to the students that the object distance of a general microscope is about 1 centimeter. Therefore, if the object distance has exceeded 1 centimeter, but the object image is still not visible, it may be that the specimen is not in the field of view or the coarse focus spiral speed is too fast. At this point, the loading position should be adjusted, and then repeat the above steps. When there is a blurry object image in the field of view, a fine focus spiral adjustment should be used to narrow the search range, Increase the speed of finding objects.
4、 The problem of objective lens conversion
After using a low-power lens and switching to a high-power lens, students often prefer to use their fingers to directly rotate the objective lens, thinking it is more labor-saving. However, this can easily cause the optical axis of the objective lens to deviate, because the material of the converter is soft and has high accuracy, and the thread is easily loosened due to uneven force. Once the thread is damaged, the entire converter will be scrapped. Teachers should guide students to hold the lower rotating plate of the converter to convert the objective lens.
5、 The problem of correct eye use
When observing an object under a microscope, both eyes should be open at the same time, and the left eye should gaze into the eyepiece. However, many students often fail to achieve this, preferring to cover their right eye with their hands or simply close it, which does not meet the observation requirements of the experiment. This habit can cause left eye fatigue and also prevent them from drawing while observing. While pointing out the student's problem, the teacher should demonstrate in detail, telling the student that their left eye should be as close to the eyepiece as possible, and their right eye should try to gaze into the field of view. This repeated training will achieve the requirement of opening both eyes for observation. Alternatively, you can do the following exercises: open your eyes, use a piece of paper or palm to stand between your eyes, with your nose in front of you, so that your left and right eyes cannot look at each other on the opposite side. Then, consciously look to the left first, then to the right. Do this 3-5 times each day in the morning and evening, and you can learn it in less than an hour.
