190 Mr. G. Pierce on Indices of 



micrometer spark-detector, he measured the indices of re- 

 fraction of fir, obtaining with the grain perpendicular to the 

 electric displacement n' = l*75, and with the grain parallel to 

 the electric displacement n n = 2*15. ]n this experiment his 

 wave-length was 66 centim. He found also that a grating, 

 with wires a particular distance apart, turned at 45° with his 

 oscillator gave sparks at the resonator turned at 90° with the 

 oscillator. From this he concluded that the grating pos- 

 sessed the anisotropic character of a bi-refrangent medium. 



Lebedew *, using a thermal-junction and galvanometer, 

 obtained such short waves that he could experiment on the 

 double-refraction of electric waves by crystals. He made two 

 prisms from two natural rhombs of sulphur (1*8 centim. high, 

 1*3 centim. broad, with a refrac ting-angle of about 25°) cut 

 with their refracting-edges parallel respectively to the smallest 

 and greatest dielectric axes (Dielectricitatsaxe) of the crystals 

 as determined by the static measurements of Boltzmann. 

 He got 



n k 



= 2-0 



as compared with Boltzmann's D^=2*18 2 and D t = l*95 2 . 



D. Mazzotto f , using Lecher's method of waves along 

 wires, obtained the indices of refraction of four specimens ot 

 wood, with the following results : — 





Fir. 



Pitch-pine. 



Pear. 



Holm-oak 



(Steineiche). 



n' 



1-568 

 1-834 



1-759 

 1-949 



1-781 

 1-813 



2244 

 2-244 



n 





He found also that the indices of refraction of fir dimin- 

 ished greatly when the fir was dried. In this table, n' is 

 the index of refraction obtained w^hen the slab of wood 

 was so oriented that the grain was perpendicular to the 

 electric displacement; n" the index of refraction with the 

 grain of the wood parallel to the electric displacement. In 

 all the specimens except the oak the latter is the larger. 



In order to get further data on the refraction of electric 

 waves by woods, I have made observations on cherry, white- 



* Wied. Ann. lvi. p. 1 (1895). 

 t Beiblatter, xxii. p. 437 (1893). 



