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Practical
Measurement and Magnification (1hr)
Introduction
It is important to know the dimensions of the specimens being observed under the microscope.
Mastering magnification and field of view is of great assistance in sizing the cells.
The purposes of this practical are to:
Equipment List
 Calculate magnification
Binocular compound microscopes
 Measure field of view
Lens cleaning tissue
 Understand the relationship between the
Minigrids and fine forceps
magnification and field of view.
Blank microscope slides
 Estimate the size of cells
Cover slips
Agapanthus sp, Onion
Instructions
Toothpicks and pipettes
Iodine and methylene blue stains
1. Calculating the Magnification
Each lens on your microscope has a magnification marked on it. Write the magnifications
of each lens in the table below.
Microscopy Tip 8
Lens
Individual Magnification
Ocular(s)
Total
Low power objective
Magnification =
Medium power objective
ocular x objective
High power objective
The total magnification is the multiplication of the individual
magnifications of the lenses you are looking through. Calculate the total magnifications for
low medium and high power and place them in the table below
2. The Field of View of your Microscope
Focus the low power of your microscope on the edge of a clear plastic ruler. Each line is
1mm. How wide is the field of view? Place your answer in the table below.
Obtain a minigrid. This is a microscope slide that contains two accurately drawn grids.
The larger grid is visible with the naked eye. Use low power to focus on this grid. It is
1mm square and is divided into 10 on each side. This makes each small box 0.1mm square.
You may see in the middle of this large square there is a smaller square also divided into
tenths.
Focus with medium power and determine the diameter of the field of view at this
magnification. Record this in the table below. Centre the tiny grid in the centre of the
field of view and turn to high power.
This tiny grid is 1.0mm square and each side is divided into 10. This makes the smallest
squares 0.1mm square. Measure the diameter of the field of view and record this in the
table below.
There are 1000micrometers or microns (μm) in 1 mm. In the table below, express the
fields of view in μm.
Total Magnification
x
x
x
Field of View Field of View
(mm)
(μm)
Microscopy Tip 9
Always remember
the fields of view
of your microscope
(a) What is the relationship between magnification and field of view?
This is actually a mathematical relationship – as the magnification doubles the field of view halves.
3. Estimating the Dimensions of Cells
Using the information about field of view, estimate the size of cells from the temporary wet
mounts you will prepare using Agapanthus sp. and onion epidermal cells, and human
epithelial cheek cells. This is easily done by estimating or counting the number of cells that
span the diameter of the field. Place the data in the table below.
Specimen
Length (μm)
Width (μm)
(a) Are cells all the same size and shape?
(b) Why do you think cells vary in size and shape?
See next page for helpful hints on how to determine the scale of your drawings.
Useful Background Information on how to determine the scale of your drawings
Determining the Magnification of your Drawing
 Estimate how many cells of the one you are intending to draw fit across the diameter of the
‘field of view’ for the objective you are using:
o You should remember the following diameters for the fields for the three objectives:
4.4 mm for the x4 objective
1.8 mm for the x10 objective
0.4 mm for the x40 objective.

If c represents the number of cells that can fit across the diameter of the field of view (mm)
for objective lens q, then the diameter of one cells is c/q mm at a magnification determined
by the magnification of the objective multiplied by the magnification of the eye piece
o e.g. if 10 cells fit across a field of view with a diameter of 0.04 mm using a x40
objective then the diameter of one cell is 0.04 mm/10 = 0.004 mm or 4.0 µm.
o Remember that 1.0 mm = 1000.0 µm

If the diameter of your drawing is 125.0 mm then you can work out the magnification of
your drawing (i.e. how many times bigger your drawing is to the cell’s actual size by
dividing 125 mm by 0.004mm = 31,250 times bigger than its actual size.

Remember that when working out the magnification of your drawing your final answer will
be just a number without UNITS.
Inclusion of a Scale line with your drawing
 If your drawing has a diameter of 125 mm and you know that your drawing represents a cell
that actually has a diameter of 0.004 mm then you can determine a scale line for your
drawing by doing the following:
o if 125 mm represents 0.004 mm (4 µm) then 25 mm represents ‘y’
o you can determine the value y by dividing 25 by 125 and multiplying it by 0.004 =
0.0008 mm or 0.8 µm.
o Therefore at the bottom of your drawing you would draw a line 25 mm long and
write 0.8 µm above the line, which tells anybody looking at your drawing that every
25 mm across your drawing actually represents 0.8 µm
 So anyone with a ruler could now work out the size of your drawing and the
size of any structures you have drawn within the cell.

Therefore if d is the diameter of your drawing and p is the length of your scale line in the
same units, and s is the actual diameter of the cell or organism then
p/d x s will give you your scale line for the drawing.
o The length of the scale line (p) you choose is up to you

For the above example you would draw a line 25mm long and state what the scale line (bar)
represents e.g.
___________ scale bar = 0.8 µm.