THEORIES OF LIGHT.
What is Light? The ancients supposed it to be something emitted by the eyes, and for ages no notion was entertained that it required time to pass through space. In the year 1676 Römer first proved that the light from Jupiter's satellites required a certain time to cross the earth's orbit. Bradley afterwards found that, owing to the velocity with which the earth flies through space, the rays of the stars are slightly inclined, just as rain-drops which descend vertically appear to meet us when we move swiftly through the shower. In Kew Gardens there is a sun-dial commemorative of this discovery, which is called the aberration of light. Knowing the velocity of the earth, and the inclination of the stellar rays, Bradley was able to calculate the velocity of light; and his result agrees closely with that of Römer. Celestial distances were here involved, but a few years ago M. Fizeau, by an extremely ingenious contrivance, determined the time required by light to pass over a distance of about 9000 yards; and his experiment is quite in accordance with the results of his predecessors.
But what is it which thus moves? Some, and among the number Newton, imagined light to consist of particles darted out from luminous bodies. This is the so-called Emission-Theory, which was held by some of the greatest men: Laplace, for example, accepted it; and M. Biot has developed it with a lucidity and power peculiar to himself. It was first opposed by the astronomer Huyghens, and afterwards by Euler, both of whom supposed light to be a kind of undulatory motion; but they were borne down by their great antagonists, and the emission-theory held its ground until the commencement of the present century, when Thomas Young, Professor of Natural Philosophy in the Royal Institution, reversed the scientific creed by placing the Theory of Undulation on firm foundations. He was followed by a young Frenchman of extraordinary genius, who, by the force of his logic and the conclusiveness of his experiments, left the Wave-Theory without a competitor. The name of this young Frenchman was Augustin Fresnel.
Since his time some of the ablest minds in Europe have been applied to the investigation of this subject; and thus a mastery, almost miraculous, has been attained over the grandest and most subtle of natural phenomena. True knowledge is always fruitful, and a clear conception regarding any one natural agent leads infallibly to better notions regarding others. Thus it is that our knowledge of light has corrected and expanded our knowledge of heat, while the latter, in its turn, will assuredly lead us to clearer conceptions regarding the other forces of Nature.
I think it will not be a useless labour if I here endeavour to state, in a simple manner, our present views of light and heat. Such knowledge is essential to the explanation of many of the phenomena referred to in the foregoing pages; and even to the full comprehension of the origin of the glaciers themselves. A few remarks on the nature of sound will form a fit introduction.
NATURE OF SOUND.
It is known that sound is conveyed to our organs of hearing by the air: a bell struck in a vacuum emits no sound, and even when the air is thin the sound is enfeebled. Hawksbee proved this by the air-pump; De Saussure fired a pistol at the top of Mont Blanc,—I have repeated the experiment myself, and found, with him, that the sound is feebler than at the sea level. Sound is not produced by anything projected through the air. The explosion of a gun, for example, is sent forward by a motion of a totally different kind from that which animates the bullet projected from the gun: the latter is a motion of translation; the former, one of vibration. To use a rough comparison, sound is projected through the air as a push is through a crowd; it is the propagation of a wave or pulse, each particle taking up the motion of its neighbour, and delivering it on to the next. These aërial waves enter the external ear, meet a membrane, the so-called tympanic membrane, which is drawn across the passage at a certain place, and break upon it as sea-waves do upon the shore. The membrane is shaken, its tremors are communicated to the auditory nerve, and transmitted by it to the brain, where they produce the impression to which we give the name of sound.
CAUSE OF MUSIC.
In the tumult of a city, pulses of different kinds strike irregularly upon the tympanum, and we call the effect noise; but when a succession of impulses reach the ear at regular intervals we feel the effect as music. Thus, a vibrating string imparts a series of shocks to the air around it, which are transmitted with perfect regularity to the ear, and produce a musical note. When we hear the song of a soaring lark we may be sure that the entire atmosphere between us and the bird is filled with pulses, or undulations, or waves, as they are often called, produced by the little songster's organ of voice. This organ is a vibrating instrument, resembling, in principle, the reed of a clarionet. Let us suppose that we hear the song of a lark, elevated to a height of 500 feet in the air. Before this is possible, the bird must have agitated a sphere of air 1000 feet in diameter; that is to say, it must have communicated to 17,888 tons of air a motion sufficiently intense to be appreciated by our organs of hearing.
CAUSE OF PITCH.