Biology 9700/12 — October/November 2019

Background Concept

An eyepiece graticule is a small glass disc with a scale etched into it that sits inside the eyepiece lens of a light microscope. Because the apparent size of an object under the microscope depends on the total magnification (eyepiece × objective), the graticule's divisions are arbitrary — one eyepiece division represents a different real length under a $\times 4$ objective than under a $\times 40$ objective.

A stage micrometer, by contrast, is a microscope slide on which a scale of known real length is printed (here, divisions $0.1\ \text{mm}$ apart). It therefore provides an absolute reference that can be used to find the real length that each graticule division represents at a given magnification. This procedure is called calibrating the eyepiece graticule.

Understanding the Question

The question lists three possible pieces of data and asks which must be collected in order to carry out the calibration. The command word "must" means we are looking for the minimum, essential data — not extra information that might also be useful.

The student already knows the real length of one stage micrometer division ($0.1\ \text{mm}$). What is missing is the link between the two scales: how many graticule divisions sit alongside one stage micrometer division when the two scales are lined up.

Approach

The standard calibration procedure is:

1. Place the stage micrometer on the stage and bring its scale into focus.
2. Rotate the eyepiece so the graticule scale lies parallel to the stage micrometer scale and the two scales overlap.
3. Find a point on the right where, say, $n$ stage micrometer divisions line up exactly with $m$ graticule divisions.
4. Calculate the real length represented by one graticule division:

$$\text{real length per graticule division} = \frac{n \times 0.1\ \text{mm}}{m}$$

So the only data the student must actually collect is the number of graticule divisions equivalent to each stage micrometer division (item 3).

Step-by-Step Reasoning

• Item 1 (magnification of the eyepiece lens): Not required. Calibration is done empirically against a known scale at whatever magnification is set, so the magnification itself is not a piece of data to collect — it is implied by the lens currently in use. Once calibrated, the graticule is valid only at that magnification, which is why calibration must be repeated for each objective.
• Item 2 (number of stage micrometer divisions in one field of view): This is a different and unrelated measurement. Knowing how many stage micrometer divisions fit across the diameter of the field of view tells you the diameter of the field but does not relate the graticule scale to a real length.
• Item 3 (number of graticule divisions equivalent to each stage micrometer division): This is precisely the comparison needed. Together with the known $0.1\ \text{mm}$ per stage micrometer division, it is sufficient to convert any graticule reading into a real length.

Only 3 is required, so the answer is D.

Key Takeaways

• The eyepiece graticule has arbitrary divisions and must be calibrated at each magnification against a stage micrometer.
• Calibration needs only one piece of information: how many graticule divisions align with one stage micrometer division (of known real length).
• Eyepiece magnification and field-of-view measurements are not part of the calibration procedure itself.

Common Mistakes

• Choosing A because the eyepiece magnification sounds like it should be involved — it is not, because calibration is empirical and bypasses the need to know magnification directly.
• Choosing B because the field of view seems relevant — counting stage micrometer divisions in a field of view gives a different quantity (field diameter) and does not link the graticule to a real length.
• Choosing C for the same reason as B, but in the absence of any comparison to the graticule.

Things to Be Careful About

• Calibration is only valid for the objective lens in use at the time of calibration; it must be repeated whenever the objective is changed.
• The stage micrometer is a real, finite-scale reference and is removed after calibration — measurements on actual specimens use only the graticule.
• The real length per graticule division is $\dfrac{\text{stage micrometer division length}}{\text{graticule divisions per stage micrometer division}}$ — divide, not multiply.