240 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1908. 



of the light in that volume divided by the square of the velocity of 

 light. Thus, when a body is radiating a portion of the mass of the 

 ether gripped by the body is carried out by the radiation. This mass 

 is, in general, exceedingly small. For example, we find by the appli- 

 cation of the rule we have just given that the mass emitted by each 

 square centimeter of surface of a body at the temperature of the sun 

 is only about 1 milligram per year. We should expect that when 

 some of the ether, bound to a body by its lines of force, is carried off 

 by the radiation, other portions of ether which will not be connected 

 with the body will flow in to take its place. Thus, in consequence of 

 the radiation which proceeds from all bodies, the ether around them 

 will be set in motion iii much the same way as if a series of sources 

 and sinks were distributed throughout the bodies. 



Though the actual mass of the ether traveling with a wave of 

 light is exceedingly small, yet its velocity is so great, being that of 

 light, that even a very small mass possesses an a]3preciable amount 

 of momentum. When the light is absorbed in its passage through a 

 medium which is not perfectly transparent this momentum will 

 also be absorbed and will be communicated to the medium, and will 

 tend to make it move in the direction in which the light is traveling ; 

 the light will thus appear to exert a pressure on the medium; the 

 pressure, which is called the pressure of radiation, has been detected 

 and measured by Lebedew, Nicols and Hull, and Poynting. All the 

 phenomena associated with this pressure may be explained very 

 simply by the view that light possesses momentum in the direction 

 in which it is traveling. The possession of momentum by light, 

 supposing light to be an electric phenomenon, has been deduced by 

 somewhat abstruse consideration. On the old Newtonian emission 

 theory it is obvious at once that this momentum must exist, for it 

 is just the momentum of the particles which constitute the light. 

 It is remarkable how recent investigations have shown that many 

 of the properties of light which might be supposed to be peculiar 

 to a process similar to that contemplated on the emission theory 

 would also be possessed by the light if it were an electric phenomenon. 

 There is one consequence of the emission theory to which I should 

 like briefly to allude, because I think it is more in accordance with 

 the actual properties of light than the view to which we should be 

 led if we took the electro-magnetic theory in the form in which it 

 is usually presented. The active agents on the emission theory are 

 discrete particles, a ray of light consisting of a swarm of such par- 

 ticles, the volume occupied by these particles being only a very small 

 fraction of the volume through which they are distributed. The 

 front of a wave of light would on this view consist of a multitude 

 of small bright specks spread over a dark ground; the wave front 

 in fact is porous and has a structure. Now on the electric theory 



