hertz's researches on electrical waves. 203 



suit tbe walls tliau to change the thickness of the walls. Thus, the 

 mirrors were pnt at 25°, which is the proper angle with the above data 

 for 10 centimeter and 20 centimeter walls. On now repeating the ex- 

 periment, better results weie obtained than I should have anticipated. 

 When the wall was 10 centimeters thick, continuous sparking was 

 easily obtained, but when 20 centimeters thick, it was onl}- after much 

 adjustment and i)atience that perhaps one slight spark could be ob- 

 tained. This was quite sufficient, considering the nature of the wall, for 

 it was only built up of plates, which afforded internal reflections, weak- 

 ening the transmitted rays, and also since it requires the sum of the 

 effect arising from the multiple reflections back and forward inside the 

 wall to completely interfere with the front, and some of these are lost 

 at the edge of the beam. 



