198 HEKTZ'S RESEARCHES ON ELECTRICAL WAVES. 



be suspended over the tauk as shown in tlie figure. The tank was first 

 tried empty, but uufortunatelj' the wooden bottom was found to reflect, 



Fig. 5. 



thus it was useless for the x>«rpose intended. I then tried what ought 

 to have been tried before constructing the tank, namely — whether or- 

 dinary boards, such as flooring, reflected. The floor was found to 

 reflect readily. This was attributed to moisture in the wood causing it 

 to conduct, specially as wood was found not to polarize by reflection. 

 Experiments were then undertaken to determine if water reflected, even 

 though in thin sheets. A large glass window was placed beneath the 

 mirrors and flooded with water; this was found to reflect well, both 

 when the mirrors were in the position shown and when rotated to the 

 position "at right angles." Thus water also acts like a metal, reflect- 

 ing the radiation however polarized. The glass had to be hardly more 

 than damp to get some reflection. 



The wooden tank being unsuitable, a glass tank was thought of, but 

 was given up for solid paraffine, which, being in slabs, could be easily 

 built up into a vertical wall of any desired thickness. Through the 

 kindness of Mr. Rathborne a large quantity of this was lent for the pur- 

 pose. 



A thin sheet of paraffine about 2 centimeters thick was found not to 

 reflect, as was expected. Next a wall 13 centimeters thick (180 centi- 

 meters long, 120 centimeters high) was tried, and found to reflect, this 

 being the thickness required in order to add another half period to the 

 retardation of the wave reflected from the back at an incident angle of 

 55°, the wave-length being taken as GG centimeters, and the index of 

 refraction being taken as 1.51, the square root of 2.29, the value taken 

 as the specific inductive capacity of paraffine. 



Then a wall twice the thickness was tried, but it also reflected, con- 

 trary to expectation. While in doubt as to the cause of this, it was de- 

 cided to make a determination by direct experiment of the index of re- 

 fraction of paraffine for these waves, by a method suggested in Nature 

 (vol. XXXIX, p. 393), which consists in interposing a sheet or wall of par- 

 affine between the resonator and the metallic reflection in the Hertzian 

 experiment of loops and nodes which are formed by the interference of 

 the reflected wave with the direct radiation; the ratio of the velocity 

 in the wall to that in the air being easily found from the observed shift- 

 ing of the loops and nodes towards the screen. 



