261 

 In all 



On the Longitudinal Component in Light. 



propagation, but not at all as regards origination, 

 cases of origination we have to do with conduction, or its 

 equivalent convection, and in most such cases we have 

 changing electrification which brings in the J term. 

 The longitudinal component at each point is 



r v r 



2z 



- z (2qsinpt — g? , + - cos pt — qi>). 



This is no doubt very small at a distance from the oscillator 



compared with the transverse component which involves — , 



and in Consequence the motion is transverse at most places. 

 On the axis of 2, however, the transverse component, which 

 is proportional to p the distance from the axis, vanishes 

 entirely. Hence along the axis there is a beam of purely 

 longitudinal vibration, of no doubt small amplitude, but 

 nevertheless existing necessarily in order that there may be 

 no compressions. This all appears on the face of Hertz's 

 investigation. He carefully studied the forces as represented 

 by the above equations, and has plotted them and shown that 

 they represent a series of whirl rings thrown off from the 

 oscillator and growing gradually thinner and thinner until at 

 a distance the rings become nearly plane waves, and the 

 opposite sides being always a 

 wave-length apart are the two 

 opposite phases of the wave. 

 The accompanying diagram 

 roughly represents this state of 

 affairs. It is evident on the most 

 cursory consideration that these 

 waves must have a longitudinal 

 region. The lines of force in 

 any one wave are up to the axis 

 along any one spherical surface 

 all round ; and if there is not to 

 be concentration anywhere, i. e. 

 if there is no electrification of the 

 medium, they must turn round 

 and be continuous with the return 

 phase of the wave. The reason 

 why they are so feebly concen- 

 trated in this return region is 

 because it is so enormously extended. If the wave-length be 

 small compared with the distance from the origin, the flows 



