When observing specimens under a microscope, scientists encounter objects too small to be measured using familiar units like centimeters or millimeters. A standard millimeter is an immense distance compared to the size of a single cell or bacterium. Measuring these microscopic structures requires a specialized unit of length that scales down to the minuscule world visible through a lens. Selecting the correct, standardized unit is the first step in accurately quantifying the size of specimens.
The Standard Unit for Light Microscopy
The standard unit of length for measuring specimens viewed with a compound light microscope is the micrometer, often called a micron. This unit is represented by the Greek letter mu and the symbol \(\mu\)m. The micrometer is an extremely small measurement, representing one-millionth of a meter.
One micrometer is precisely one-thousandth of a millimeter. This minute scale is appropriate for measuring structures in the microscopic world. For example, a typical bacterium, such as Escherichia coli, is approximately 1 to 10 \(\mu\)m in length.
Many common biological cells fall within this range. A human red blood cell has a diameter of about 6 to 8 \(\mu\)m. Larger cells, such as those found in human tissue, are typically in the range of 10 to 100 \(\mu\)m. The micrometer provides a standardized way to express the dimensions of objects observed in routine light microscopy.
Practical Measurement Techniques
Establishing the micrometer as the unit of measure requires specialized tools to perform the actual measurement under the microscope. The primary instruments used are the ocular micrometer and the stage micrometer. The ocular micrometer is a small glass disk placed inside the eyepiece, engraved with an arbitrary, uncalibrated scale.
The stage micrometer is a microscope slide with a precisely known, fixed scale, typically with divisions separated by 0.01 mm or 10 \(\mu\)m. Since magnification changes with each objective lens, the ocular micrometer must be calibrated against the known scale of the stage micrometer for every lens used. This calibration determines the exact distance in micrometers that one ocular division represents for a specific magnification.
The calibration involves aligning the zero mark of the ocular scale with the zero mark of the stage scale. Scientists then find a point where another pair of lines perfectly superimposes. By counting the number of ocular divisions and stage micrometer divisions between these two points, a calculation factor is derived. This factor converts the number of ocular divisions spanned by a specimen into its actual size in micrometers.
Units for Extreme Magnification
When specimens are too small to be accurately measured in micrometers, such as at the limits of light microscopy, a smaller unit is required. The nanometer (nm) is the standard unit for measuring structures at this extreme level of magnification. The nanometer is one-thousandth of a micrometer, or one-billionth of a meter.
This unit is frequently used in electron microscopy, which achieves much higher resolution and magnification than traditional light microscopy. The nanometer scale is necessary to measure structures invisible to the light microscope, such as viruses and molecular components. For instance, most viruses, including the SARS-CoV-2 coronavirus, have a diameter in the range of approximately 20 to 300 nm.
The width of the DNA double helix is also measured in nanometers. The nanometer extends the range of precise measurement into the realm of ultrastructure, allowing scientists to quantify the size of the smallest biological entities and components. The shift from micrometers to nanometers reflects the progression from measuring whole cells and large bacteria to quantifying the minute details of cellular machinery and viruses.