Biology 9700/13 — May/June 2014

Background Concept

An eyepiece graticule is a small circular glass disc, etched with a fine scale (typically divided into 100 equal units), that is placed inside the eyepiece lens of a light microscope. When you look down the microscope, the graticule scale is superimposed on the image of the specimen, so you appear to see the specimen laid against a ruler.

A second tool, the stage micrometer, is a microscope slide on which is engraved a scale of known length (usually 1 mm divided into 100 parts of 10 µm). The stage micrometer is used to calibrate the eyepiece graticule, because one graticule division corresponds to a different actual length at every objective lens. The graticule is the ruler; the stage micrometer is what gives the ruler its numbers.

Understanding the Question

The question gives three statements about the eyepiece graticule and asks which combination is correct. The candidates must judge each statement on its biological/technical accuracy:

• 1: claims the graticule can directly measure actual cell length.
• 2: claims the graticule helps in drawing cells with correct proportions.
• 3: claims the graticule itself changes size when the objective lens is changed.

The command is "which statements are correct", so a single wrong statement rules out any option that includes it.

Approach

Apply two key facts about the eyepiece graticule:

1. It is a fixed physical scale, so its appearance cannot change with the objective lens (it is the specimen's image that changes).
2. It must be calibrated against a stage micrometer before any real length can be read off it; it does not, by itself, give true measurements.
3. The visible graticule divisions are a useful frame of reference for copying outlines of cells while drawing.

Use these to judge each statement in turn and then match the surviving set to the answer options.

Step-by-Step Reasoning

• Statement 1 (It allows you to measure the actual length of cells) — INCORRECT. The graticule scale is in arbitrary units. To convert those units into micrometres or millimetres, you must first calibrate the graticule at that specific magnification using a stage micrometer. Without calibration, the graticule is just a relative scale, not a measure of actual length. (A common alternative reading is that the graticule is a measuring device, but in its raw form it does not give true lengths; calibration is essential.)
• Statement 2 (It allows you to draw cells with correct proportions) — CORRECT. The graticule divisions are superimposed on the specimen image, so when the user copies the image onto paper, the graticule provides a fixed reference grid. Counting and reproducing graticule divisions while drawing keeps the cell's proportions faithful to what is on the slide.
• Statement 3 (It changes in size as the objective lens changes from ×10 to ×40) — INCORRECT. The graticule is a physical disc fixed inside the eyepiece. Changing the objective lens changes the magnification of the specimen, not the size of the graticule divisions. A related misconception is that "more divisions now fit across the cell" means the graticule has shrunk; in fact, the cell image has simply been enlarged more. This is exactly why the graticule must be recalibrated at every magnification: the number of graticule units covering a real distance changes with the objective.

Only statement 2 is correct, so the answer is D: 2 only.

Key Takeaways

• The eyepiece graticule is a fixed arbitrary scale; it does not change with magnification.
• Calibration with a stage micrometer is required at each objective before the graticule can be used to measure actual size.
• The graticule is, however, very useful as a visual reference when drawing cells in correct proportion.

Common Mistakes

• Believing the graticule directly reads in µm or mm — it does not, until it has been calibrated.
• Thinking the graticule physically changes size when you switch objectives — only the specimen's magnification changes.
• Confusing the eyepiece graticule with the stage micrometer; remember the graticule is in the eyepiece and the micrometer is on the stage.

Things to Be Careful About

• The phrasing "changes in size" in statement 3 is a deliberate trap: candidates must distinguish the graticule (fixed) from the image of the specimen (which changes with magnification).
• Statement 1 is the classic "needs calibration" trap — credit the graticule for allowing measurement only after it has been calibrated, not on its own.
• In exam answers, do not credit the graticule with giving units such as µm without stating that a stage micrometer has been used to calibrate it.