For the microscopic examination of an animal tissue or organ, we usually collect small representative fragments of a tissue or organ. In a laboratory, these fragments will be subjected to a series of procedures at the end of which a section of a few square millimeters in area will be produced. You can see in the figure at your right that, in most cases, we are observing a very small fraction of an organ.
When observing images directly under the microscope or printed images or images on a monitor screen, it is difficult for someone unexperienced to assess at what magnification the structures of the preparation are being visualized.
The unit of measurement used for the dimensions of objects seen under a light microscopy is the micrometer. Its abbreviation is µm. One thousand µm fit into 1 milimeter (mm), 10 mm fit into one centimeter (cm) and one thousand mm as well as one hundred cm fit into one meter.
One micrometer, therefore, corresponds to one thousandth of a millimeter and one millionth of a meter.
