296 Prof. Hertz on Rays of Electric Force, 



secondary conductor which also was not interfered with by 

 the closing of the door. If the reflecting screen was turned 

 through about 10° from the right position, the sparks ceased. 

 The reflexion is therefore regular, and the angles of incidence 

 and reflexion are equal. That the action was transmitted 

 from the source to the plane mirror and from there to the 

 secondary conductor could also be shown by interposing 

 shadow-giving screens at different points of this path, when 

 the secondary sparks at once ceased, whilst the screen could 

 be placed anywhere else in the room without effect. With 

 the aid of the circular secondary conductor it is possible to 

 determine the position of the wave-surface in the ray; this 

 was at right angles to the ray, both before and after reflexion, 

 so that in the reflexion it suffers a deviation of 90°. 



So far the focal lines of the concave mirrors have been 

 vertical, and the plane of vibration consequently at right angles 

 to the plane of incidence. In order to produce reflexion 

 with the vibrations in the plane of incidence, I adjusted the 

 focal lines of both concave mirrors in the horizontal plane. 

 I observed the same phenomena as in the former case, and 

 was moreover not able to perceive a difference in the intensity 

 of the reflected rays in the two cases. If, on the other hand, 

 the focal line of the one mirror is vertical and that of the 

 other horizontal, no secondary sparks are to be perceived. 

 The inclination of the plane of vibration to the plane of inci- 

 dence is therefore not altered by reflexion so long as this 

 inclination has one of the values mentioned ; but this state- 

 ment cannot be taken as generally true. It may even be 

 . considered as open to doubu whether generally the ray after 

 reflexion is plane-polarized. The interferences which the 

 intersecting systems of waves produce before the mirror and 

 which, as I observed, give rise to characteristic phenomena in 

 the circular conductor, may possibly lead to conclusions in the 

 current problems of the optician as to change of phase and 

 amplitude upon reflexion. 



I will mention one other experiment on reflexion by an 

 isotropic surface. The two curved mirrors were again placed 

 side by side as in the experiment on reflexion first described, 

 and opposite to them as a reflecting wall the screen, constructed 

 of parallel copper wires, was placed. 



It was seen that the secondary spark-interval remained 

 dark when the wires intersected the direction of the vibrations 

 at right angles, but became brighter as soon as the wires coin- 

 cided with the direction of vibration. The analogy between 

 our surface conducting in one direction and the tourmaline plate 

 is therefore limited to the transmitted portion of the ray. The 



