Overview
This lecture covers the basic parts, operation, and practical use of a compound microscope, including magnification calculations and slide preparation.
Microscope Parts and Functions
- The ocular lens (eyepiece) is where you look through, usually 10x magnification.
- Objective lenses (scanning/red 4x, low/yellow 10x, high/blue 40x) are rotated to change magnification.
- The arm connects the top to the base and is used for carrying the microscope.
- The stage holds the slide in place; stage knobs move the slide side to side or forward/back.
- The condenser focuses light onto the slide; the diaphragm adjusts the amount of light.
- The coarse adjustment knob raises/lowers the stage quickly, used for rough focusing.
- The fine adjustment knob is for precise focusing, especially on high power.
- The base supports the microscope and often houses the light source.
- The on/off switch and brightness control manage microscope power and illumination.
Using the Microscope
- Always carry the microscope with two hands: one under the base, one on the arm.
- Plug in and turn on the microscope before use.
- Place the slide on the stage and secure it; never move the slide by hand—use stage knobs.
- Start with the scanning (4x) objective and bring the image into focus with the coarse adjustment, then fine adjustment.
- Rotate to low (10x) and high (40x) power as needed; only use fine adjustment at high power to avoid breaking slides.
- When finished, lower the stage, remove the slide, and return the microscope properly.
Magnification and Measurement
- Total magnification = ocular lens x objective lens (e.g., 10x x 4x = 40x).
- Field of view decreases as magnification increases, showing less of the specimen.
- To estimate specimen size, divide the field of view diameter by the number of specimens fitting across.
- Example: 2 mm field of view/6 cells = approx. 0.33 mm per cell.
- Formula for field diameter: (low power field diameter x low power total magnification) / high power total magnification = high power field diameter.
Types of Slides
- Permanent slides are pre-made, glued, and labeled for repeated use.
- Wet mount slides are prepared fresh for each use using water or stain and a coverslip—dispose of sample, rinse and keep the slide.
Key Terms & Definitions
- Ocular lens (Eyepiece) — lens you look through, typically 10x power.
- Objective lens — interchangeable lenses with different magnifications.
- Coarse adjustment — knob for broad focus changes.
- Fine adjustment — knob for detailed focus.
- Condenser — focuses light onto the specimen.
- Diaphragm — regulates light intensity.
- Field of view — visible area through the lens at a given magnification.
- Wet mount — a temporary slide preparation using liquid and a coverslip.
Action Items / Next Steps
- Review names and functions of microscope parts in your lab manual/module.
- Practice focusing and changing objectives on your microscope.
- Complete any exercises measuring field of view with a ruler and calculating specimen size.
- Prepare and observe a wet mount slide as demonstrated.