(b. 1768; d. Edinburgh, Scotland, 2 September 1851)
optics, petrology, paleontology.
Although he achieved fame in physics as the inventor of the first polarizer, the Nicol prism, Nicol was primarily a geologist who made important but unappreciated contributions to petrology and paleontology.
Little is known about Nicol’s early career at the University of Edinburgh, where he lectured in natural philosophy, for he did not publish until he was fifty-eight. His first known scientific research dealt with the structure of crystals, and it was undoubtedly in connection with this work that he invented the prism. Nicol constructed his device by splitting a parallelepiped of calcite spar along its shorter diagonal and then cementing the halves together with Canada balsam, a substance with an index of refraction intermediate to the two indices of the doubly refracting calcite spar. The balsam allows the extraordinary ray to pass almost undeviated through the prism while it reflects the ordinary beam (see Figure 1). The two beams emerging from the prism are so widely separated they can be used independently. When it was invented (1828) Nicol’s device was the most convenient means of producing polarized light, and it became an important tool in physical optics and petrography.
Nicol’s inventive talents were equally well displayed in geology, although he received less credit than he deserved for his work in this field. To aid his early studies of crystals and rocks, he developed a technique for preparing transparent slivers for viewing directly through a microscope. Previous microscopic studies of minerals had been done with reflected light, which could reveal only surface qualities. Nicol’s technique was to cement the mineral in question to a glass plate and then grind it down to extreme thinness, thereby making possible for the first time direct microscopic investigation of the innermost structure of rocks and crystals.
Unfortunately, the potential of Nicol’s new method was not realized in petrology for more than forty years after its invention, in or around 1815. Nicol himself was partly to blame for this, since he never published any structural studies of his slide specimens (indeed he may never have made any such studies), except for two papers on fluid cavities in crystals. Furthermore, the first printed account of his technique did not appear until 1831, and then in a book on fossil woods (Witham’s Observations on Fossil Vegetables), which few petrologists were likely to read. Thus, it is not altogether surprising that this promising new method was not incorporated into the science of petrology until 1853, when Henry Sorby obtained Nicol’s slides and showed how they could divulge the secrets of mineral structure.
Nicol had far more success in the field of paleontology. He found that the same slidemaking technique could be used in the study of fossil woods in order to obtain a view of the minute cell structure with a microscope. Knowledge of the cell pattern thus obtained could be used as a basis for classifying and identifying the specimens being examined. Nicol made these identifications for a large number of fossil woods and displayed the arrangement of the cells in Witham’s Observations. Yet he seems not to have been accorded full recognition for this important work either.
Nicol’s articles include “Observations on the Fluids Contained in Crystallized Minerals,” in Edinburgh New Philosophical Journal, 5 (1828), 94–96; “On a Method of So Far Increasing the Divergency of the Two Rays in Calcareous Spar That Only One Image May Be Seen at a Time,” ibid., 6 (1829), 83–84; “On the Cavities Containing Fluids in Rock Salt” ibid., 7 (1829), 111–113; ibid., 10 (1831), 361–364; ibid., 14 (1833), 153–158; “On the Anatomical Structure of Recent and Fossil Woods,” in British Association for the Advancement of Science Report (1834), 660–666; Edinburgh New Philosophical Journal, 18 (1835), 335–339; and “Observations on the Structure of Recent and Fossil Coniferae,” ibid., 29 (1840), 175.
The only biographical material on Nicol is in Poggendorff, II, 151. The account of his mounting technique is in H. T. M. Witham, Observations on Fossil Vegetables (Edinburgh, 1831).
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.