2.1 · Cell Structure
Measuring Objects with a
Light Microscope
By the end of this topic you should be able to demonstrate and apply your knowledge of preparing and
examining microscope slides, and the use and manipulation of the magnification formula — including using
graticules to make real measurements.
✦ OCR A Spec Points
the preparation and examination of microscope slides for use in light microscopy
the use and manipulation of the magnification formula
✦ Key Definitions
Eyepiece graticule Key idea: the eyepiece graticule scale is arbitrary —
A measuring device placed in the eyepiece of a it represents different real-world lengths at different
microscope. It acts as a ruler when you view an magnifications. The specimen looks bigger at higher
object under the microscope. magnification, but it hasn't actually grown! The
eyepiece scale must be calibrated separately for
Stage graticule each objective lens.
A precise measuring device — a small scale placed
on the microscope stage — used to calibrate the
value of eyepiece divisions at different
magnifications.
✦ Using Graticules
A microscope eyepiece can be fitted with a graticule — transparent, with a small ruler etched onto it.
As the specimen is viewed, the eyepiece graticule scale is superimposed on top of it, and the specimen
can be measured just as you'd measure an object with a ruler — in eyepiece units (epu).
A stage graticule is used only to calibrate the eyepiece graticule — never to measure the specimen
directly.
✦ Calibrating the Eyepiece Graticule — Method
1 Insert an eyepiece graticule into the ×10 eyepiece (a ruler with 100 divisions).
2 Place a stage graticule on the stage; bring it into focus using the low-power (×4) objective. Total magnification is
now ×40.
3 Align the eyepiece graticule and stage graticule scales.
4 Example: the stage graticule (1 mm / 1000 μm) lines up with 40 eyepiece divisions.
5 So one eyepiece division = 1000 ÷ 40 = 25 μm (at total magnification ×40).
6 Switch to the ×10 objective (total magnification ×100) and refocus on the stage graticule.
7 Align both scales again. Example: 100 eyepiece divisions now line up with 1000 μm.
8 So one eyepiece division = 1000 ÷ 100 = 10 μm (at total magnification ×100).
Measuring Objects · OCR A Biology · page 1 of 4
Measuring Objects with a
Light Microscope
By the end of this topic you should be able to demonstrate and apply your knowledge of preparing and
examining microscope slides, and the use and manipulation of the magnification formula — including using
graticules to make real measurements.
✦ OCR A Spec Points
the preparation and examination of microscope slides for use in light microscopy
the use and manipulation of the magnification formula
✦ Key Definitions
Eyepiece graticule Key idea: the eyepiece graticule scale is arbitrary —
A measuring device placed in the eyepiece of a it represents different real-world lengths at different
microscope. It acts as a ruler when you view an magnifications. The specimen looks bigger at higher
object under the microscope. magnification, but it hasn't actually grown! The
eyepiece scale must be calibrated separately for
Stage graticule each objective lens.
A precise measuring device — a small scale placed
on the microscope stage — used to calibrate the
value of eyepiece divisions at different
magnifications.
✦ Using Graticules
A microscope eyepiece can be fitted with a graticule — transparent, with a small ruler etched onto it.
As the specimen is viewed, the eyepiece graticule scale is superimposed on top of it, and the specimen
can be measured just as you'd measure an object with a ruler — in eyepiece units (epu).
A stage graticule is used only to calibrate the eyepiece graticule — never to measure the specimen
directly.
✦ Calibrating the Eyepiece Graticule — Method
1 Insert an eyepiece graticule into the ×10 eyepiece (a ruler with 100 divisions).
2 Place a stage graticule on the stage; bring it into focus using the low-power (×4) objective. Total magnification is
now ×40.
3 Align the eyepiece graticule and stage graticule scales.
4 Example: the stage graticule (1 mm / 1000 μm) lines up with 40 eyepiece divisions.
5 So one eyepiece division = 1000 ÷ 40 = 25 μm (at total magnification ×40).
6 Switch to the ×10 objective (total magnification ×100) and refocus on the stage graticule.
7 Align both scales again. Example: 100 eyepiece divisions now line up with 1000 μm.
8 So one eyepiece division = 1000 ÷ 100 = 10 μm (at total magnification ×100).
Measuring Objects · OCR A Biology · page 1 of 4
