Nicol, William

1768–1851
SCOTTISH
MINEROLOGIST, PHYSICIST

William Nicol (aka William Nichol) was born, lived his entire life, and died in Edinburgh, Scotland. He was considered a quiet and unassuming professor at the University at Edinburgh who had a profound effect on the forensic sciences by pioneering the production of polarized light and the creation of the Nicol prism.

Nicol used the double refraction properties of Iceland spar to produce polarized light in 1825 and in 1829 created an optical device called the Nicol prism, a precursor to the polarizing light microscope. Essentially, the Nicol prism consists of a crystal of calcite or Iceland spar that is cut into two equal pieces at an angle; the pieces are then rejoined with Canada balsam. When a beam of light enters the crystal, it undergoes double refraction (birefraction). That is, the beam is split into two parts, each of which is differentially affected. The first part, called the ordinary ray, undergoes total reflection at the Canada balsam joint and is shifted off course to pass out of one side of the Iceland spar crystal. The other part, called the extraordinary ray, continues on through the crystal. By means of the Nicol prism, a beam of light could be polarized or a beam of already polarized light can be subjected to analysis. William Nicol utilized his prism to investigate the optical properties of minerals and other substances. He created lenses by grinding semiprecious stones, and used those to investigate fossilized wood and fluid-filled cavities in crystals.

Nicol prisms were first used to measure the polarization angle of birefringent compounds, which led to new developments in the scientific understanding of interactions between polarized light and crystalline substances. (Optical birefringence is when light enters a nonequivalent axis in an anisotropic crystal and is refracted into two rays, each of which are polarized with the vibration directions oriented at right angles to one another, and traveling at different velocities. Anisotropic crystals have crystallographically different axes that interact with light differently, depending on the angle in which the incident light reaches the surface of the crystal.)

Nicol's work set the stage for development of the polarizing light microscope, an important forensic tool. The purpose of the polarizing light microscope is to view and photograph specimens visible due to their anisotropic characteristics. Polarized light is scientifically and forensically useful because it enhances contrast and improves the image quality of birefringent materials when compared to other techniques such as darkfield and brightfield illumination, phase contrast, and fluorescence. As a forensic investigative tool, polarized light microscopy permits access to a great deal of information not obtainable with any other optical microscopy technique: because it exploits optical properties of anisotropy, it can reveal minutely detailed information about the structure and composition of materials. This is of critical importance for crime scene/criminal identification, as well as for forensic diagnostic purposes.

SEE ALSO Alternate light source analysis; Identification; Microscopes; Minerals.