424 Prof. 0. Lodge on Absolute Velocity and 



naturally obeyed by the action on the one body and the 

 reaction on the other. But during the growth of the stress, 

 during the generation of the energy-filled region, i.e. durino- 

 the time required by the light to advance from source to sink, 

 there is something exceptional and very instructive. Suppose 

 the light has travelled half-way, and consider a parallel beam 

 for simplicity ; half the space is full of energy, the other half 

 is empty, the boundary between the two spaces is like an 

 immaterial piston advancing with the velocity of light. On 

 one side of this piston the stress must be complete and must 

 extend from the source (or parabolic mirror &c.) to the 

 boundary ; the force on the source is manifest, where is the 

 corresponding reaction ? 



I reply, on the advancing boundary between the active and 

 inert regions : on the first wave-front. Here there is an 

 electromagnetic disturbance combined with a sudden transi- 

 tion ; on one side of the boundary, energy H 2 /47r per unit 

 volume, half of it magnetic, half of it electrostatic ; on the 

 other side, no energy at all. But the energy is growing, the 

 clear space in front is receiving energy, how does it receive 

 it ? By the performance of work. The front-wave force, p 

 on every unit area, is advancing with the speed v and doing- 

 work at the rate pv } filling a cylinder of length v with energy 

 every second ; filling every unit cube therefore with energy^. 

 This mechanical force acts along the ray, doing work on the 

 a?ther, which it displaces or shears, not normally but tangen- 

 tially, giving rise to, or at least accompanying, the co-phasal so- 

 called electric and magnetic displacements in the wave-front. 



The energy is propagated along the ray in the direction of 

 the normal force, it exists wherever there is electric accele- 

 ration, and not only where there is obvious radiation ; and 

 the value of the mechanical force in general is the vector 

 product of the electric and magnetic forces, corresponding 

 with Poynting's transmission of energy. 



The front-wave surface may be likened to a liquid skin ; 

 on one side of a liquid boundary is cohesion force, on the other 

 side nothing ; a residual consequence of the internal mole- 

 cular (normal) pressure is a tangential tension in the surface. 

 Somewhat similarly, a result of the internal astherial shearing- 

 stresses in the aether is a normal pressure at the boundary. 



A piece of matter encountering the rays may feel the force, 

 and experience mechanical acceleration in the direction of the 

 force. If it interrupts their progress sharply and discon- 

 tinuously, either by reflexion or absorption, it feels the whole 

 force ; if it reflects or absorbs a little, it feels a little ; if it 

 is perfectly transparent but delays the rate of transmission, 

 then it feels a temporary force appropriate to the difference in 

 speed. For now the aether inside the body receives the energy 



