The auricle, or exterior ear, is also represented, but we need not go into any minute description of the parts. We will just name them (fig. 171).
Sound is the motion imparted to the auditory nerve, and we shall see in a moment how sound is produced. The undulations enter the auditory canal, having been taken up by the auricle; the waves or vibrations move at the rate of 1,100 feet a second, and reach the drum-head, which has motion imparted to it. This motion or oscillation is imparted to other portions, and through the liquid in the labyrinth. The impressions of the sound wave are conveyed to the nerve, and this perception of the movement in the water of the labyrinth by the nerve threads and the brain causes what we term “hearing.”
Fig. 171.—1. Pit of anti-helix. 2, 6, 10. Curved edge of the auricle. 3. Mouth of auditory canal. 4. Tragus. 5. Lobe. 7. Anti-helix. 8. Concha. 9. Anti-tragus.
Let us now endeavour to explain what sound is, and how it arises. There are some curious parallels between sound and light. When speaking of light we mentioned some of the analogies between sound and light, and as we proceed to consider sound, we will not lose sight of the light we have just passed by.
Sound is the influence of air in motion upon the hearing or auditory nerves. Light, as we have seen, is the ether in motion, the vibrations striking the nerves of the eye.
There are musical and unmusical sounds. The former are audible when the vibrations of the air reach our nerves at regular intervals. Unmusical sounds, or irregular vibrations, create noise. Now, musical tones bear the same relation to the ear as colours do to the eye. We must have a certain number of vibrations of ether to give us a certain colour (vide table). “About four hundred and fifty billion impulses in a second” give red light. The violet rays require nearly double. So we obtain colours by the different rate of the impingement of impulses on the retina. The eyes, as we have already learned, cannot receive any more rapidly-recurring impressions than those producing violet, although as proved, the spectrum is by no means exhausted, even if they are invisible. In the consideration of Calorescence we pointed this out. These invisible rays work great chemical changes when they get beyond violet, and are shown to be heat. So the rays which do not reach the velocity of red rays are also heat, which is the effect of motion.
Thus we have Heat, Light, and Sound, all the ascertained results of vibratory motion. The stillness of the ether around us is known as “Darkness”; the stillness of the air is “Silence”; the comparative absence of heat, or molecular motion of bodies is “Cold”!
In the first part we showed how coins impart motion to each other.
| VELOCITY OF LIGHT WAVES. | ||
|---|---|---|
| According to Sir J. Herschel. | ||
| Colour of the Spectrum. | No. of Undulations in an inch. | No. of Undulations in a second. |
| Extreme Red | 37,640 | 458,000,000,000,000 |
| Red | 39,180 | 477,000,000,000,000 |
| Intermediate | 40,720 | 495,000,000,000,000 |
| Orange | 41,610 | 506,000,000,000,000 |
| Intermediate | 42,510 | 517,000,000,000,000 |
| Yellow | 44,000 | 535,000,000,000,000 |
| Intermediate | 45,600 | 555,000,000,000,000 |
| Green | 47,460 | 577,000,000,000,000 |
| Intermediate | 49,320 | 600,000,000,000,000 |
| Blue | 51,110 | 622,000,000,000,000 |
| Intermediate | 52,910 | 644,000,000,000,000 |
| Indigo | 54,070 | 658,000,000,000,000 |
| Intermediate | 55,240 | 672,000,000,000,000 |
| Violet | 57,490 | 699,000,000,000,000 |
| Extreme Violet | 59,750 | 727,000,000,000,000 |