HKRTZ'S KESEAKCIIES ON ELECTRICAL WAVES. 180 



indications of reflection from each of the walls. He was also able to 

 obtain distinct evidence of reflection from one of the iron columns in the 

 room, and of the existence of electro dynamic shadows on the side of 

 the column remote from the primary. 



Jn the preceding experiments the secondary conductor was always 

 placed between the wall and the primary conductor; — that is to say, in 

 a space in which the direct and reflected rays were travelliug" in oppo- 

 site directions, and gave rise to stationary waves by their iuterference. 



He next placed the primary conductor between the wall and the sec- 

 ondary, so that the latter was in a space in which the direct and re- 

 flected waves were traveling in the same direction. This would neces- 

 sarily give rise to a resultant wave, the intensity of which would depend 

 on the difference in phase of the two interfering, waves. In order to 

 obtain distinct results it was necessary tbat the two waves should be 

 of approximately equal intensities, and therefore the distance of the 

 primary from the wall had to be small in comparison with the extent of 

 the latter, and also in comparison with its distance from the secondary. 



To fulfill these conditions the secondary was placed at a disadvantage 

 of 14 meters from the reflecting wall, and therefore about 1 meter 

 from the opposite one, with its plane in the plane of vibration, and its 

 air space directed towards the nearest wall, in order to make the con- 

 ditions as favorable as possible for the production of sparks. The 

 primary was placed parallel to its former position, and at a perpendic- 

 ular distance of about 30 centimeters from the center of the reflecting 

 metallic plate. The sparks observed in the secondary were then very 

 feeble, and the air sj)ace was increased until they disaj)peared. The 

 primary conductor was then gradually moved away from the wall, when 

 isolated sparks were soon observed in the secondary, passiug into a 

 continuous stream when the primary was between 1.6 and 2 meters from 

 the wall; — that is, at the point B. This might have been supposed to 

 be due to the decrease in the distance between the two conductors, ex- 

 cept that as the primary conductor was moved still further from the 

 wall the sparking again diminished, and disappeared when the primary 

 was at the point C. After passing this point the sparking continually 

 increased as the primary approached nearer to the secondary. These 

 experiments were found to be easy to repeat with smaller apparatus, and 

 the results obtained confirmed the former conclusion, that the position 

 of the nodes depeuds only on the dimensions of the conductor, and not 

 on the material of the reflecting wall. 



Dr. Hertz points out that these phenomena, which are exactly anal- 

 ogous to the acoustical experiment of approaching a vibratiug tuning- 

 fork to a wall, when the sound is weakened in certain positions and 

 strengthened in others, and also to the optical phenomena illustrated in 

 Lloyd's form of Fresuel's mirror experiments; and as these are accepted as 

 arguments tending to prove that sound and lightare due to vibration, hif| 



