548 Dr. G. Pierce on the Double 



directly with the bands obtained in vacuo or in air for the 

 same path. The results of experiments now in progress 

 demonstrate the greater simplicity and accuracy of the method 

 as compared with others. — D. B. Brace.] 



Physical Laboratory, 

 University of Nebraska, Lincoln. 



LI. Note on the Double Refraction of Electric Waves. 

 By George Pierce, Ph.D.* 



IN the February number of the Philosophical Magazine I 

 have described an experiment on the measurement of the 

 refractive indices for electric waves of a number of doubly- 

 refractive media: namely, various woods. These media were 

 all also doubly-absorptive. 



In the present brief note I wish to examine the question as 

 to whether, on Maxwell's theory, the double absorption by 

 these media is sufficient to account for the double refraction ; 

 and, indeed, whether both of these properties can be ascribed 

 merely to differences of conductivity of the woods along and 

 across the grain. The treatment that follows, except the 

 application, is taken largely from Boltzmann's Vorlesungen 

 uber Maxwells Uieorie, § 9(5 (Leipzig, 1891). 



Let/, g, h be the components of electric polarization ; then 

 in a conducting medium, of conductivity C, the equations of 

 the field are (Boltzmann eq. 81) 



and similar equations for g and h. 



Now let us assume a plane wave propagated in the direction 

 of the Z-axis, and let the electric displacement be in the 

 direction of the X-axis ; then g = h = 0, and / is a function of 

 t and z alone ; whence equation (1) becomes 



*3*W$-% • • • • (2) 



A particular solution of (2) i; 



H?«+(H-»!t> 



/=B« T , (3) 



where B, T, £, and rj are constants, to determine which we 



* Communicated by the Author. 



