592 Stoney — Cause of Double Lines in SiHctra. 



CHAPTER V. 



ILLUSTRATIONS. 



When a chord is played by an orchestra, a very complicated undulation spreads 

 around it through the air. This involved motion admits of two successive resolu- 

 tions into simpler elements, first by regarding it as the coexistence and super- 

 position of undulations emanating from the several instruments each of which 

 by itself would produce the effect on the ear of a musical note ; and next by 

 the further resolution of the note emitted by each separate instrument into its 

 pendulous elements whose coexistence is what gives to the tone of that instrument 

 its special quality. So long as the undulation advances through the air, these 

 elements are not separated from one another ; but their presence is indicated 

 analytically in the fullest detail by Fourier's theorem, and the ear of a highly 

 trained musician is a means by which they may in practice be partially distinguished. 

 Such a person can completely effect the first resolution, i.e. can distinguish each 

 separate note, and can imperfectly effect the second resolution, i.e. he can distinguish 

 the qualities or clangs of these separate notes ; but his ear is powerless to complete 

 the resolution by distinguishing the individual partials or pendulous elements, 

 whose presence is what determines the distinctive sound of each instrument. Now 

 that which the ear does imperfectly in the case of sound the spectroscope does 

 fully in the case of light. 



In this comparison between light and sound, each molecule of the gas cor- 

 responds to the entire orchestra ; and the orbits described within it by its several 

 electrons are what correspond to the vibrations of the sounding-boards, columns of 

 air, &c., of the instruments, these being the parts of the orchestra which act 

 directly on the sm*rounding medium. The undulation in the luminiferous eether 

 which emanates from a molecule of the gas corresponds to the waves that fill the 

 atmosphere when a chord is being played by the instruments of the orchestra. 

 That we can resolve this into the notes emitted by the individual instruments cor- 

 responds to our being able in some cases to divide the lines of the spectrum of a 

 gas into groups, each of which may be attributed to the motion of one of its 

 electrons. The patterns which we may observe in these groups of lines correspond 

 to the clangs or qualities of the notes of the orchestra ; and, finally, the individual 

 lines themselves supply us directly with the intensities and periodic times of the 

 individual partials which are the ultimate elements into which Fourier's theorem 

 resolves every undulation of the aether, however complex. 



