Microscope Use and Onion Cell Observation

Overview

This lecture covers the use of a light microscope for biological experiments, specifically focusing on preparing, observing, and measuring onion cells as part of a required practical.

Parts of the Microscope

• The microscope has a light source, mirror, stage (platform), handle, and turret with objective lenses.
• Always use the handle and base for carrying the microscope.
• The stage holds the slide with specimen clips.
• Objective lenses usually provide 4x, 10x, and 40x magnification.
• The eyepiece lens typically provides an additional 10x magnification.

Setting Up for Observation

• Use the smallest (lowest magnification) objective lens to start.
• Adjust the mirror for maximum light through the stage.
• Place the specimen slide on the stage and secure with clips, ensuring the cover slip is facing up.

Preparing the Onion Skin Slide

• Peel a thin layer of onion skin and lay it flat on the glass slide.
• Add a few drops of iodine to stain the cells and make them visible.
• Carefully lower a glass cover slip to avoid air bubbles; use blotting paper to remove excess stain if needed.

Focusing the Microscope

• Start with both the lens and stage as close together as safely possible.
• Use the coarse focus dial for large adjustments, then the fine focus dial for clarity.
• Adjust only slightly after changing objective lenses, especially when moving to higher magnifications.
• Never focus downwards while looking through the eyepiece to avoid damaging the slide or lens.

Measuring Cell Size

• At medium magnification, align a row of whole cells across the diameter of the field of view.
• Use a transparent ruler under the microscope to measure the field of view in millimeters.
• Convert millimeters to micrometers (1 mm = 1000 micrometers).
• Divide the field width (in micrometers) by the number of cells to estimate average cell size.

Key Terms & Definitions

• Stage — The platform where the slide is placed.
• Objective lens — Lenses with different magnification powers (e.g., 4x, 10x, 40x).
• Eyepiece lens — The lens you look through, usually adds 10x magnification.
• Coarse focus dial — Adjusts focus quickly with large movements.
• Fine focus dial — Makes small, precise adjustments for sharp focus.
• Micrometer/micron (μm) — Unit of length; 1 mm = 1000 μm.

Action Items / Next Steps

• Practice preparing and staining an onion skin slide.
• Measure the field of view under the microscope and calculate average cell size.
• Draw labeled diagrams of your observed cells, including size estimation.