Refraction for Electric Waves. 187 



Other determinations of this curve gave about the same 

 position of the maxima and minima, and also the same general 

 outline of the curve. 



If we give equal weight to the several values of half wave- 

 length averaged above, the probable error is about *05 cm., 

 or 2 per cent. 



Index of Refraction of Paraffin. — If a piece of dielectric 

 ([/, tig. 4) with an index of refraction different from that 

 of' air be interposed between the oscillator and the reflector, 

 with the arrangement of the apparatus above, the stationary 

 wave will be displaced. This affords a method of measuring 

 the index of refraction of dielectrics for short electric waves 

 A working formula is derived by the following simple 

 analysis: — 



Suppose a flat piece of dielectric of thickness ^ to be 

 placed in the position indicated by the dotted outline D, 



Let n = the index of refraction of the dielectric referred to 

 air ; then 



n=V/Y'; 



if V and V are respectively the velocity of propagation in 

 air and in the dielectric. 



Let r = the distance from the oscillator to the reflector for 

 a given minimum with air as dielectric; r x = this distance 

 when the block of other dielectric is interposed ; then, since 

 the time of traversing these respective paths is the same, we 

 have 



r/y v V' 



r — '/ = x(n — l); 



.-. »=.— + 1, 



X 



where D is the displacement of the given minimum. 



In determining the index of refraction of paraffin, for con- 

 venience, the third nodal point of the stationary wave above 

 was observed. In locating this node enough points were 

 determined in its neighbourhood to give the form of the curve 

 on either side of the node. On account of an accident to 

 the apparatus, the air- wave had to be redetermined (Curve V.) . 

 Three thicknesses of paraffin were used, for which the corre- 

 sponding curves are numbered II., III., and IV. In the 



2 



