Double refraction in a crystal had been observed as early as 1669 by the Danish naturalist Erasmus Bartholin (1625 to 1698). Using a crystal of Iceland spar he showed that an incident beam of light was split into two rays by what he called ordinary and extraordinary refraction. He could give no theoretical explanation for this. Christiaan Huygens (1629 to 1695) advocated a wave theory of light that would explain double refraction as the formation of spherical and spheroidal waves. However, since Huygens considered the waves to be longitudinal rather than transverse oscillations, his wave theory could not account for many of the other observations of light phenomena, hence was not generally accepted. The readily observed phenomena of double refraction thus remained an enigma into the eighteenth century.
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