A compound microscope uses multiple lenses and transmitted light to produce a highly magnified, detailed view of tiny objects, such as cells or microorganisms. The objective lens creates a primary magnified image of the specimen, which is then further enlarged by the eyepiece, or ocular lens. This makes the compound microscope an indispensable tool in biology and medicine for studying minute structures. Proper operation is necessary for obtaining clear, accurate observations.
Preparing the Microscope and Specimen
Safely move the microscope by grasping the arm with one hand and supporting the base with the other. Once placed on a stable surface, plug in the power cord and switch on the light source, or illuminator. The stage, the platform where the specimen rests, uses clips or a mechanical holder to secure the glass slide.
Before mounting the specimen, rotate the revolving nosepiece so the lowest power objective lens clicks into position. This shortest objective, often 4x, ensures the largest field of view for initial orientation. Place the prepared slide onto the stage, ensuring the specimen is centered directly over the aperture where the light passes through. Starting with the lowest magnification minimizes the risk of damaging the lens or the slide.
Locating and Focusing the Image
With the low-power objective engaged, look at the microscope from the side and use the coarse adjustment knob to bring the stage as close to the objective lens as possible. This visual check helps prevent damage, even if the microscope has a built-in stop. Looking through the eyepiece, slowly turn the coarse adjustment knob to move the stage downward, away from the objective lens.
The specimen should come into focus as the stage is lowered; the coarse knob is used only at this lowest magnification setting. Once the image is roughly visible, stop using the coarse adjustment knob. All subsequent adjustments for clarity are made with the fine adjustment knob, which moves the stage by small increments to achieve a sharp focus. After focusing, use the stage controls to center the specific area of interest precisely in the field of view before increasing magnification.
Optimizing High Magnification
Moving to higher magnification is accomplished by rotating the nosepiece until the next objective lens (e.g., 10x or 40x) clicks into place. Most compound microscopes are parfocal, meaning that once an image is focused at a lower power, it remains nearly in focus when switching to a higher power objective. This allows the transition to be made without drastic refocusing.
When a higher power objective is engaged, only the fine adjustment knob should be used to achieve sharp focus. Using the coarse adjustment risks driving the objective lens directly into the glass slide, damaging both the lens and the specimen. As magnification increases, the field of view becomes smaller, and the working distance between the objective lens and the slide decreases.
Achieving optimal image quality at high power requires careful management of illumination. The objective lens aperture decreases as magnification increases, reducing the amount of light reaching the eye. Adjust the light intensity and contrast using the condenser and the iris diaphragm, which are located beneath the stage. The iris diaphragm controls the light beam’s diameter, and opening it further increases brightness. Proper adjustment ensures the image is neither too dim nor too bright, which could wash out detail and reduce resolution.
Post-Use Care and Storage
Once viewing is complete, rotate the objective lens back to the lowest power setting before removing the slide. Turn off the light source and allow the illuminator to cool for a few minutes before storage. If immersion oil was used, clean that lens immediately using only specialized lens paper to prevent the oil from hardening.
Gently wipe down all other objective and ocular lenses with lens paper, as regular tissues can scratch the optical surfaces. Wrap the power cord neatly around its designated storage bracket. Finally, cover the compound microscope with its dust cover to protect the optics and mechanical parts from dust and debris.