MECHANICS AND OPTICS. 223 



cannon recoils because the projectile on which it has 

 acted reacts upon it. 



But here the case is not the same. What we have 

 fired away is no longer a material projectile ; it is 

 energy, and energy has no mass — there is no counter- 

 part. Instead of an excitator, we might have con- 

 sidered simply a lamp with a reflector concentrating 

 its rays in a single direction. 



It is true that if the energy emanating from the 

 excitator or the lamp happens to reach a material 

 object, this object will experience a mechanical thrust 

 as if it had been struck by an actual projectile, and 

 this thrust will be equal to the recoil of the excitator 

 or the lamp, if no energy has been lost on the way, 

 and if the object absorbs the energy in its entirety. 

 We should then be tempted to say that there is still 

 compensation between the action and the reaction. 

 But this compensation, even though it is complete, is 

 always late. It never occurs at all if the light, after 

 leaving the source, strays in the interstellar spaces 

 without ever meeting a material body, and it is 

 incomplete if the body it strikes is not perfectly 

 absorbent. 



Arc these mechanical actions too small to be 

 measured, or are they appreciable by experiment ? 

 They are none other than the actions due to the 

 Maxwell-Bartholi pressures. Maxwell had predicted 

 these pressures by calculations relating to Electro- 

 statics and Magnetism, and Bartholi had arrived at 

 the same results on Thermodynamic grounds. 



it is in this way that tails of comets are explained. 

 Small particles are detached from the head of the 

 comet, they are struck by the light of the Sun, which 



