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 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.]