A compound light microscope uses a combination of lenses and visible light passing through a specimen to magnify and resolve minute structures and cells invisible to the naked eye. This instrument is widely used in biological and medical studies. Learning to operate it properly ensures effective viewing and maintains the longevity of the precision optics. This guide provides a step-by-step process for safely and effectively using a compound light microscope.
Understanding the Basic Components
The optical components include the eyepiece (ocular lens), which provides $10\text{x}$ magnification. The revolving nosepiece holds the objective lenses, typically including low-power ($4\text{x}$), medium-power ($10\text{x}$), and high-power ($40\text{x}$) options. Total magnification is calculated by multiplying the power of the eyepiece by the power of the objective lens in use.
The specimen slide rests on the stage, secured by stage clips over a central aperture. Light is projected up through the specimen from the illumination source, usually a built-in lamp in the base. The coarse adjustment knob moves the stage significantly for initial, rough focusing. The fine adjustment knob permits minute movements of the stage for achieving sharp clarity once the image is generally in view.
Preparing the Specimen Slide
Observation begins by preparing the sample, which must be thin enough for light to pass through. For fresh samples, a wet mount is the most common method. This involves placing the specimen on a clean glass slide and adding a single drop of water or saline solution.
To prevent air bubbles, lower the coverslip carefully at a $45$-degree angle so one edge touches the liquid first. Slowly drop the coverslip over the specimen, allowing surface tension to spread the liquid underneath. Position the prepared slide centrally on the stage, ensuring the specimen is directly over the light source opening, and secure it using the stage clips.
Step-by-Step Focusing and Viewing
Rotate the lowest-power objective lens (often $4\text{x}$) into position until it clicks securely. This lens provides the widest field of view, making it easier to locate the specimen. While looking at the stage from the side, use the coarse adjustment knob to raise the stage until the objective lens is very close to the coverslip, stopping just short of contact.
Turn on the illumination and adjust the diaphragm to control the amount of light reaching the specimen. Looking through the eyepiece, slowly turn the coarse adjustment knob in the opposite direction, lowering the stage until the specimen comes into general view. Once the image is roughly focused, the coarse knob should not be touched again.
Use the fine adjustment knob exclusively to bring the image into sharp, detailed focus. After achieving clarity, center the specimen in the field of view before increasing magnification. To switch to a higher power, rotate the nosepiece until the next objective lens clicks into place. Because microscopes are parfocal, the image will remain close to focus, requiring only a slight turn of the fine adjustment knob for resolution.
Proper Care and Storage
Conclude the session by rotating the nosepiece back to the lowest-power objective lens to maximize the distance between the lens and the stage. Turn off the light source and remove the slide from the stage clips. Lenses should only be cleaned with approved lens paper and solution, as other materials can scratch the delicate glass coating.
Allow the built-in lamp to cool for a few minutes to extend the life of the bulb. Cover the microscope with its dust cover and store it in a cool, dry place. Always transport the instrument with one hand supporting the base and the other gripping the arm to protect the alignment of the components.