action of the prisms and that of
the two plates of tourmaline. The magnified images of these plates,
with their axes at right-angles to each other, are now before you.
Introducing between them a film of selenite, you observe that by
turning the film round it may be placed in a position where it has no
power to abolish the darkness of the superposed portions of the
tourmalines. Why is this? The answer is, that in the gypsum there are
two directions, at right angles to each other, in which alone
vibrations can take place, and that in our present experiment one of
these directions is parallel to one of the axes of the tourmaline, and
the other parallel to the other axis. When this is the case, the film
exercises no sensible action upon the light. But now I turn the film
so as to render its directions of vibration _oblique_ to the two
tourmaline axes; then, you see it exercises the power, demonstrated in
the last lecture, of partially restoring the light.

[Illustration: Fig. 36.]

Let us now mount our Nicol prisms, and cross them as we crossed the
tourmaline. Introducing our film of gypsum between them, you notice
that in one particular position the film has no power whatever over
the field of view. But, when the film is turned a little way round,
the light passes. We have now to understand the mechanism by which
this is effected.

First, then, we have a prism which receives the light from the
electric lamp, and which is called the _polarizer_. Then we have the
plate of gypsum (supposed to be placed at S, fig. 36), and then the
prism in front, which is called the _analyzer_. On its emergence from
the first prism, the light is polarized; and, in the particular case
now before us, its vibrations are executed in a horizontal plane. We
have to examine what occurs when the two directions of vibration in
the interposed gypsum are oblique to the horizon. Draw a rectangular
cross (A B, C D, fig. 37) to represent these two directions. Draw a
line (_a_ _b_) to represent the amplitude of the horizontal vibration
on the emergence of the light from the first Nicol. Let fall from each
end of this line two perpendiculars (_a_ _c_, _a_ _f_, _b_ _d_, _b_
_e_) on the two arms of the cross; then the distances (_c_ _d_, _e_
_f_) between the feet of these perpendiculars represent the amplitudes
of two rectangular vibrations, which are the _components_ of the first
single vibration. Thus the polarized ray, when it enters the gypsum,
is reso