Preparing Temporary Mounts and Staining
- When observing cells under a microscope, the first step is to prepare a slide.
- A temporary mount enables you to examine fresh or living specimens without the need for complex preparation techniques.
These mounts are quick to assemble and are ideal for observing dynamic processes such as cell division or the movement of organelles.
How to Prepare a Temporary Mount
- Obtain the Specimen: Select a thin layer of cells or tissue, such as the lower epidermis of a leaf or cells scraped from the inside of your cheek.
- Place on the Slide: Lay the specimen flat on a clean microscope slide.
- Add a Drop of Water or Stain: Use water to keep the cells hydrated, or apply a stain like methylene blue or iodine to enhance visibility by highlighting specific structures (e.g., the nucleus or starch granules).
- Lower the Cover Slip: Using forceps, gently lower a cover slip at an angle to minimize the risk of trapping air bubbles.
- Remove Excess Liquid: Use a paper towel to blot away any excess water or stain from the edges of the cover slip.
- Trapping air bubbles under the cover slip is a common error.
- Lower the cover slip slowly and at an angle to avoid this.
- For best results, use methylene blue to stain animal cells (e.g., cheek cells) and iodine to stain plant cells (e.g., banana cells).
- These stains highlight key structures like the nucleus or starch granules.
Focusing the Microscope
- Focusing correctly is essential for obtaining a clear image.
- Always start with the lowest magnification (e.g., 40×) and follow these steps:
- Position the Slide: Place the prepared slide on the stage and secure it with stage clips. Ensure the area of interest is centered over the light source.
- Coarse Focus: Use the coarse focusing knob to bring the specimen into view. Move the stage away from the objective lens to prevent accidental damage.
- Fine Focus: Once the image is nearly clear, switch to the fine focusing knob to sharpen the details.
- Starting with the high-power objective lens is a frequent mistake.
- Always begin with low power to locate the specimen before switching to higher magnifications.
Measuring Sizes Using an Eyepiece Graticule
Definition
Eyepiece graticule
An eyepiece graticule is a small scale etched onto the eyepiece lens of a microscope.
- It allows you to measure the size of structures in "eyepiece units" (EPU).
- However, the actual size represented by each EPU varies with magnification, so calibration is required.
How to Calibrate the Graticule
- Use a Stage Micrometer: Place a slide with a known scale (e.g., 1 mm divided into 100 divisions) on the stage.
- Align the Scales: Focus the microscope and align the stage micrometer scale with the eyepiece graticule.
- Calculate the Value of One EPU:
- Count how many EPUs align with a known length on the stage micrometer.
- Divide the known length by the number of EPUs.
