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Biology LibreTexts

3: Microscopy

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    A microscope is an instrument that magnifies an object so that it may be seen by the observer. Because cells are usually too small to see with the naked eye, a microscope is an essential tool in the field of biology. In addition to magnification, microscopes also provide resolution, which is the ability to distinguish two nearby objects as separate. A combination of magnification and resolution is necessary to clearly view specimens under the microscope. The light microscope bends a beam of light at the specimen using a series of lenses to provide a clear image of the specimen to the observer.

    In this lab, parts of the microscope will be reviewed. Students will learn proper use and care of the microscope and observe samples from pond water.

    Parts of the microscope

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    Your microscope has 4 objective lenses: Scanning (4x), Low (10x), High (40x), and Oil Immersion (100x). In this lab you will not use the oil immersion lens; it is for viewing microorganisms and requires technical instructions not covered in this procedure.

    In addition to the objective lenses, the ocular lens (eyepiece) has a magnification. The total magnification is determined by multiplying the magnification of the ocular and objective lenses.

      Magnification Ocular lens Total Magnification
    Scanning 4x 10x 40x
    Low Power 10x 10x 100x
    High Power 40x 10x 400x
    Oil Immersion 100x 10x 1000x

    General Procedures

    1. Make sure all backpacks, purses, etc. are off the benchtop.

    2. Carry microscope by the base and arm with both hands.

    3. Store with cord wrapped around microscope and the scanning objective clicked into place.

    Focusing Specimens

    1. Plug your microscope in to power supply and switch on illuminator.

    2. Always start with the stage as low as possible and using scanning objective (4x). Odds are, you will be able to see something on this setting (sometimes it’s only a color). Use the coarse knob to focus: the image may be small at this magnification, but you won't be able to find it on the higher powers without this first step. Move the mechanical stage until your focused image is also centered.

    3. Once you've focused using the scanning objective, switch to the low power objective (10x). Use the coarse knob to refocus and move the mechanical stage to re-center your image. Again, if you haven't focused on this level, you will not be able to move to the next level.

    4. Now switch to the high power objective (40x). At this point, ONLY use the fine adjustment knob to focus specimens.

    5. If the specimen is too light or too dark, try adjusting the diaphragm.


    1. Store microscope with the scanning objective in place and the stage in its lowest position.

    2. Wrap cords around microscope.

    3. Replace slides to original slide tray.


    Occasionally you may have trouble with working your microscope. Here are some common problems and solutions.

    1. Image is too dark!

    • Adjust the diaphragm, make sure your light is on.

    2. There's a spot in my viewing field- even when I move the slide the spot stays in the same place!

    • Your lens is dirty. Use lens paper, and only lens paper to carefully clean the objective and ocular lens. The ocular lens can be removed to clean the inside.

    3. I can't see anything under high power!

    • Remember the steps, if you can't focus under scanning and then low power, you won't be able to focus anything under high power.

    4. Only half of my viewing field is lit, it looks like there's a half-moon in there!

    • You probably don't have your objective fully clicked into place.

    5. I see my eyelashes!

    • You’re too close to the objectives. Move your head back a little.

    6. This is giving me a headache!

    • Relax. Try adjusting the ocular distance, check that the intensity of your light isn’t too high or too low. Take breaks if needed!