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How to Measure with a Microscope

Measuring with the microscope can be a f rustrating endeavor f or beginner biologists. In f act, I rarely teachmeasuring in my f reshman classes. Advanced (AP) biology does look at how to measure with themicroscope. The entire process involves f ocusing on high power, using math to solve a ratio problem, andin the end making an estimation of the f ield of view of your microscope. The section on measuring isincluded in the advanced microscopy lab, but the lab itself doesnt go into much detail (pref erring insteadf or the students to work it out. The steps and process Im including here as an extra aid f or students(and teachers) who need a ref resher on measuring with the microscope.

Disclaimer: Numbers are rough estimates based on a specific microscope. Microscopes can vary in theirmagnifications and viewing field diameter.

Estimating Viewing Field with a Ruler

Step 1: Use a clear ruler with a cm/mm scale to measure the diameter of your viewing f ield at scanning(40x). On our scopes, we estimated the viewing f ield to be about 4 mm across.

Step 2: Repeat the process on low power (100x). We estimate ourlow power viewing f ield to be about 2 mm across.

Convert mm to microns f or both the scanning and low power. Thereare 1000 microns in a millimeter. So Scanning = 4000 microns; Low Power = 2000 microns.

You can at this point use that measurement to measure anything inyour viewing f ield that you can see with low or scanning power. Themeasurement is an estimation though, and probably not veryaccurate. For instance, if you have a slide with the word are across it, use your measurement of the viewing f ield to estimate thedistance of each of the letters. In the drawing below, I havesuperimposed the slide over the millimeters of the ruler so you canmake your estimation. Based on the ruler marks, the letter r is about 1100 microns, and the entire word,which takes up most of the viewing f ield is about 4000 microns across. In reality, you cannot place yourslide over the ruler, so you have to make a guess based on how large your microscopes viewing f ield is.

Calculating High Power Field of View

Measurements on High Power can be a litt le more complicated. If youtry to use the clear ruler technique, youll f ind that you cannot see theindividual ruler marks. This is where math comes in, the values youestimated above can be used to solve a ratio problem and determinethe size of your viewing f ield on high power.

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Solving f or High Power Field of View .

X / 2000 = 100/400

X (high power f ield of view) = 500 microns

Now that you have 500 microns as an estimate of your viewing f ield, any object you are viewing under highpower can be estimated based on that. I tell my students to look at a paramecium and guess how manyparamecia can f it end to end on their slide. They guess they can f it two, then the length of theirparamecium is about 250 microns.

Using a Stage (Slide) Micrometer

These are slides you buy that contain a tiny ruler on them. The ruler is marked as .01 mm, which meanseach litt le etch mark on the ruler is .01. The total length of the scale on most stage micrometers is 1 mm,which then means that there are 1000 microns measured by that scale.

Students tend to get conf used by the scale and metric conversions. If each each mark is .01 mm then thatis equal to 10 microns. The picture below gives you an idea of how the micrometer looks under low power(100x). The image is enlarged a litt le more due to the camera used to photograph it, so you dont see thef ull viewing f ield. The paramecium was superimposed on the shot to illustrate the scale.

In the image above, you would estimate the length of the paramecium to be about 230 microns. In reality,you cannot place a paramecium slide over a stage micrometer. Instead, you use the micrometer to estimateyour viewing f ield size which can then be used to determine the size of the specimen (similar to estimatingwith a ruler f rom steps 1&2). The advantage of using a stage micrometer is that you are more likely to getan accurate measurement of the size of your viewing f ield. The disadvantage is that these slides are closeto $100.00 and may not be worth the cost f or a simple lab on measurement in a high school setting.

How to Measure with a MicroscopeEstimating Viewing Field with a RulerUsing a Stage (Slide) Micrometer