Microscope Techniques and Biological Drawings

Preparing a Microscope Slide

You cannot simply place a thick piece of tissue under a lens and expect to see its cells. Light must be able to pass through the biological specimen, meaning it must be prepared carefully into a layer just one cell thick.

• Plant Cells (Onion): Use forceps to peel a thin, transparent layer of epidermis from the inner curve of an onion leaf. Place this flat on a glass slide and add a drop of iodine solution. This is a staining technique that colours the starch, cell walls, and nucleus to increase contrast.
• Animal Cells (Cheek): Gently scrape the inside of your cheek with a sterile cotton bud and smear the collected cells onto the centre of a slide. Add a drop of methylene blue to stain the nucleus and cell membrane. As a critical safety protocol, immediately place the used cotton bud in a beaker of disinfectant (such as Virkon) to mitigate biohazard risks.
• Adding the Coverslip: Use a mounted needle to slowly lower a glass coverslip over the specimen at a 45° angle. Lowering it at an angle gently pushes the air out and prevents air bubbles from becoming trapped over your cells.

Operating a Light Microscope

Searching for a microscopic specimen feels like trying to find a specific house from an airplane. You must start wide before zooming in on the details.

• Initial Setup: Always begin by clicking the lowest power objective lens (usually x4) into place. This provides the widest field of view, making it significantly easier to locate your specimen on the slide.
• Placing the Slide: Place your prepared slide onto the stage. Use the stage clips to secure the slide firmly in place so it does not move during focusing.
• Pre-Focusing: Look at the microscope from the side (not through the eyepiece). Turn the coarse adjustment knob to move the stage as close to the objective lens as possible. Watching from the side ensures you do not accidentally crush the slide.
• Focusing: Now, look through the eyepiece lens. Turn the coarse adjustment knob to move the stage away from the lens until the cells come into rough focus.
• Fine-Tuning: Slowly turn the fine adjustment knob to sharpen the image clarity.
• High Power: Once focused on low power, rotate the nosepiece to a higher magnification (e.g., x40). From this point on, you must only use the fine adjustment knob. The working distance is so small that using the coarse knob could smash the lens into your glass slide.

Magnification vs. Resolution

It is important to understand the difference between how large an image appears and how much detail you can actually see.

• Magnification: This describes how many times larger an image is compared to the real object.
• Resolution: This is the ability to distinguish between two separate points that are close together. A higher resolution provides a clearer image with more visible detail.
• Limits of Light Microscopy: A standard light microscope is limited by the wavelength of light. Its maximum resolution is approximately 200 nm. If two objects are closer than 200 nm, they will appear as a single blurred point, regardless of how much you increase the magnification.

Calculating Magnification

A drawing of a cell might be the size of an apple, but the real biological structure is far too small to see with the naked eye.

The overall power of a microscope is found by multiplying the power of its two lenses together:

$$\text{Total Magnification} = \text{Eyepiece Lens} \times \text{Objective Lens}$$

To find the magnification of a micrograph or drawing compared to the real object, you use the standard magnification formula. You must ensure that both the Image Size and Actual Size are in the exact same units before you calculate.

$$\text{Magnification} = \frac{\text{Image Size}}{\text{Actual Size}}$$

Worked Example

A student draws a high-power cellular drawing of an onion cell. The length of the cell in their drawing is 60 mm. The actual length of the onion cell is 200 $\mu$m. Calculate the magnification of the drawing.

Step 1: Convert the units so they match. (Remember: $1\,\text{mm} = 1000\,\mu\text{m}$)

• $60\,\text{mm} \times 1000 = 60{,}000\,\mu\text{m}$

Step 2: Substitute the identical units into the magnification equation.

• $\text{Magnification} = \frac{60{,}000}{200}$

Step 3: Calculate the final answer, ensuring you include the magnification symbol.

• $\text{Magnification} = \times 300$

Biological Drawing Conventions

In science, a drawing is not an art project—it is a precise, factual record of what you observe.

When producing a scientific biological drawing, you must follow a strict set of rules:

• Equipment: Always use a sharp HB or 2H pencil on plain, unlined white paper. Ink and colours are strictly forbidden.
• Line Quality: Draw clear, single, and continuous lines. Overlapping, sketchy, or "hairy" lines are not acceptable.
• Shading: Never use shading, colouring, or stippling (dots). To indicate a dark area in a cell, simply draw its outline.
• Accuracy: Draw exactly what you see, keeping the proportions accurate. If a cell has an irregular shape, draw it irregularly rather than making it look like a perfect textbook diagram.
• Labelling: Label lines must be drawn straight using a ruler and must touch the exact structure being identified. Lines must never cross each other, and they must never have arrowheads. Write the label text horizontally.
• Metadata: Every drawing must include a descriptive, underlined title alongside either the total magnification or a drawn scale bar.

You may be asked to draw a low-power tissue plan (which outlines tissue layers without drawing individual cells) or a high-power cellular drawing (which details a few specific cells and their internal structures).