In order to increase the precision of visual settings with the class of instruments employing the Sénarmont compensator, two types of half-shade eyepieces have been utilized. The first is the biquartz, placed between the quarter-wave plate and analyzer, which rotates the plane of polarization by a fixed amount in opposite directions on the two sides of the field. The second consists of a single thin plate of birefringent material such as quartz placed before the quarter-wave plate and arranged to cover half of the field. The dividing line is made to appear very narrow by employing the principle used in the Lippich prism, namely, that if both sides of the edge are illuminated by the full cone of rays the edge will tend to disappear. The theory of the optimum half-shade angle has been applied to determine the thickness of the plate in each type of eyepiece.
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