On the Transparency of the Ether. 1 1 



In the actual case of solid bodies, the decay of a vibration 

 does not seem to follow either equation (14) or equation {12). 

 Sir William Thompson found that the relative diminution in 

 the amplitude of vibrating wires of different periods was less 

 than it would be if it were due to viscosity alone, and greater 

 than it would be if due to imperfect elasticity or the Elastic 

 After-effect alone. However, the diminution was more rapid 

 for short periods than for long ones, indicating a dependence 

 on the rate of shear, as well as on imperfect elasticity. The 

 law of decay, if due to both these causes, would be expressed 

 by the equation 



^ = Ae ^''^^'^^'^''coS2 7rp_^\ (16) 



where 6■^ and 6^ are unknown, but may be determined by 

 experiment for different substances subjected to different 

 rates of distortion. The three formulae 



II. ^=Ar^^\o^2 7rU-^\ 



III. i = Az ^"^ '"'-' COS2 7r(^X~T 



indicate the absorption as depending on the wave-length. If 

 absorption takes place in the ether in a way analogous to that 

 in ponderable substances, it must follow one of these laws, 

 which include all modes of absorption for ordinary bodies, 

 and hence coloration should occur in varying amounts with 

 the distance. The equation II. represents the case of mini- 

 mum coloration. 



From what is known of the decay of vibrations in material 

 bodies, it seems most probable that the conditions of the 

 problem are most nearly satisfied by I. When the rate of 

 distortion in solid bodies is considerable, the viscous resist- 



II 



