Refraction for Electric Waves. 



189 



values of >*, as well as for the largeness of the mean in com- 

 parison with other experimenters' results. 



Index of Refraction of Hard Rubber. — In a similar manner 

 data were obtained for locating the nodes of a stationary wave 

 in air, and of the same wave when displaced by the interpo- 

 sition of a sheet of hard rubber between the oscillator and the 

 reflector. 



By plotting the corresponding curves (VII. and VI., not 

 given here) the following results were obtained : — 



Thickness of specimen = 13 millim. 



i Third Node 



I 



i Second Node 



Curve VII 



13-8 

 11-45 



Curve VI. 



12-8 

 10-5 



Disr 



100 

 95 



n. 



177 

 1-73 



Mean value of ?i = 1*75. 



Indices of Refraction of Woods. — The refraction of electric 

 waves by woods is a subject of interest because woods give 

 evidence of being bi-refracting media. 



In 1891 Righi came to the conclusion that the wood of the 

 fir-tree showed double refraction of electric waves. His 

 experiment was as follows : — He turned his oscillator and 

 resonator at right angles to each other about their common 

 optical axis. In this position there were no sparks at the 

 resonator. When, however, a slab of fir was interposed 

 between the oscillator and the resonator with its grain in- 

 clined at an angle of 45° to the former, the resonator showed 

 sparks that in general were not extinguished for any angle 

 of the resonator, but merely went through maxima and 

 minima of vivacity when the resonator had two certain orien- 

 tations. When this experiment was repeated with the fibre 

 of the wood turned, not at 45°, but perpendicular or parallel 

 to the oscillator, the emergent wave preserved its rectilinear 

 character. Experimentally he found that a slab of this wood 

 of thickness 13*7 centim. acted as a quarter-wave plate for his 

 10*6-centim. waves ; and from this he calculated the difference 

 between the two principal indices of refraction. 



K. Mack *, with a wave of length 50 or 60 centim ., repeated 

 Eighi's experiment with the same result. Also later f, by 

 the method of stationary waves, using a Hertz oscillator and 



* Wied. Ann. liv. p. 342 (1895). 

 t JftwUvi. p. 177(1895). 



