Microscopy Scales: measurement in the micro-small world.

Text by Alan Maude, Images by Maurice Smith 1996


How big?

The Universe is so big that it appears infinite in size. Our earthly measurements dwindle into insignificance when applied to it. Miles or kilometres just run into too many zeros when recording distances between galaxies. To make things easier, new measures on a scale equal to these vast distances are required: light-years and parsecs! A light year is the distance that light would travel in the time it takes the earth to orbit the sun for one complete revolution: the distance light travels in space in one earth year.
 
 

How small?

At the microscopic level, a similar problem exists. Creatures, invisible to the eye, are often less than a single millimetre in length. Take a look at a ruler to see just how small this is!

Measurement: a scientific tool.

Measuring things is an important aspect in all scientific disciplines. It gives us the capacity to relate and model the things we observe. For example, how would you like a five minute wrestling bout with another person who was 3 metres tall and 1 metre across? Probably, you wouldn't! Knowing the size of something helps us to assess cause and effect.

Many people may scoff at such a simple device as a ruler. Yet it is probably the first thing we will use in our formal education as a way to understand and help us record details about the things we observe. At Micscape, we are mainly involved with observing and discussing things which are not easily seen with the naked eye. We need a way to measure very small things and we need to understand the scale of the things we observe to consider part of their relationship with other things in a similar environment.

The metre

Our measurement of distance at the human scale of existence revolves around the root distance called the metre. This is the Internationally accepted unit of length and is one of the seven agreed International System of Units (SI).

Most people can retain a mental model of what a metre is (a long stride by an adult), and can judge distances based on its root. A 'mental-model' is important. It gives us a 'feel' for size which enables rapid judgements to be made in the absence of accurate data. How high can you jump: 1 metre, 3 metres or 9 metres? As soon as the question is asked, you start imagining a 'height' and your capacity to leap it.

The kilometre

Distances on our planet can normally be described in 1000 metre sections: the kilometre. If you had a journey to make, you would probably wish to know how far you were going (in kilometres) because it would help you to assess the time it would take to get to where you were going to, the cost in transport terms, and if far enough - climatic changes you can expect.

The scientific world needs to be certain of the size of things and this requirement is equally important to the microscopist. Measurements taken of subjects smaller than our ability to see with the unaided eye still need to be relative to our standard root measure of 1 metre.

A microscopist's units are as follow:-

1 centimetre (cm) = 0.01 metres (m) or 1 x (10 to the power of -2) metres.

The single unit of 1 centimetre is visible. Take a look at a ruler to see how easy this length is to see. You will notice a centimetre has tiny divisions, in fact - 10 to each centimetre. Each division represents 1 tenth of a centimetre and is called a millimetre.

1 millimetre (mm) = 0.001 metres (m) or 1 x (10 to the power of -3) metres.

Notice just how small but still visible a millimetre is on a ruler. A millimetre is about the size of objects that we humans can still manage to manipulate with our hands without using devices and tools to help us.

It is the sub-division of this tiny unit which is used to begin measuring forms normally invisible, or nearly invisible, to the naked eye: a micron

The micron (um)

Note: The 'u' would normally have a long tail in the left vertical bar, but for the sake of showing this character on all web browsers, we have used the character 'u' instead! The picture below shows the correct shorthand for the micron.

1 micron = 0.000001 metres (m) or 1 x (10 to the power of -6) metres.

The name 'micron' is actually derived from micrometre. It is very important to remember that there are 1000 microns to a millimetre! When you see the size of a microscopic subject quoted as, say - 800 microns in length and 500 microns in width, you can mentally realize this is nearly a millimetre long and half a millimetre across; if you look again at what a millimetre looks like on your ruler, you will see that a subject (or object) is still visible to the naked eye. However, something which is 10 microns x 10 microns could not be seen without a microscope!

Scale at the microscopic level

To put things in perspective, we need to consider some microscopic subjects and their size. Most people will have heard of an Amoeba. The typical size of this microscopic animal is 0.8 mm ( 8/10 of a millimetre) long by 0.4mm (4/10 of a millimetre) wide: just about visible to the naked eye! Measured in microns, the creature is quite large - 800 microns by 400 microns. It is a good reference size to consider smaller life-forms and objects against.

Euglena, a protozoan, is typically 130 x 50 microns - much smaller than our large amoeba. It uses a whip like tail called a flagellum to propel itself through water at the rate of 20 to 200 microns per second. Compare this speed to its own body-length and than consider this to the speed of a Paramecium, another protozoan "weighing-in" at 240 x 80 microns; the latter uses cilia (tiny hairs) to move through the water at 400 to 2000 microns per second.

The Paramecium moves 10 times faster than the Euglena in real terms! Is it still faster in relative terms, e.g.. speed relative to the length of each creature's body?

As a rough guide, cells typical to plants and animals are called eukaryotic cells and range in size from 10 to 150 microns.

Smaller and smaller

Bacteria, which are much smaller than any of the other forms and cells discussed so far, are typically between 1 micron to 10 microns in length; the latter being the length of the rod-shaped bacteria. If you look at the millimetre scale on your ruler, you can try and visualize 1000 bacteria lined up in a chain between the two marks indicating a millimetre width... or maybe just 8 Euglena "nose-to-tail".

An interesting point to note is that a single human sperm is only 2.5 microns across its widest part! Around 400 of them could line up across the 1 mm notch of a ruler.

 

Beyond light

The optical microscope can resolve images of microscopic forms up to around 1600x magnification. Some subjects (object?) are just too small to be made visible using this type of instrument. A top quality optical light microscope can resolve two points that are ca. 0.25 microns apart.

A lower level of the microscopical world exists which is still populated with living forms called viruses. These are so tiny - typically around 0.1 microns - that they defy measurement when using the micron as a standard. Instead of the micron, we need to switch to using another unit called the nanometre (nm).

1 nanometre (nm) = 0.000000001 metres (m) or 1 x (10 to the power of -9) metres.

There are 1000 nanometres to a micron! Therefore, our virus of 0.1 microns is actually 100 nanometres (nm) long. To see a virus, you would need to use an electron microscope, which utilizes a fine beam of electrons to probe the subject and resolve an image up to 50,000x that of an optical light microscope.
  by Alan Maude  

 

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