Photographic Action of Stationary Light-Waves. 99 



excited by simple translation, by means of the device of an 

 outer shell imbedded in the aether and containing inside it 

 masses with spring connexions ; and such a system might also 

 be adjusted so as to respond to simple rotation, and therefore be 

 excited at the nodes of the wave-train instead of the antinodes. 



A theory based in this manner on difference of inertia must 

 take the density of the aether to be very minute compared 

 with that of matter ; therefore if the molecule is to have free 

 periods of the same order of magnitude as the periods of the 

 incident light-waves, the elastic forces acting- between the 

 atoms and concerned in these periods must be very intense. 

 But Lord Kelvin's well-known estimate of the rigidity of the 

 aether on this hypothesis makes it very small compared with 

 the ordinary rigidity of material bodies*. In fact on 

 Pouilletfs data, which imply a considerable underestimate, 

 the energy of the solar radiation near the sun's surface is 

 about 4 x 10 -5 ergs per cubic centimetre; it easily follows that 

 if the amplitude of the sethereal disturbance is, say, e times 

 the wave-length, the density of the aether must be about l/10' 22 e, 

 and its rigidity, which is equal to the density multiplied by 

 the square of the velocity of propagation, therefore 1/1 Oe 2 . 

 On an elastic solid theory it is desirable to have the density 

 very small : thus if we adopt 10 -2 as the maximum likely 

 value of e, the density of the aether comes out 10 -18 of that of 

 water, and its rigidity about 10 3 , whereas the rigidity of steel 

 or glass is of the order 10 u . 



Now at first sight it would appear that the elastic tractions 

 exerted by an aether of such small rigidity on an imbedded 

 molecule swaying backwards and forwards in it, would be 

 vanishingly small compared with the elastic forces between 

 its constituent atoms which are concerned with free vibrations 

 of the kind of period under consideration ; and that therefore 

 they would be quite incompetent to produce violent disturb- 

 ance in the molecule. But on a closer examination this 

 difficulty may to a considerable extent be evaded. 



Let us imagine an imbedded rigid nodule of linear dimen- 

 sion L, and let the force necessary to displace it in the aether 

 in any manner through a distance x be \jx. Let us compare 

 with it a similar nodule of linear dimensions kL displaced 

 through a distance kx. There is complete dynamical simi- 

 larity between the two cases ; the strains at corresponding 

 points in the aether are equal, and therefore so are the trac- 

 tions per unit area. Thus the forcive necessary in the latter 

 case to produce the displacement kx is k 2 Jjx, and therefore to 

 produce the same displacement x as in the previous case a 

 * Cf. Maxwell, Ennjc. Brit., article "^Ether." 



H2 ' 



