OPTICAL ACTIVITY
��A niool polariser p.
A plate of quartz or mica oat parallel to the optical axis, and of auch a thickness as to produce a difference of half a wave-length between the two rays which it gives by double refraction. This plate covers half of the opening o, as shown in the end view immediately below.
A tube k, closed at its ends by glass plates, to hold the liquid to be examined.
A nicol analyser a.
A small telescope L, focussed on the opening o.
A graduated circle c, and an index-pointer fastened to the analyser and rotating over c.
Suppose the plane of vibration ■ of the nicol P be indicated in fig. 14 by the straight line oh, forming the
����angle a with the principal axis oa of the quartz plate ; then for the half of the field of vision to the right, the luminary vibration ob may be considered as the resultant of the two components oa and ob. For the half-field to the left (covered by the quartz plate) we have the com- ponents oa and ob', the latter being opposed to its corresponding component ob on account of the retardation of half a wave-length produced by the quartz. The com- ponents oa and ob' give the resultant ob'. If now we place the analyser in such a position that its plane of vibration is perpendicular to the direction ob (as indicated by cc' in fig. 15), the half of the field to the right will be
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