within each molecule the atoms are in motion. Each atom, again, has its own internal movement. But, if the world were made up of atoms and molecules alone, we should never know of their existence; and, to explain the phenomena of the universe, we must recognize the presence of a continuous universal medium penetrating all space and all bodies. This medium, which we call the luminiferous ether, or simply the ether, serves to keep up the connection between atoms or molecules. All communications from one atom to another, and from one molecule to another, are made through this ether. The internal motions of one atom are communicated to this medium, propagated through space, until they reach another atom; attraction, repulsion, or some other manifestation takes place; and, if you examine any of the changes which you see constantly going on around you, and follow it backward through its various stages, you will always find the motion of atoms or molecules at the end of the chain.
The importance of studying the motion of molecules is therefore clear; and it is the special domain of the modern spectroscopy to investigate one kind of these motions.
When a tuning-fork or a bell is set in vibration, its motion is taken up by the surrounding air, waves are set up, they spread and produce the sensation of sound in our ears. Similarly, when an atom vibrates, its motion is taken up by the ether, waves are set up, they spread, arid if of sufficient intensity produce the sensation of light in our eyes. Both sound and light are wave-motions. A cursory glance at a wave in water will lead you to distinguish its two most prominent attributes. You notice at once that waves differ in height. So the waves both of light and sound may differ in height, and to a difference in height corresponds a difference in the intensity of the sound you hear or of the light you see. The higher the wave the greater its energy, the louder is the sound or the brighter is the light. But, in addition to a difference in height, you have noticed that in different waves the distance from crest to crest may vary. The distance from crest to crest is the length of the wave, and waves not only differ in height but also in length. A difference in the length of a wave of sound corresponds to a difference in the pitch of the sound; the longer a sound-wave is, the lower is the tune you hear. In the case of light a difference in the length of the wave corresponds to a difference in the color you see. The longest waves which affect our eyes produce the sensation of red, then follow orange, yellow, green, blue, and the shortest waves which we ordinarily see seem violet. If a molecule vibrates, it generally sends out a great number of waves which vary in length. These fall together on our retina, and produce a compound sensation which does not allow us to distinguish the elementary vibrations, which we want to examine. A spectroscope is an instrument which separates the waves of different lengths before they reach our retina; the elementary vibrations, after having passed through a spectroscope, no longer
