It is well known that poisons will affect sight, and belladonna applied to the eyes will so affect them as to render the sight nil, by enlargement of the “pupil.” If one is out of health there is practically a poisoning of the system, and when we have a “bilious headache” we see colours and stars because there is a pressure upon the blood-vessels of the eye. The effects of a disordered stomach, induced by drinking too much, are well known; objects are seen double, and most ghosts may be traced to a disordered state of health of mind or body, brought on by excitement or fatigue. We could relate a series of ghost stories—some in our own experience, for we have seen a ghost equally with our neighbours—but this is not the place for them. Although many apparently incontrovertible assertions are made, and many spectres have been produced to adorn a tale, we must put on record our own opinion, that every one could be traced to mental impression or bodily affection had we only the key to the life and circumstances of the ghost-seer. Many celebrated conjurers will convince us almost against our reason that our pocket-handkerchief is in the orange just cut up. They will bring live rabbits from our coat-pockets or vests, and pigeons from our opera-hats. These are equally illusions. We know what can be done with mirrors. We have seen ghosts at the Polytechnic, but we must put down all apparitions as the result of mental or bodily, even unconscious impressions upon the retina of the eye. There are numerous illusions, such as the Fata Morgana, the Spectre of the Brocken, etc., which are due to a peculiar state of the atmosphere, and to the unequal reflection and refraction of light. Those, and many other optical phenomena, will, with phenomena of heat and sound, be treated under Meteorology, when we will consider the rainbow and the aurora, with many other atmospheric effects.
CHAPTER XV
ACOUSTICS.
THE EAR, AND HEARING—PHYSIOLOGY OF HEARING AND SOUND—SOUND AS COMPARED WITH LIGHT—WHAT IS SOUND?—VELOCITY OF SOUND—CONDUCTIBILITY—THE HARMONOGRAPH.
Before entering upon the science of Acoustics, a short description of the ear, and the mode in which sound is conveyed to our brain, will be no doubt acceptable to our readers. The study of the organs of hearing is not an easy one; although we can see the exterior portion, the interior and delicate membranes are hidden from us in the very hardest bone of the body—the petrous bone, the temporal and rock-like bone of the head.
Fig. 170- 1. Temple bone. 2. Outer surface of temple. 3. Upper wall of bony part of hearing canal. 4. Ligature holding “hammer” bone to roof of drum cavity. 5. Roof to drum cavity. 6. Semicircular canals. 7. Anvil bone. 8. Hammer bone. 9. Stirrup bone. 10. Cochlea. 11. Drum-head cut across. 12. Isthmus of Eustachian tube. 13. Mouth of tube in the throat. 14. Auditory canal. 15. Lower wall of canal. 16. Lower wall of cartilaginous part of canal. 17. Wax glands. 18. Lobule. 19. Upper wall of cartilaginous portion of canal. 20. Mouth of auditory canal. 21. Anti-tragus.
The ear (external) is composed of the auricle, the visible ear, the auditory canal, and the drum-head, or membra tympani. The tympanum, or “drum,” is situated between the external and the internal portions of the ear. This part is the “middle ear,” and is an air cavity, and through it pass two nerves, one to the face, and the other to the tongue. The internal ear is called the “labyrinth,” from its intricate structure. We give an illustration of the auditory apparatus of man (fig. 170).
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
|