Biology 9700/13 — October/November 2014

Background Concept

A light microscope can be fitted with two measuring aids: an eyepiece graticule (a small scale etched onto a disc inside the eyepiece, so it sits in the optical path and is visible superimposed on the specimen) and a stage micrometer (a glass slide on which a scale of known, fixed length is etched, e.g. 1 mm divided into 100 divisions of 10 µm each). The graticule's divisions have no fixed real length — they look the same size regardless of the objective lens because the graticule is in the eyepiece and is not re-magnified by the objective. Therefore the graticule cannot be used to give actual lengths until it has been calibrated against a scale of known length at each magnification. The stage micrometer provides that known length, which is why it must be used to calibrate the graticule at every objective lens being used.

Understanding the Question

The question asks which of three statements about the stage micrometer (not the eyepiece graticule) are correct. We must judge each statement on the role of the stage micrometer and how it behaves when the objective lens is changed.

Approach

Think about what the stage micrometer physically is (a fixed scale on a slide of known real length), what it is used for (calibration of the graticule, not direct cell measurement), and how its image appears at different magnifications (larger apparent divisions at higher magnification). Each statement is then tested against these points.

Step-by-Step Reasoning

• Statement 1 — "It allows you to measure the actual length of cells." The stage micrometer is a reference scale; the cell sits on top of it on the stage. You cannot lay the cell on the micrometer divisions in a meaningful way to read off a real length directly. Instead, the micrometer is used to find how many graticule units correspond to a known real length, after which the graticule is used to measure the cell. So statement 1 is incorrect.
• Statement 2 — "It allows you to calibrate the eyepiece graticule." Yes — this is exactly the purpose of the stage micrometer. The two scales are aligned on the same field of view and the known real length of a stage micrometer division is compared with the number of graticule divisions it spans. The result (e.g. 1 graticule division = 2.5 µm at ×40) is then used to convert graticule readings into real lengths. So statement 2 is correct.
• Statement 3 — "It changes in size as the objective lens changes from ×10 to ×40." The physical etchings on the slide are fixed, but the image of those etchings is magnified by the objective lens. At ×40 the image of each division is four times larger than at ×10, so the apparent size of the stage micrometer (as seen through the eyepiece) does change with the objective. This is precisely why recalibration is needed for each objective. So statement 3 is correct.

Only statements 2 and 3 are correct → answer B.

Key Takeaways

• Stage micrometer = fixed real-length scale on a slide; used to calibrate the eyepiece graticule.
• Eyepiece graticule = scale inside the eyepiece; used to measure specimens after calibration.
• Calibration must be repeated at each magnification because the apparent image size of both scales changes with the objective lens, even though the graticule divisions themselves are unchanging.

Common Mistakes

• Saying the stage micrometer is what measures the cell. It is the calibrated eyepiece graticule that does the measurement; the micrometer only calibrates it.
• Thinking the stage micrometer's apparent size stays the same when the objective changes. Its physical size is fixed, but its image size scales with magnification.
• Choosing option A because all three statements sound plausible, when in fact statement 1 confuses calibration with measurement.

Things to Be Careful About

• The graticule divisions are not magnified by the objective (they are in the eyepiece), so they look the same size at every objective — that is why the stage micrometer is needed at every objective to give a real-length reference.
• "Size" in statement 3 refers to the image size, not the physical size of the etched scale.