Refraction for Electric Waves. 



191 



wood, Georgia pine, white pine, mahogany, black walnut, 

 and white oak, with the same general result ; namely, that n" 

 is greater than n'. 



Headings were taken and curves plotted as in the experiment 

 with paraffin. The indices of refraction of the woods were 

 obtained for the two principal orientations of the grain of the 

 specimen; t. e., with the grain of the wood perpendicular (n!) 

 to the electric displacement, and with the grain of the wood 

 parallel (n") to the electric displacement. A synopsis of the 



results is given in the following table. 



Substance. 



Thickness 

 in mm. 



mm. Displacement 

 of Nodes. 



n". 



n . 



Grain par. 

 11-8 



Grain perp. 



8-8 



Cherry 



20-8 



1-57 



1-42 



White wood ... 



22-5 



110 



7-6 



1-49 



1-32 



Pitch-pine 



20 2 





12-6 





1-62 



White pine ... 



21-8 



10-0 



70 



1-46 



1-32 



Mahogany 



22-0 



10-5 



6-5 



1-48 



1-30 



Black walnut ... 



20-0 



140 



9-5 



1-70 



1-40 



White oak 



21-3 



140 



110 



160 



1-52 



The transparencies of these specimens for electric waves 

 were also measured, and these together with the indices of 

 refraction are here collected : 



Wood. 



Thickness 

 in mm. 



20-8 



Transparency. 



Index of 

 Befraction. 





Grain par. 



Grain perp. 



n". 

 1-57 



n' . 

 1-40 



Cherry .. 



•61 



•81 



White wood ... 



22-5 



•64 



•82 



1-49 



1*32 



Pitch-pine 



202 



•25 



•54 





1-62 



White pine ... 



21-8 



•52 



•77 



1-46 



132 



Mahogany 



220 



•84 



•85 



1-48 



1-30 



Black walnut. 



200 



•46 



•71 



1-70 



1-49 



White oak 



213 



•60 



•70 



1-66 



1-52 



Whence it appears that the indices of refraction and the 

 transparencies of these woods differ respectively in the two 

 orientations of the grain of the wood. With all the woods 



