Cell size can be measured using an eyepiece graticule. The graticule has a scale ruled on it.
You must find out the distance measured for each division of the graticule. You can then use the graticule to measure cells. The distance will be different for each objective.
The distance of 250 μm on the stage micrometer lines up against two divisions at 10 and 59 on the eyepiece graticule.
59 - 10 = 49 divisions on the eyepiece graticule are equivalent to 250 μm on the stage micrometer.
One division on the eyepiece graticule is equivalent to μm on the stage micrometer: = 5.1 μm (to two significant figures).
Therefore one division is equal to 5.1 μm.
Using the same calibrated eyepiece graticule to measure a cell:
The width of the cell highlighted = 50 - 40 = 10 eyepiece graticule divisions.
The real width of the cell is 10 × 5.1 μm = 51 μm (to two significant figures).
What would be the length of a plant cell, to two significant figures, that was 35 divisions on this graticule?
Each eyepiece graticule division is 5.1 μm
The real width of the cell is 35 × 5.1 μm = 178.5 μm
To two significant figures, this is 180 μm
How many graticule divisions would a single celled organism that was 240 μm take up?
Each graticule division is 5.1 μm
An organism that measured 240 μm would take up divisions = 47 divisions