126

Q UA NT IT A TIVE AflRlCriT 17M /, .-1 AM L )',S7X

while the "extraordinary" ray, n, passes through. This ray Ls
polarized in a plane which is perpendicular to the " optical
principal plane77 of the prism, a term which need not be defined
here.

This Nicol prism, properly fixed in place in the end of the
polarimeter nearest the light source, forms the polarizer. The
analyzer is another Nicol of similar construction. When this
is turned so that the optical principal plane is parallel to that of
the polarizer, maximum brilliancy of transmitted light is ob-
served. If these two planes are perpendicular to each other,

Fro. 38.—Nicol pram.
total extinction results because the extraordinary ray from the
polarizer is now in the plane for the ordinary ray for the analyzer
and it is therefore reflected to the side of the latter and there
absorbed.
Method of Making Observations.—In practice it in not easy
to determine when either maximum brightnasB or maximum
extinction of entering light occurs. Accordingly most polarim-
eters are constructed with an additional device to aid in making
the reading. In "half-shadow" instruments the field in divided
into halves by interposition of a thin plate of quartz which covers
half of the diaphragm of the polarizer. The thicknejw, method of
grinding and position of this plate are such as to cause a small
difference between the angles of maximum intensity or extinc-
tion for the two halves. That w, an the analyzer is rotated, one
half of the field gains in intensity while the other half diminishes.
The zero of the instrument is the position of the analyzer which
gives a uniformly lighted field.
By use of a somewhat similar principle triple fields may be
produced. The arrangement of the Nicol prisms is different
in instruments using this principle but the effect IB «uch that the
field is divided into three parts. The Hides have alwavs like