Prof. Hertz on Bays of Electric Force. 295 



cannot doubt that the electric vibrations are accompanied by- 

 vibrations of magnetic force, which take place in the hori- 

 zontal plane and become zero in the vertical plane. The 

 question in which of the two planes the vibrations of our ray- 

 take place does not, then, admit of an answer without data to 

 determine whether the question is of electric or of magnetic 

 vibrations. That the failure to decide an old optical dispute 

 is explained by this consideration was first clearly pointed 

 out by Herr Kolacek*. 



Re/ieaaion. 



We have already proved the reflexion of the wave at a con- 

 ducting surface by the interference 'of the reflected and direct 

 waves, and have made use of it in the construction of our 

 concave mirrors. But it is now possible for us to separate 

 the two wave-systems. I placed the two concave mirrors 

 in an open space side by side, so that their apertures faced 

 the same way, and that their axes converged upon a point 

 about 3 metres off". The spark-space of the receiving mirror 

 of course remained dark. Now I placed a vertical plane 

 wall of thin zinc plate 2 metres high and 2 metres broad at 

 the intersection of the axes, so as to be equally inclined to 

 them both. I then obtained a long stream of sparks result- 

 ing from the reflected rays. The stream of sparks ceased as 

 soon as the wall was turned about a vertical axis either way 

 through about 15° from the correct position ; hence the re- 

 flexion is regular, and not diffuse. If the wall was removed 

 from the mirrors, their axes being made to converge always 

 upon the wall, the sparks diminished very slowly. I was stiU 

 able to recognize sparks when the wall was distant 10 metres 

 from the mirrors ; the waves had then to traverse a path of 

 20 metres. This arrangement might be employed with ad- 

 vantage if it were desired to compare the velocity of radiation 

 in the air with other slower velocities of propagation, e. g. 

 that by means of a cord. 



In order to obtain the reflexion of the ray at incidences 

 other than normal, I arranged the ray parallel to a wall in 

 w^hich there was a door, and in the neighbouring room to 

 which the door led I placed the receiving mirror so that its 

 optic axis traversed the centre of the door and cut the direc- 

 tion of the ray at right angles. If, now, the plane conducting 

 screen was placed vertically at the point of intersection so as 

 to make angles of 45° both with the ray and with the axes of 

 the receiving mirror, a stream of sparks was produced in the 



* Wiedemann's Annalen, xxxiv. p, 676. 



