Appendix 3: Measurement for Microscopy

Though the millimeter is a convenient unit for measurement with magnification, it is too large for measuring microscopic objects conveniently.  For microscopic measurement the millimeter is divided into 1000 equal parts, each part being called a micrometer or a micron.  The symbol is um.  A micrometer, then, is one millionth part of a meter.  The smallest object under the most favorable background conditions that can be observed with the unaided eye is of the dimensions of 0.1 millimeter or 100 um.

Under the ideal background conditions the human ovum and the largest of nerve cells are barely visible.  Objects which are below the size for visual examination with the unaided eye and are yet large enough to be seen with a bright field microscope are said to be microscopic.  The range is from 0.1 millimeter to 0.0001 mm or from 100 um and 0.1 um.  Particles from 0.1 um to 0.004 um in size are called submicroscopic (or ultrastructural) particles.

A still finer unit of measurement is the angstrom unit (A).  Its value is 0.000,000,1 of a mm.  The angstrom unit has recently been abandoned in favor of the nanometer, but many references to it remain.