| Calcite, or Iceland Spar, is a form of calcium
carbonate. In 1669 Erasmus Bartholin (1625-98) discovered that it was doubly
refracting; when a naturally-occuring crystal is placed over some writing,
the writing is doubled. For a beam of light perpendicular to the surface
of the crystal, one ray (the O or or Ordinary ray) is transmitted without
being refracted at the surface, while a second ray (the E or Extraordinary
ray) has a non-zero angle of refraction and emerges parallel to the incoming
In 1678 Christiaan Huygens (1629-95) discovered that the E and O rays were polarized at right angles to each other.
| William Nicol (1770-1851) of Edinburgh developed
what is now called the Nicol prism in 1828. The problem with using calcite
as a polarizer is the presence of two beams of polarized light. In principle,
the E ray can be eliminated by using a narrow crystal, long enough so that
the E ray can be sufficiently displaced from the O ray to allow it to be
masked off. Nicol used the now classic technique of slicing the crystal diagonally
at QS and fastening the two halves back together with a cement (such as canada
balsam) of such in index of refraction that the E ray is totally reflected
at the internal interface, leaving the O ray to emerge alone from the crystal.
The late 19th century nicol prism used for demonstration purposes at Kenyon College shows the typical angle of the front and rear faces of 68° with respect to the long axis of the crystal. The nicol prism is colorless, unlike tourmaline , but its basic long, narrow construction unfortunately gives it a small angular field of view. It was almost always used as an analyzer; only in the Saccharimeter was it used as a polarizer.
At the right is a Nicol Prism at Union College in Schenectady, New York. It was also made by Duboscq.