Calibrate the eyepiece graticule
About 4 minutes. The scale you see in the has no unit. You it with a : a special slide with a scale of known length.
Align a blue line with 0 of the black scale. Drag the orange frame to find where the lines meet exactly again, then calculate what one eyepiece unit is in µm.
★ Key rule Calibrate the graticule for each objective lens you use.
✓ Do Align the two 0 lines first. Then look along the scales for the next lines that meet exactly.
✗ Don't Use a value from another microscope or from another objective lens.
The picture shows two scales, with their names on it. The (black, numbers above) is inside the eyepiece, so it stays still. Its divisions are (EPU). The (blue, numbers below) is a slide on the , and it moves with the stage. Each of its divisions is 10 µm.
Why not a normal ruler? Its smallest marks are 1 mm apart, and at ×400 the whole circle you see is less than half a millimetre across.
- Focus on the stage micrometer at ×400 (the ×40 ).
- Drag the blue scale (the stage micrometer), or use the Stage micrometer arrows under the picture, until one of its lines sits exactly on 0 of the black scale.
- Drag the orange frame along the scales, or use the Lens arrows. The lens under the scales shows what is inside the frame four times larger. Find the next place where a black line and a blue line meet exactly, in one straight line. Near 0 some lines nearly meet: they do not count, so keep looking.
- Count the eyepiece units and the micrometer divisions from 0 to that place.
1 eyepiece unit = (number of micrometer divisions × 10 µm) ÷ number of eyepiece units
On a real microscope, choose a match far along the scale (30–100 EPU): a small error in your reading then counts for less.
Measure cells with the graticule
About 3 minutes. The stage micrometer is gone, and cheek cells stained with are on the stage. The is still in the eyepiece, so at ×400 one is still 2.35 µm.
Measure cells A, B and C along their longest axis, in eyepiece units. Change each length to µm, then calculate the mean.
★ Key rule Length in µm = number of eyepiece units × the length of one eyepiece unit.
✓ Do Measure at least three cells and calculate a .
✗ Don't Use your ×400 calibration at ×100: calibrate each objective.
Drag the slide to move a cell onto the scale. Use ⟲ and ⟳ under the picture to turn the eyepiece. Count the eyepiece units from one edge of the cell to the other.
Each reading has an : you read each edge to the nearest division, so a length is correct to about ± 1 EPU.
Another way to find a mean size: measure a whole row of cells, then divide by the number of cells.
Scale bars: magnification and actual size
About 4 minutes. A micrograph in an exam often has a . You measure lengths on the page with a ruler, in mm, then calculate.
Use the ruler on the picture: measure the scale bar and the Paramecium. Calculate the magnification and the actual length. Then calculate a scale bar for your own drawing.
★ Key rule = ÷ , both in the same unit.
✓ Do Change mm to µm first (× 1000). Show your working, and give the unit.
✗ Don't Divide a length in mm by a length in µm.
1 mm = 1000 µm · 1 µm = 1000 nm
actual size = image size ÷ magnification
Drag the ruler onto the scale bar. Put its 0 mark on one end of the bar and read the other end. Measure the Paramecium the same way, from one end of its body to the other; leave out the cilia, the fine hairs round its edge. “Turn 90°” turns the ruler upright.
What is wrong with these drawings?
About 3 minutes. A good biological drawing follows a short list of rules: SOLD (Size, Outline, Labels, Detail), and a scale. Recall them, then find the mistakes in three drawings.
Recall the rules from memory. Then type the three mistakes in each of drawings A, B and C.
★ Key rule No scale, no size: without a , nobody can tell how large the cells really are.
✓ Do Recall the rules first; then check each drawing against them. Drawing D follows every rule.
✗ Don't Copy a drawing from a book or a photo: draw the cells on your own slide.
Now press A, B, C or D under the picture, and find what is wrong.
Take a photo through the eyepiece
About 1 minute. A photo keeps what you saw, with the scale beside it.
Step by step
- Focus on your cells at ×400, with the graticule scale across one cell.
- Hold the phone's camera lens close to the , in the middle.
- Move the phone slowly until the circle of light fills the middle of the screen.
- Tap the cells on the screen to focus. Do not use the zoom.
- Check the photo: the cells and the scale must both be sharp.
Safety
Carry a microscope with two hands. Hold slides and cover slips by their edges. Tell your teacher about broken glass at once.
That is all. Whenever you use a microscope, calibrate it yourself: every microscope and every objective lens has its own value.