2.3 Instruments of Microscopy

Microscopy Fundamentals

Microscopy is the foundation of microbiology. Without it, we'd have no way to observe organisms too small for the naked eye. The brightfield compound microscope is the workhorse of most microbiology labs, and understanding its components is the first step toward using it well.

Components of Brightfield Microscopes

A brightfield microscope has three systems working together: illumination, magnification, and mechanical positioning.

Illumination system provides and focuses light on your specimen:

• The light source (halogen lamp or LED) generates illumination from below the stage
• The condenser lens sits beneath the stage and concentrates that light onto the specimen, improving both brightness and contrast
• The iris diaphragm (built into or near the condenser) controls how much light passes through the specimen. Opening it wider lets in more light; closing it increases contrast

Magnification system enlarges the image in two stages:

• Objective lenses are the primary magnifying lenses, mounted on a rotating nosepiece. Standard magnifications are 4x, 10x, 40x, and 100x. Higher-magnification objectives have shorter focal lengths and smaller fields of view. The 100x oil immersion objective requires a drop of immersion oil between the lens and the slide to prevent light refraction and maintain image clarity.
• The ocular lens (eyepiece) provides a second round of magnification, typically 10x. It takes the real image produced by the objective and creates a magnified virtual image that you see when you look through the microscope.

Mechanical components hold and position everything:

• The mechanical stage securely holds the slide and allows precise movement in the x and y directions using stage control knobs
• Coarse focus moves the stage (or body tube) in large increments for initial focusing
• Fine focus makes small, precise adjustments to sharpen the image

Magnification in Compound Microscopes

Total magnification is calculated by multiplying the objective lens power by the ocular lens power:

$\text{Total Magnification} = \text{Objective Magnification} \times \text{Ocular Magnification}$

For example, a 40x objective with a 10x ocular gives you $40 \times 10 = 400\text{x}$ total magnification.

You can also work backward to determine the actual size of a structure. If a cell measures 8 mm across in your field of view at 400x total magnification:

$\text{Actual Size} = \frac{\text{Measured Size}}{\text{Total Magnification}} = \frac{8 \text{ mm}}{400} = 0.02 \text{ mm} = 20 \text{ µm}$

In practice, different magnifications serve different purposes. Lower magnifications (40x–100x) help you scan a slide and locate regions of interest. Higher magnifications (400x–1000x) let you examine cell morphology, identify microorganisms, and detect structural details in tissue samples.

Advanced Microscopy Techniques

Light vs. Electron vs. Scanning Probe Microscopes

These three categories differ in what they use to "see" the specimen and what kind of detail they reveal.

Light microscopes use visible light and glass lenses. They max out around 1000x magnification because resolution is limited by the wavelength of visible light (about 400–700 nm). The best resolution you can achieve is roughly 0.2 µm. Several specialized techniques fall under this category:

• Brightfield: standard illumination; staining usually needed for contrast
• Darkfield: light hits the specimen at an angle so only scattered light enters the objective, making organisms glow against a dark background
• Phase contrast: converts differences in refractive index into visible contrast, useful for observing live, unstained cells
• Differential interference contrast (DIC): similar to phase contrast but produces a 3D-like image with better detail

A major advantage of light microscopy is that you can observe living cells in real time.

Electron microscopes replace light with a focused beam of electrons, which have much shorter wavelengths. This allows magnification up to ~1,000,000x and resolution down to about 0.05 nm. Specimens must be fixed and placed in a vacuum, so living cells cannot be observed.

• Transmission electron microscope (TEM): electrons pass through ultra-thin sections of the specimen. Reveals detailed internal structures like organelles, membranes, and viruses (this is how early images of coronavirus structure were captured).
• Scanning electron microscope (SEM): electrons bounce off the specimen's surface, generating detailed 3D images of external morphology. Useful for examining surface features like biofilm architecture or bacterial appendages.

Scanning probe microscopes use a physical probe that scans across the specimen's surface at extremely close range, achieving nanoscale resolution.

• Atomic force microscope (AFM): measures tiny forces between the probe tip and the sample surface. Can image living cells and biological molecules (like proteins) under normal physiological conditions, which is a major advantage over electron microscopy.
• Scanning tunneling microscope (STM): measures electrical current (quantum tunneling) between the probe and the surface. Works only on conductive or semi-conductive samples. Used for studying molecular structures like DNA on prepared surfaces.

Both AFM and STM produce 3D surface topography maps and can reveal molecular interactions at scales no optical system can reach.

Microscopy Principles and Techniques

Two concepts are central to all microscopy: magnification and resolution.

• Magnification is how much larger the image appears compared to the actual specimen.
• Resolution is the ability to distinguish two closely spaced objects as separate. You can magnify an image endlessly, but without sufficient resolution, it just becomes a bigger blur.

Resolution in light microscopy depends on two factors: the wavelength of light used (shorter wavelengths give better resolution) and the numerical aperture (NA) of the objective lens (higher NA collects more light and improves resolving power). The relationship is:

$d = \frac{0.61\lambda}{NA}$

where $d$ is the minimum resolvable distance and $\lambda$ is the wavelength of light.

Specimen preparation varies by technique:

• Fixation preserves cell structure (common for electron microscopy)
• Staining adds contrast in light microscopy (Gram stain, acid-fast stain, etc.)
• Sectioning cuts specimens into thin slices, especially important for TEM where electrons must pass through the sample

Fluorescence microscopy is a specialized light technique that uses fluorescent dyes or fluorescent proteins (like GFP) to label specific cellular components. When excited by a particular wavelength of light, these molecules emit light at a longer wavelength, making targeted structures visible against a dark background. This is widely used to track gene expression, locate specific proteins, and visualize cellular processes in real time.