1881.] on the Teachings of Modern Spectroscopy. 495 



nrouiul yoii, aiul follow it backwards through its various stages, you 

 will always fiuil 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 spectrosco2)y to 

 invcstigatato one kind of these motions. 



When a tuning-fork or a bell is set into 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 uji by the ether, waves are set up, they 

 spread, and if of sufficient intensity produce tlie 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 tlie 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 j'-ou hear. In the case 

 of light a difference in the length of the wave corresponds to a 

 difference in the colour you see. The longest waves which affect our 

 eyes produce the sensation of red, then follows 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 

 w^hich 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 overlap, but produce their im- 

 pressions side by side of each other, and their examination and 

 investigation is therefore rendered possible. 



The elements of si^ectroscopy will be familiar to most of you, but 

 you will forgive me if I briefly allude to some points, which, though 

 well known, are of special importance in the considerations which I 

 wish to bring before you to-night. 



When a body is sufficiently hot it becomes luminous, or to speak 

 in scientific language, the vibrations which are capable of pro- 

 ducing a luminous sensation on our retina, are increased in intensity 

 as the temperature is raised, until they produce such a sensation. 

 By means of a strong electric current I can in the electric lamp raise 

 a piece of carbon to a high temperature. When looked at with the 

 unaided eye it seems white hot, but when I send the rays through 



2 M 2 



