196 hertz's researches on electrical waves. 



these mirrors, whether the "vibrator" was j^erpendicular to the plane 

 of reflection or in it. But with a wall 3 feet thick reflection was ob- 

 tained, when the " vibrator " was perpendicular to the plane of reflec- 

 tion; but none, at least at the polarizing angle,* when turned through 

 90° so as to be in it. 



This decides the point in question, the magnetic disturbance being 

 found to be in the plane of polarization, the electric at right angles. 

 Why the glass did not reflect was probably due to its thinness, the re- 

 flectiou from the front interfering with that from the back, this latter 

 losing half a wave-length in reflection at a surface between a dense and 

 a rare medium ; and, as Mr. Joly pointed out, is in that case like the 

 black spot in Newton's rings, or more exactly so, the black seen in very 

 thin soap-bubbles. 



Fig. 4. 



Hertz has pointed out several important things to be guarded 

 against in making these experiments* Ultra-violet light, for exam- 

 ple, falling on the "vibrator," prevents it working properly, the spark- 

 ing in the resonator ceasing or becoming poor. Also the knobs of the 

 " vibrator" must be cleaned of burnt metal, and polished every quarter 

 of an hour at least, to prevent a like result. 



JBoth these effects probably arise, as suggested by Mr. Fitzgerald, 

 from a sort of initial brush discharging (either ultra-violet light or 

 points being capable of doing this), which prevents the discharging im- 

 pulse being sufliciently sudden to start the oscillation in the " vibra- 

 tor." For to start a vibration, the time of impulse must be short com- 

 pared with the time of oscillation. These precautions therefore become 

 especially needful when working with small-sized " vibrators." Possibly 

 charging the "vibrator" very suddenly, after the manner of one of 

 Dr. Lodge's anti-lightning-rod experiments, would save the irksome 

 necessity of repeatedly cleaning the knobs of the " vibrator." 



Several important problems seem to be quite within reach of solution 

 by means of these Hertziau waves, such for instance as dispersion. 

 Thus it could be tried whether placing between the reflector and the 

 "resonatoi" conducting bodies of nearly the same period of vibration 

 as the waves used would necessitate the position of the "resonator" 



* Slight reflectiou was obtained at an incidence of 70°. 



