294 Prof. Hertz on Rays of Electric Force. 



consists in transverse vibrations, and can be " plane-polarized " 

 in the optical sense. This fact, however, may also be shown 

 by experiment. If the receiving mirror be rotated round the 

 ray as axis until its focal line, and consequently also the 

 secondary conductor, have attained the horizontal position, 

 the secondary sparks disappear gradually, and no sparks are 

 to be obtained when the two focal lines are at right angles, 

 even if the two mirrors are made to approach each other 

 closely. The two mirrors behave like the polarizer and 

 analyzer of a polarizing apparatus. I had made an octagonal 

 frame of wood 2 metres high and 2 metres broad, and wound 

 copper of 1 millim. thick wire over this, so that the wires 

 were all parallel and 3 centim. from each other. If now 

 the two mirrors were arranged with parallel focal lines, 

 and the wire screen was moved about in the ray at right 

 angles to its direction so that the wires were at right angles 

 to the plane of the focal lines, the screen produced practically 

 no effect upon the secondary sparks ; but if the screen was 

 placed so that its wires were parallel to the plane of the focal 

 lines, the ray disappeared altogether. Thus, in regard to the 

 transmitted energy, the screen behaves to the ray exactly 

 like a tourmaline plate to a plane-polarized ray. The focal 

 line of the receiving mirror was then placed horizontal, in 

 which position, as already mentioned, no sparks were ob- 

 tained, nor were sparks produced by the introduction of the 

 screen into the ray, so long as its wires were either vertical or 

 horizontal. But if the wire frame was placed in either of 

 the two possible positions so that its wires made an angle of 

 45° with the horizontal line, the introduction of the screen 

 caused the production of sparks. Evidently the screen pro- 

 duces the resolution of the vibrations into two components, 

 and transmits only that vibration which takes place at right 

 angles to the direction of the wires. This component is 

 inclined at an angle of 45° to the focal line of the second 

 mirror, and when once more resolved by the mirror produces 

 the action upon the secondary conductor. The phenomenon 

 is exactly similar to the brightening produced in the dark 

 field of two crossed nicols by a tourmaline plate introduced 

 in the proper position. 



The following remarks upon the subject of polarization may 

 be permitted. With the means employed in the present in- 

 vestigation we are only able to recognize electric force. Its 

 vibrations undoubtedly take place, for a vertical position of 

 the primary vibration, in the vertical plane passing through 

 the ray, and are wanting in the horizontal plane. But from 

 the phenomena we observe with slowly changing currents we 



