Prof. Hertz on Rays of Electric Force. 297 



part not transmitted is absorbed by the tourmaline plate but 

 reflected by our surface. If in the last experiment the focal 

 lines of the two mirrors are crossed, we can obtain no sparks 

 in the secondary conductor by reflexion at an isotropic sur- 

 face ; but I convinced myself that the experiment succeeds 

 with reflexion at an anisotropic wire screen, if so placed that 

 its wires are inclined at an angle of 45° to both focal lines. 

 The experiment is easily understood from what has been said. 



Refraction. 



In order to try if a bending of the ray could be observed 

 upon its passing from air into another insulating medium, I 

 had constructed a large prism of so-called hard pitch, an 

 asphalt-like material. The base was an equilateral triangle 

 1*2 metre in the side, and with a refracting angle of about 

 30°. The height of the prism with its refracting edge 

 placed vertical was about 1'5 metre. 



But since the prism weighed about 12 cwt. and altogether 

 was inconveniently heavy, it was constructed in three pieces, 

 each about 0"5 metres high, placed one on the other. The 

 mass was cast in wooden boxes, which, since they would have 

 no prejudicial effect, were left round the mass. The prism 

 was placed in a support at such a height that the centre of its 

 refracting edge was at the same height as the primary and 

 secondary spark-intervals. After I had convinced myself 

 that refraction did occur, and had formed an opinion as to its 

 amount, I arranged the experiment as follows: — The produ- 

 cing mirror was placed at a distance of 2*6 metres from the 

 prism, turned towards the one refracting surface, so that the 

 axis of the ray passed through the centre of gravity of the 

 prism and intersected the refracting surface at an angle of 

 65°. Two conducting-screens were placed near the refracting 

 edge of the prism and near the opposite side, which cut off" 

 from the ray every other part than that through the prism. 

 On the side of the emerging ray a circle was traced upon the 

 floor of 2*5 metres radius, with the centre of gravity of the 

 prism as centre. The receiving mirror was moved about 

 upon this, so that its aperture was always directed towards 

 the centre of the circle. If the mirror were placed in 

 the line of the incident ray, no sparks were to be obtained 

 in it; in this direction the prism threw a perfect shadow. 

 Sparks, however, appeared when the mirror was moved 

 towards the second surface of the prism, and first when the 

 angular displacement was about 11°. The sparks increased 

 in intensity up to a deviation of about 22° and then decreased 

 again, becoming imperceptible at about 34°. If the mirror 



