470 THE POPULAR SCIENCE MONTHLY. 



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 in- 

 vestigate 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 diifer in height, and to a difference in height cor- 

 responds 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 differ- 

 ence 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 element- 

 ary vibrations, after having passed through a spectroscope, no longer 



