Measuring and Counting with a Light Microscope
Microscopes make excellent measurement instruments, provided that the user knows how to calibrate them. A calibration device called a stage micrometer consists of a thick glass slide with a very precise scale etched into the surface. A typical scale is about 2 mm long, with a line marking each tenth of a millimeter. Usually, one end of the scale is more finely etched, into divisions of 0.01 - 0.02 mm. These etchings are included on only one end of the scale to keep the cost of micrometers—which are far more expensive than plain glass slides—a bit lower. We don’t place specimens directly on a stage micrometer, but rather use it to calibrate the true diameter of the field of view and/or to calibrate another scale, called a reticule, that is built into one ocular.
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