Having regard to what we have now seen, I do not think you will have any difficulty in seeing how it is that the biconvex sound-lens, filled with carbonic acid gas, is able to render divergent sound-rays parallel; in other words, can convert a spherical sound wave into a plane sound wave. Consider what the effect really must be. Let the sound-lens be represented in section by AB (see Fig. 49), and let W be the whistle sending out spherical sound waves, represented by the dotted lines. When the spherical wave meets the lens, the central portion of the wave passes into a retarding medium, whilst the right and left wings of the wave are still in air. Hence, as before, the wings gain on the centre. Again, at emergence the wings emerge before the centre of the wave, and hence again the wings gain on the centre. After complete emergence the spherical wave-surface has been flattened out and made into a plane wave. Hence the sound-rays diverging from the whistle are rendered parallel or even convergent, provided that the whistle is properly placed with regard to the lens. You will see, therefore, that we can use a gas denser than the air, contained in a transparent bag or vessel of collodion, as the means of changing the form and direction of sound waves. We can make lenses and prisms of carbonic acid gas which act on rays of sound just as do lenses and prisms of glass on rays of light. There is, however, one great difference between the operation of a carbonic acid prism on rays of sound, and that of a glass or other prism on rays of light. In the lectures on æther waves it will be made clear to you that what we call light really consists in waves in a medium known as the æther. But when such light waves are propagated through a transparent material like glass, the speed of transmission depends on the wave-length, just as in the case of water waves. But as regards sound waves there is no difference between the velocity of propagation or speed with which waves of different wave-lengths move. Hence a bass note travels just as fast as a treble note, and the sound waves from a flute have a speed of the same value as that from a trumpet or bassoon. If it were not so, it would be impossible for us to hear music or song at a distance, because the notes would arrive all in the wrong order, and the most familiar melody would be unrecognizable. It follows from this that air waves, no matter what their wave-length, are equally refracted on passing from one medium to another of different density. We shall see later on that this is not the case with waves of light and æther waves generally.