The species of undulations are also different in the cases of Light and Sound. In common light the vibrations of the molecules of ether take place in a line at right angles to the direction of propaga tion of the ray; in sound the particles of air vibrate in the direction of propagation. The undulations of light may be compared to those produced by throwing a stone into a smooth and deep pond of water ; those of sound to the waves produced in a field of corn when the wind sweeps over it. In the former case, that of the water set in motion by the stone, each particle of water moves up and down in a vertical line ; in the case of the field of corn, each ear of corn moves backwards and forwards in a horizontal direction, or something ap proaching to it. The illustration is certainly far from being perfect, but it may assist the reader in forming a true conception of the dif ference between light and sound. There is also a great difference between the velocity of propagation, and length of the wave in the cases of light and sound. Light travels in vacuo at the enormous rate of 192,000 miles in a second, and the average length of a wave of light is about the one fifty-thousandth part of an inch; while sound only travels at the rate of about 1100 feet in a second, and the length of a wave of sound capable of affecting the auditory nerves of man, lies between a few inches and several feet,—the short wave most frequently repeated giving the high note, the long wave less frequently repeated, the low note.
But before carrying these analogies any further, we will endeavour to explain more clearly the precise nature of an undulation of light.
A luminous body is supposed to be a material substance the particles of which are in a state of intense agitation. These set in motion the molecules of ether next to them, and this motion is com municated from molecule to molecule, along a line of molecules, with amazing velocity, and in the following manner : Suppose AB to be the direction of a ray of light, and bed atoms of ether; then, the atom b oscillates oevibrates up and down along the line el, at right angles to AB ; the atom c along the line fm d along the line gn ; and so on.
The velocity of the atom at b gradually decreases as it moves towards e, and on arriving at e altogether vanishes ; the molecule then returns towards b, acquiring fresh velocity as it approaches b, attaining its maximum of velocity at b, then gradually losing it again as it approaches /, and losing it entirely at /; after which it returns to e as before ; and so on. Similarly with respect to every
atom along the line AB ; and it must be clearly understood that no atom actually travels from A towards B, but merely continues to vibrate through an exceedingly small space in a line at right angles to AB.
This being understood, we have to explain how an undulation is produced and propagated. It will be remembered that although light travels with amazing velocity, yet that velocity is measurable and finite, so that a molecule of ether, b, at one part of the line AB may be vibrating, while another molecule, d, is at rest. Suppose then we consider the state of the vibrating atoma which constitute a ray of light, at a particular epoch of time. It is evident that one atom may be at e, while another is at p, another at o, another at q, another at f, and so on. If then we draw a curved line through the instantaneous positions of the atoms, epoqf, that curve will represent an undulation, or wave of light ; and the particles e and f, and 8, are said to be in the same " phase" of undulation ; the length of the wave being the distance ef, or qs.
A ray of common light is composed of undulations which are propagated in the manner described in all possible planes passing through AB. A ray of " plane polarized light" is one in which the undulations are propagated in only one plane which passes through AB. A ray of " circularly polarized light" is one in which the curved outline of the undulation, instead of lying on a plane, forms a spiral round AB like the thread of a corkscrew, and called a " helix." A ray of " elliptically polarized light" is one in which the spiral, instead of being coiled, so to speak, round a circular cylinder, as in the former case, is coiled round an elliptical cylinder. The subject of polarized light will be discussed presently.
The effect produced by a ray of light is due to the blow of the last vibrating atom against the material substance upon which it is incident. As the undulations are propagated by the luminous body continuously, these blows follow one another in rapid succession, and a vast number of very small blows thus administered produce an appreciable effect in a finite time. This effect is moreover con siderably increased when a number of rays axe brought to a focus, and act upon the same point.