Measuring with the microscope can be a frustrating endeavor for beginner biologists. In fact, I rarely teach measuring in my freshman classes. Advanced (AP) biology does look at how to measure with the microscope. The entire process involves focusing on high power, using math to solve a ratio problem, and in the end making an estimation of the field of view of your microscope. The section on measuring is included in the advanced microscopy lab, but the lab itself doesn’t go into much detail (preferring instead for the students to work it out. The steps and process I’m including here as an “extra” aid for students (and teachers) who need a refresher on measuring with the microscope.
Disclaimer: Numbers are rough estimates based on a specific microscope. Microscopes can vary in their magnifications 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 field at scanning (40x). On our scopes, we estimated the viewing field to be about 4 mm across.
Step 2: Repeat the process on low power (100x). We estimate our low power viewing field to be about 2 mm across.
Convert mm to microns for both the scanning and low power. There are 1000 microns in a millimeter. So… Scanning = 4000 microns; Low Power = 2000 microns.
You can at this point use that measurement to measure anything in your viewing field that you can see with low or scanning power. The measurement is an estimation though, and probably not very accurate. For instance, if you have a slide with the word “are” across it, use your measurement of the viewing field to estimate the distance of each of the letters. In the drawing below, I have superimposed the slide over the millimeters of the ruler so you can make 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 field is about 4000 microns across. In reality, you cannot place your slide over the ruler, so you have to make a guess based on how large your microscope’s viewing field is.
Calculating High Power Field of View
Measurements on High Power can be a little more complicated. If you try to use the clear ruler technique, you’ll find that you cannot see the individual ruler marks. This is where math comes in, the values you estimated above can be used to solve a ratio problem and determine the size of your viewing field on high power.
Solving for “High Power Field of View” ….
X / 2000 = 100/400
X (high power field of view) = 500 microns
Now that you have 500 microns as an estimate of your viewing field, any object you are viewing under high power can be estimated based on that. I tell my students to look at a paramecium and guess how many paramecia can fit end to end on their slide. They guess they can fit two, then the length of their paramecium 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 means each little 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 confused by the scale and metric conversions. If each each mark is .01 mm then that is 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 little more due to the camera used to photograph it, so you don’t see the full viewing field. 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 estimate your viewing field size which can then be used to determine the size of the specimen (similar to estimating with a ruler from steps 1&2). The advantage of using a stage micrometer is that you are more likely to get an accurate measurement of the size of your viewing field. The disadvantage is that these slides are close to $100.00 and may not be worth the cost for a simple lab on measurement in a high school setting.