PRESIDENTIAL ADDRESS, B.A. 1871. 171 



(2) That the ultimate atom of sodium is sus- 

 ceptible of regular clastic vibrations, like those of a 

 tuning-fork or of stringed musical instruments ; 

 that like an instrument with two strings tuned to 

 approximate unison, or an approximately circular 

 elastic disk, it has two fundamental notes or vibra- 

 tions of approximately equal pitch ; and that the 

 periods of these vibrations are precisely the periods 

 of the two slightly different yellow lights consti- 

 tuting the double bright line D. 



(3) That when vapour of sodium is at a high 

 enough temperature to become itself a source of 

 light, each atom executes these two fundamental 

 vibrations simultaneously ; and that therefore the 

 light proceeding from it is of the t.wo qualities 

 constituting the double bright line D. 



(4) That when vapour of sodium is present in 

 space across which light from another source is 

 propagated, its atoms, according to a well-known 

 general principle of dynamics, are set to vibrate 

 in either or both of those fundamental modes, if 

 some of the incident light is of one or other of 

 their periods, or some of one and some of the 



