Biology 9700/12 — May/June 2023

Background Concept

Two pieces of equipment are used to measure the size of an object under a light microscope:

• Eyepiece graticule: a small circular glass disc bearing a fine scale (e.g. 100 divisions across a 1 cm line). It is inserted into the eyepiece lens, where it appears superimposed on the specimen image. Because the apparent size of its divisions changes with objective magnification, the graticule must be calibrated before each objective is used.
• Stage micrometer: a glass slide bearing a scale of known length (typically 2 mm in 0.01 mm divisions). It is placed on the microscope stage like a normal slide, viewed through the same objective as the specimen, and used to calibrate the graticule.

A calibrated graticule then lets the observer read the size of a specimen directly from the graticule scale and convert it to a real measurement (in mm, µm or nm) by multiplying by the calibration factor for the objective in use.

Understanding the Question

The diagram shows a light microscope with three numbered parts:

• 1 — the eyepiece (ocular) lens at the top of the body tube
• 2 — the objective lens, on the nosepiece, pointing down at the stage
• 3 — the stage, the flat platform that holds the slide

The question asks where the graticule and the stage micrometer should each be placed, and which of the four answer combinations is correct.

Approach

Match each piece of equipment to the part of the microscope it belongs in:

• The graticule is built into / inserted into the eyepiece → position 1.
• The stage micrometer is a slide placed on the stage → position 3.

Therefore the answer is the row graticule = 1, micrometer = 3.

Step-by-Step Reasoning

1. The graticule must be viewed at the same plane as the specimen, so it sits inside the eyepiece (position 1). It cannot sit in the objective or on the stage because it would not be seen as a superimposed scale on the image.
2. The stage micrometer is itself a microscope slide, so it must be clipped onto the stage (position 3) in place of a specimen slide during calibration.
3. The objective lens (position 2) is a fixed optical component with no slot or space for either item.
4. Combining these, graticule = 1 and micrometer = 3, which is option B.

Key Takeaways

• Graticule → eyepiece; stage micrometer → stage.
• The stage micrometer is a reference slide of known length, used only to calibrate the graticule.
• Calibration must be repeated for every objective, because the apparent length of graticule divisions changes with magnification.
• The conversion used is: actual size = (number of graticule divisions × calibration factor for that objective).

Common Mistakes

• Confusing the two and placing the graticule on the stage: the stage micrometer goes on the stage, not the graticule.
• Placing the stage micrometer in the eyepiece: the eyepiece graticule goes in the eyepiece, not the micrometer.
• Thinking the objective (position 2) is involved: the objective is a sealed optical element; neither item is inserted there.

Things to Be Careful About

• Remember the direction of the question: "graticule" then "micrometer scale"; the columns of the table are in that order, so the correct row must put 1 in the graticule column and 3 in the micrometer column.
• Final answers in µm (or even nm for very small structures such as ribosomes) — always convert mm to µm ($\times 10^3$) or nm ($\times 10^6$) when calculating actual sizes.