Hie Study of the Properties of Electric Waves. 57 



1). Phenomena of interference. 



1. Determination of the wave-length by curved 



gratings. 



2. Bi-prism experiments. 



E. Double refraction and polarization. 



1. 



Polarizing gratings. 



2. 



„ crystals. 



3. 



Double refraction produced by crystals. 



4. 



„ „ other substances, 



5. 



,, „ by strain. 



6. 



Circular polarization. 



7. 



Magnetic rotation. 



8. 



Electro-polariscope and polari meter. 



In the list of experiments above-mentioned, the determina- 

 tion of the wave-length by curved gratings has been carried 

 out w T ith a larger apparatus (vide Proc. Roy. Soc. vol. lx., 

 " On the Determination of the Wave-length of Electric 

 Radiation""). Experiments with circular polarization and 

 magnetic rotation and with the bi-prism are still in progress. 

 All the others have been repeated with the apparatus to be 

 described below. 



The complete apparatus consists of : — (1) A radiating 

 apparatus emitting electric waves of short length ; (2) A 

 receiver used as a detector of electric radiation; and 

 (3) Various accessories for the study of the different phe- 

 nomena. 



I used various methods for the production of oscillatory 

 discharge. One method was to imbed a row of metallic 

 beads, with small spark spaces, in solid paraffin, the end beads 

 being in connexion with the electric generator. Another 

 method was to have the two sparking-balls immersed in kero- 

 sene ; this is effective, but troublesome. The simplest method, 

 however, is Prof. Lodge's arrangement of two side balls 

 and an interposed sphere. 



Electric oscillation is produced by sparking between two 

 beads of platinum and an interposed sphere of the same 

 metal. The discharge ceases to be oscillatory when the 

 ball is roughened, and a platinum ball resists, to a great 

 extent, the disintegrating action of the sparks. Two jointed 

 electrodes carry the two beads at their ends. The distance 

 between the beads and the interposed sphere can thus be 

 adjusted. This is a matter of importance, as the receiver 

 does not properly respond if the spark length is too large. 

 It is more convenient to use short electric waves, and these are 



