r example, is a specimen of heart's-ease, the colours of which you
might safely defy the artist to reproduce. By turning the front Nicol
90 degrees round, we pass through a colourless phase to a series of
colours complementary to the former ones. This change is still more
strikingly represented by a rose-tree, which is now presented in its
natural hues--a red flower and green leaves; turning the prism 90
degrees round, we obtain a green flower and red leaves. All these
wonderful chromatic effects have definite mechanical causes in the
motions of the ether. The principle of interference duly applied and
interpreted explains them all.

Sec. 3. _Colours of Crystals in Polarized Light explained by the
Undulatory Theory_.

By this time you have learned that the word 'light' may be used in two
different senses: it may mean the impression made upon consciousness,
or it may mean the physical cause of the impression. It is with this
cause that we have to occupy ourselves at present. The luminiferous
ether is a substance which fills all space, and surrounds the atoms
and molecules of bodies. To this inter-stellar and inter-atomic medium
definite mechanical properties are ascribed, and we deal with it in
our reasonings and calculations as a body possessed of these
properties. In mechanics we have the composition and resolution of
forces and of motions, extending to the composition and resolution of
_vibrations_. We treat the luminiferous ether on mechanical
principles, and, from the composition and resolution of its
vibrations we deduce all the phenomena displayed by crystals in
polarized light.

[Illustration: Fig. 35.]

Let us take, as an example, the crystal of tourmaline, with which we
are now so familiar. Let a vibration cross this crystal oblique to its
axis. Experiment has assured us that a portion of the light will pass
through. The quantity which passes we determine in this way. Let A B
(fig. 35) be the axis of the tourmaline, and let _a_ _b_ represent the
amplitude of an oblique ethereal vibration before it reaches A B. From
_a_ and _b_ let the two perpendiculars _a_ _c_ and _b_ _d_ be drawn
upon the axis: then _c_ _d_ will be the amplitude of the transmitted
vibration.

I shall immediately ask you to follow me while I endeavour to explain
the effects observed when a film of gypsum is placed between the two
Nicol prisms. But, prior to this, it will be desirable to establish
still further the analogy between the