366 A CENTUEY OF SCIENCE 



Ized light these vibrations are confined to a single 

 direction. This supposition explained so many of the 

 puzzling results of experiment, that it was accepted at 

 once and led to the complete vindication of the undula- 

 tory theory. 



Elastic Solid Theory. Shortly afterwards Poisson 

 succeeded in solving the differential equation which 

 determines the motion of a wave through an elastic 

 medium. His solution shows that such a medium is 

 capable of transmitting two types of wave one longi- 

 tudinal, the other transverse. If k denotes the volume 

 elasticity, -n the rigidity and p the density of the medium, 

 the velocities of the two waves are respectively 



+ l and 

 p 



Now a solid has both compressibility and rigidity, 

 and transmits in general both types of wave. A 

 fluid, on the other hand, on account of its lack of 

 rigidity, cannot support a transverse vibration. Hence 

 it was natural that Green, in searching for a dynamical 

 explanation of the ether, should have proposed in a paper 

 read before the Cambridge Philosophical Society in 

 1837 that the ether has the elastic properties of a solid. 

 One great difficulty presented itself; disturbances 

 inside an elastic solid must give rise to compressional as 

 well as to transverse waves. But no such thing as a 

 compressional wave had been found in the experimental 

 study of light. Green attempted to overcome this diffi- 

 culty by attributing an infinite volume-elasticity to the 

 ether. The expression above shows that longitudinal 

 waves originating in such an incompressible medium 

 would be carried away with an infinite velocity, and it 

 may be shown that the energy associated with them 

 would be infinitesimal in amount. The next step was to 

 calculate the coefficients of transmission and reflection 

 for light passing from one material medium to another. 

 Here the elastic solid theory is not altogether successful. 

 If the ether is supposed to have different densities in the 

 two media, as in FresnePs theory, but the same rigidity, 

 certain of these coefficients fail to give the values 



