On tJie Transparency of the Ether. 9 



Substituting now in (4), 



or, omitting tlie imaginary part, 



■2TT-V , ^ ^iTT-V , . 



^ = Ae -^^''cos(^^-//j = ^e~^COS2 7r(|-Q. (ll) 



2 77=.' ./ 

 ^_y + Z - y 



Thus F=€ "''' " approximately. (12) 



Stokes ^ has advanced the view that a fluid may be an ex- 

 tremely plastic body which admits of "a finite, but exceed- 

 ingly small amount of constraint before it is relieved from its 

 state of tension by its molecules assuming new positions of 

 equilibrium." He suggests that the ether may be like a very 

 plastic substance, which allows of the free motions of solids 

 through it, but which also admits of small amounts of con- 

 straint without permanent distortion. He instances ^ as an 

 illustration a mixture of jelly and water in varying propor- 

 tions, which will admit of a given amount of constraint with- 

 out dislocation, this constraint being less and less as the 

 mixture is made thinner. Now all solid bodies in nature 

 seem to possess to a degree more or less marked the quality 

 of " Elastic After-effect," as observed by Weber, Kohlrausch, 

 and others, where the elastic recovery, as well as the stress 

 produced by any strain, depends on the time. Extending 

 this quality to the ether on the supposition that it is an ex- 

 tremely plastic body and should possess the same kind of 

 qualities as other plastic substances, it would seem that a loss 

 of energy could take place from this cause. In this case F 

 will have a somewhat different form from (12). 



Suppose that the stress at any time is independent of the rate 

 of shear, but depends on the duration and magnitude of the 

 strain, as it would if the " Elastic After-effect " were present. 



1 Collected Papers, Vol. I., page 125. 



2 Collected Papers, Vol. II., page 12. 



9 



