Using a light microscope Knowledge-Based Study

Study notes

1. Overview of the Light Microscope

   A compound light microscope uses visible light transmitted through or reflected from a specimen, combined with a system of lenses, to magnify small biological structures. Understanding the function of each mechanical and optical component is essential for effective microscopy.

   Key Optical Components

   • Ocular lens (Eyepiece): The lens at the top of the microscope through which the viewer looks, typically providing an inherent magnification of 10x.
   • Objective lenses: Mounted on a rotating nosepiece, these lenses are positioned closest to the specimen. Common standard objectives include scanning power (4x), low power (10x), and high-dry power (40x).
   • Condenser: A lens assembly located beneath the stage that concentrates and focuses light onto the specimen.
   • Iris diaphragm: An adjustable shutter integrated with the condenser that regulates the amount of light entering the objective lens, controlling image contrast and resolution.

   Key Mechanical Components

   • Stage and stage clips: The flat platform where the microscope slide is placed and secured. Mechanical stages allow precise horizontal (X-Y) movement of the slide.
   • Coarse adjustment knob: Moves the stage or nosepiece rapidly up or down to bring the specimen into general focus; used strictly with low-magnification lenses.
   • Fine adjustment knob: Moves the stage in minute increments to sharpen the image details, particularly necessary at higher magnifications.

   Calculating Total Magnification

   In a compound microscope, image enlargement occurs in two distinct stages: first by the objective lens, and second by the ocular lens. Therefore, the total magnification is calculated by multiplying the magnifying power of the ocular lens by that of the objective lens:

   Total Magnification = Ocular Lens Power × Objective Lens Power

   • Scanning power: 10x ocular × 4x objective = 40x total magnification.
   • Low power: 10x ocular × 10x objective = 100x total magnification.
   • High-dry power: 10x ocular × 40x objective = 400x total magnification.

   Field of View and Inversion

   The observable circular area visible when looking through the eyepiece is called the field of view (FOV). As magnification increases, the diameter of the field of view decreases proportionally. Consequently, less of the total specimen is visible at high power, although that portion is seen in far greater detail.

   Additionally, compound microscopes produce an inverted and reversed virtual image. If a specimen moves toward the upper right on the stage, its image appears to move toward the lower left in the field of view.

   Proper Focusing Sequence

   To prevent slide breakage and lens scratching, microscope operation must always follow a standardized sequence:

   1. Start on the lowest objective: Rotate the revolving nosepiece until the lowest-power objective (such as 4x or 10x) clicks into position over the stage aperture.
   2. Place and center the slide: Secure the slide on the stage and ensure the biological specimen is positioned directly over the light beam coming from the stage opening.
   3. Coarse focus adjustment: While observing from the side, raise the stage using the coarse adjustment knob until it is near the top of its range. Then, looking through the eyepiece, slowly lower the stage with the coarse adjustment knob until the specimen comes into approximate focus.
   4. Fine focus refinement: Rotate the fine adjustment knob back and forth slightly until the cellular structures appear crisp and well-defined.
   5. Adjust lighting: Modify the iris diaphragm to balance illumination and contrast. An open diaphragm provides maximum light but can wash out thin, unstained specimens.

   Switching to High Power

   Most modern compound microscopes are parfocal, meaning that when an image is focused using one objective, it remains nearly in focus when rotating to a higher-power objective. When switching to a higher objective:

   • Ensure the target region is centered in the field of view before rotating the nosepiece.
   • Rotate the higher objective into alignment without moving the coarse focus knob.
   • Use only the fine adjustment knob to bring the image into sharp focus at high magnification. The distance between the objective lens and the slide (the working distance) is extremely small; using the coarse knob can crush the slide or damage the lens.

   Preparing a Wet Mount

   Living cells and thin biological tissues are routinely examined using a wet mount preparation to preserve hydration and optical clarity:

   1. Place a clean microscope slide on a flat work surface.
   2. Add a drop of water, physiological saline, or an appropriate stain (such as iodine or methylene blue) to the center of the slide.
   3. Carefully place a thin specimen into the liquid droplet.
   4. Hold a square coverslip at an approximate 45-degree angle against the slide, gently touching the edge of the droplet.
   5. Slowly lower the coverslip over the specimen to prevent the entrapment of air bubbles, which appear as thick black-bordered circles under illumination.

   Care and Handling Guidelines

   Reliable microscopy requires systematic care and cleaning practices:

   • Transport: Always carry the instrument with two hands—one hand gripping the arm securely and the other supporting the base.
   • Optical surfaces: Clean glass lenses exclusively with specialized lens paper. Paper towels, tissues, or shirt sleeves contain abrasive fibers that permanently scratch optical coatings.
   • Storage: When returning the microscope to storage, switch the power off, lower the stage completely, rotate to the lowest objective lens, remove any prepared slide, and cover the instrument with a dust jacket.