Microscopy Lab Notes: Compound Light Microscope, Parfocal, and Oil Immersion

Light-based image sharpening in microscopy

  • Objective: sharpen our image to look bigger, clearer, and more detailed using light-based methods; this is called compounding or condensing light to sharpen an image.
  • Context: using a compound light microscope in the microscopy lab to condense light and produce a clear, sharp image.
  • For bigger structures (e.g., tissue pieces): a dissecting microscope is available but not used in this room; it is milder and intended for larger samples.
  • Typical specimen size in this course: about 10 microns on average; microbe size is around ~10 μm, hence focus on small-scale specimens with the compound light microscope.

Microscopy types: overview

  • Compound light microscope (this course): uses light to illuminate and form an image; produces heat due to light emission, which can eventually affect or kill organisms in slide preparations.
  • Dissecting microscope: used for larger structures; not used in the current room.
  • Electron microscopes (TEM and SEM): more powerful than light microscopes; do not use visible light but electrons; proposed to users:
    • TEM (transmission electron microscope): transmits electrons through the specimen to reveal internal structures and organelles; internal arrangement and DNA organization visible.
    • SEM (scanning electron microscope): scans the surface to reveal 3D surface texture and morphology; provides detailed surface features and textures, including flagella arrangements.
  • General notes:
    • Electron microscopes are large, expensive, and require more space and infrastructure.
    • Most textbook images of tiny microbes come from electron microscopy.
    • The images shown in class for TEM vs SEM differ in the type of information they provide (internal vs surface).

Compound light microscope: key components and workflow

  • Core idea: we use light to refract or reflect through the specimen to observe a magnified image.
  • Light path: light source at the bottom shines upward through the specimen and into the eyepiece.
  • Image formation depends on two main parts: ocular lens and objective lenses.
  • The host microscope is designed for teaching and learning, with several aids to help beginners learn the basics.
  • Orientation: the specimen sits on the mechanical stage and is clipped by stage clips to prevent slipping and unintended movement.

Optical path and basic terms

  • Ocular lens (eyepiece): magnification power of 10×; the part you look through to visualize the specimen.
  • Objective lenses (on the rotating nosepiece): four lenses total in the lab, color-coded to indicate power:
    • Scanning lens (short, often used for initial focus) – exists but is not always used in this course; not shown in every slide.
    • Low power lens: yellow-coded; magnifies 10×.
    • High dry lens: blue-coded; magnifies 40×.
    • Oil immersion lens: white-coded; magnifies 100×.
  • Magnification relationship: total magnification = ocular magnification × objective magnification.
    • Low power: M<em>extocular=10, M</em>extobjective=10⇒Total Magnification=10×10=100M<em>{ ext{ocular}} = 10, \, M</em>{ ext{objective}} = 10 \Rightarrow \text{Total Magnification} = 10 \times 10 = 100
    • High dry: Mextobjective=40⇒Total Magnification=10×40=400M_{ ext{objective}} = 40 \Rightarrow \text{Total Magnification} = 10 \times 40 = 400
    • Oil immersion: Mextobjective=100⇒Total Magnification=10×100=1000M_{ ext{objective}} = 100 \Rightarrow \text{Total Magnification} = 10 \times 100 = 1000
  • Working distance (WD): the distance from the objective lens to the slide; as magnification increases, WD decreases; at oil immersion, the lens approaches the slide very closely and should be in contact with oil.
  • Light source placement: bottom-mounted lamp; light is directed up through the specimen to the ocular lens.
  • Stage and stage controls: mechanical stage offers precise front-back and left-right movement; stage is secured by two silver stage clips.
  • Coarse vs fine focus knobs (located on both sides of the base for teaching):
    • Coarse adjustment: rough focusing; physically moves the stage closer or farther from the objective lens.
    • Fine adjustment: precise focusing; bends light to sharpen the image without moving the stage.
  • Parfocal concept: a key operating rule for microscopy.
    • Parfocal means you start with the lowest power lens and move directly to higher power lenses without adjusting focus in between.
    • In practice: as you switch from low to high power, you should not move the stage between lens changes; the slide should be nearly in focus and only refocused with fine adjustment when needed.
    • Important nuance: with higher power lenses, you will be extremely close to the slide; aim to have the slide touching the lens region or be very near it by the time you reach oil immersion, as this is part of parfocal practice.
  • Practical caution: do not use coarse focus once you switch to blue (high dry) or white (oil immersion) lenses, because coarse adjustments can crack or misalign high-power lenses.
  • If you lose focus at high power: go back down to low power, refocus, then work back up to regain the sharp image.

The four objective lenses and their practical use

  • Scanning lens (not always in use; exists to help find the specimen) – lowest power; not illustrated in every slide.

  • Low power lens (yellow): magnifies 10×; allows both coarse and fine focus because its shorter distance to the slide allows movement of the stage.

  • High dry lens (blue): magnifies 40×; near the slide; use only with fine focus; coarse focus is not recommended here due to proximity.

  • Oil immersion lens (white): magnifies 100×; requires a drop of oil on the slide; light refraction improves with oil; is used with only fine focus; coarse focus is prohibited to avoid lens damage.

  • Total magnifications:

    • Low power: 100×100\times
    • High dry: 400×400\times
    • Oil immersion: 1000×1000\times

Parfocal sequence and handling tips

  • Start with the lowest power lens and proceed to higher powers without adjusting focus in between.
  • As you increase magnification, you should keep the slide aligned and close to the objective, but avoid moving the stage between power changes beyond the intended sequence.
  • If the image becomes blurry at a higher power, first bring the slide back into roughly the same focal plane using a lower power, then re-focus carefully with fine adjustment as you move back up.
  • When switching to oil immersion, remember:
    • Place a small drop of oil on the slide, not on the lens.
    • The oil improves refraction and image clarity, allowing a higher numerical aperture to be used.
    • The objective will be in very close contact with the oil and should only use the fine focus knob.
    • Do not use coarse focus with oil immersion; it can damage the lens.

Working distance and oil immersion details

  • Working distance decreases as magnification increases; the distance between the objective and the slide shortens from low power to high power to especially near the slide with oil immersion.
  • Oil immersion requires physical contact between the lens and the oil layer on the slide; this is what enables the high resolution and the 1000× total magnification.
  • If you lose focus at high power, return to low power, refocus, and reattempt at higher power.

Lab setup, slides, and safety considerations

  • Lab setup:
    • Microlab benches with drawers, access left/right depending on seating; each student has a personal drawer with their prepared slides for the semester.
    • Prepared slides include BSL-2 infectious agents that are dead/killed for safe viewing (examples mentioned: Mycobacterium tuberculosis, Giardia, Toxoplasma gondii, and blood).
  • Safety and handling:
    • Prepared slides are dead and safe for handling under supervision; still observe standard lab safety and instruction.
    • The lab context mentions BSL-2 level materials; these are prepared slides and are to be viewed safely as instructed.
  • Slide handling specifics:
    • You will be working with two slides today: blood and bacterial types.
    • Blood slide is used to practice focusing from low power to oil immersion.
    • The bacterial types slide features three morphologies to observe:
    • Bacillus (rod-shaped)
    • Staphylococcus (coccus, spherical)
    • A spirochete-like morphology described as a spider shed in the transcript (likely a mispronunciation of “spirochete” or “spirillum”); the slide is meant to illustrate Lyme disease organism morphology.
  • Materials access:
    • Each student buddy pair selects a microscope and two slides to examine; the instructor assigns two slides per group (blood and bacterial types).
    • The slides and a set of questions are in pages 8 and 9 of the lab book; page 8 contains definitions, page 9 contains multiple-choice questions.
  • Assignment workflow for today:
    • Pages 6 and 7: Blood slide and bacterial types slide activities; practice focusing from low power to oil immersion.
    • Take pictures or sketches of what you observe to aid memory; use phone camera by carefully aligning with the ocular lens or sketch the images.
    • Pages 8 and 9: Answer definitions and multiple choice questions related to today’s material; submission of a compact assignment.
  • Next steps:
    • Next week, students will work on more advanced handling and familiarization with the microscopes and slides.
    • The lab will continue with more complex samples and techniques.

Quick tips, student workflow, and memory aids

  • Buddy system: pair up with a partner to share microscopes and slides; decide which microscope to use.
  • Practice tips:
    • Begin with blood slide to build comfort with low power and oil immersion.
    • Move to the smaller bacterial-type slide to simulate observing tiny organisms.
    • For memory, take pictures or draw quick sketches of what you observe.
  • Common pitfalls and remedies:
    • Don’t move the stage when switching from low to higher power — parfocal rule.
    • If the image is not in focus at high power, revert to low power, refocus, then proceed again.
    • Avoid using coarse adjustment with blue (high dry) or white (oil immersion) lenses to prevent lens damage.
    • Oil immersion requires the lens to be very close to the slide; handle carefully to avoid lens damage.
  • Documentation: prepare to submit a short report comprising two slide observations and the answers to pages 8–9 questions; include any photos or sketches.

Summary of key formulas and terms

  • Total magnification formula: Total Magnification=M<em>ocular×M</em>objective\text{Total Magnification} = M<em>{\text{ocular}} \times M</em>{\text{objective}}
    • Low power: 100×100\times (10 × 10)
    • High dry: 400×400\times (10 × 40)
    • Oil immersion: 1000×1000\times (10 × 100)
  • Key terms to memorize:
    • Parfocal: switch from low to high power without refocusing in between.
    • Working distance: distance from the objective lens to the slide; decreases with higher magnification.
    • Coarse focus: moves the stage; used only at low power.
    • Fine focus: adjusts optical focus by refining the light path; used at all higher magnifications, and exclusively for blue and white lenses.
    • Oil immersion: 100× objective lens with oil on the slide; enhances resolution and light gathering.

Quick recall prompts (for study)

  • What is parfocal, and why is it important when using multiple objective lenses?
  • What happens to working distance as you move from low power to oil immersion?
  • Why is coarse focus avoided on blue and white lenses?
  • How do TEM and SEM differ in the type of information they provide about microbes?
  • What samples are prepared for today’s observation, and what is the safety context for these slides?