Nicol Prism
   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 ray..

   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. 

   The prism at the left below is from the Garland Collection of Classical Physics Apparatus at Vanderbilt University in Nashville. Prof. William Tobin of the University of Canterbury in Christchurch, New Zealand, has noted that this is probably an example of Fresnel's Prism, which has a shorter overall length. The prism, by Duboscq of Paris (400 f, or about $80), is unusually large, with faces of 4.5 and 4.7 cm in diameter. It was designed to be used in a solar microscope for demonstrations with polarized light by projection. The apparatus was purchased about 1875. 

   At the right is a Nicol Prism at Union College in Schenectady, New York. It was also made by Duboscq.

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