Differential     interference
   contrast  (Nomarski) works by separating a polarised light source into
   two beams taking slightly different paths through the sample.
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   When  recombined  the two beams interfere and enhance the variation of
   optical density of the sample.
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   Nomarski microscopy:  <A ClAsS=MID Href="../../../med/Vid/Campbell7e/TurgidElodea-V.swf" oNClick="popurlt('../../../../med/Vid/Campbell7e/TurgidElodea-V.swf');txtcolor('cell_differential-contrast1','gray');return false;" Id=cell_differential-contrast1><iMg sRc="/images/shockwave.gif" aLt='' boRDer=0></A>