on the Mechanical Function of anJEthev. 425 



more slowly than before, and yet energy is poured in at the ori- 

 ginal rate; the difference p{v — v f ) is being taken by the material 

 particles or electrons which share in the conveyance of light. 

 But the matter only feels the force p(v — v f ) for a moment; 

 when the light emerges the pressure behind is the same as 

 that in front, and the matter transmits its portion of the stress 

 like any other transparent region and experiences no per- 

 sistent acceleration. (Divergence of rays matters nothing, 

 for Stokes's solenoidal law, p A = constant, of course holds.) 

 But an opaque body may receive the whole energy on its 

 molecules, and in that case will experience the whole pressure; 

 and the activity pv now represents the rate at which heat is 

 being persistently generated in the body per unit of illumi- 

 nated area. A reflecting body of course leaves the energy 

 in the aether, and starts a new reaction force travelling back- 

 wards with the new advancing wave, the mirror itself taking 

 up the reaction both of the old and the new streams. 



The force at a free wave-front is not to be thought of as if 

 operating on a yielding or elastic body, but, after the manner 

 of all such cases, as generating and leaving energy in one 

 region of the aether after another as it sweeps over it. 



If, for whatever reason, there is a boundary between active 

 and inert aether, that boundary must experience the pressure; 

 if the boundary is the surface of black matter, the transition is 

 maintained by absorption, and the energy generated is that of 

 heat ; if the boundary is free aether, the transition can only 

 be maintained by rapid translation, and the energy generated 

 is that of radiation. If the activity of the source is at any 

 instant stopped, a definite block of radiation, with its longitu- 

 dinal stress, travels bodily forward ; the front of it acts precisely 

 as before, but the pressure at its back surface is opposed to the 

 motion, and thus corresponds with the wiping out of the energy 

 of the aether left behind. So long as the source was in action 

 it kept this otherwise back region constantly replenished. 



When the force impinges on matter it accelerates it 

 normally, and may move with the matter at any arbi- 

 trary rate ; though if the matter move away with the 



speed u, the force on it is diminished in the proportion v . 



So also if a source moves with speed + u, the reaction on it 

 is likewise altered, and with it its radiating power, which at 

 once becomes p(V ±u), or m general p(V + ucos 6) in any 

 direction. It is simply a question of relative velocity; and 

 the balance of force is such as to oppose the motion. What- 

 ever force a retreating black target thus avoids is taken up by 

 the aether recently inside it, which thus has energy given to 

 it at the rate pu, while the matter takes the rest ; but while 

 mechanical yield is along, absorption yield is across, the force. 



