Prof. Hertz on Rays of Electric Force. 293 



the space at my disposal I could recognize the sparks from 

 one end to the other. The greatest distance to which — by 

 making use of an open door — I could trace the ray was 16 

 metres ; but the results of the experiments on reflexion, to 

 be described immediately, leave no doubt that in open spaces 

 sparks could be obtained up to at least 20 metres. For the 

 remaining experiments so great a distance is not necessary, 

 and it is more convenient if the secondary spark is not too 

 weak ; a distance of 6-10 metres is the most advantageous for 

 most experiments. We will now describe the simple experi- 

 ments which may be made with the ray without difficulty. 

 When the opposite is not expressly remarked, the focal lines 

 of both mirrors are to be supposed vertical. 



Rectilinear Radiation. 



If a screen of zinc plate 2 metres high and 1 metre broad 

 be placed in the straight line joining the two mirrors at right 

 angles to the direction of the ray, the secondary sparks com- 

 pletely disappear. A screen of tin-foil or of gilt paper gives 

 an equally perfect shadow. A frame placed across the ray 

 leaves the secondary spark-interval dark whenever it intercepts 

 the ray, and permits the sparks to appear whenever it allows tbe 

 ray to pass. Insulators do not intercept the ray — it passes 

 through a screen of wood or through a wooden door, and it 

 is not without surprise that we see the sparks produced inside 

 a closed room. If two conducting screens, each 2 metres 

 high and 1 metre broad, are placed diametrically right and 

 left near the ray and at right angles to its direction, they 

 do not influence the secondary sparks at all, so long as the 

 breadth of the slit which they leave between them is not 

 smaller than the aperture of the mirror, that is 1*2 metres. 

 If the slit is made narrower than this, the sparks diminish 

 and disappear when the breadth of the slit becomes less than 

 0'5 metre. If the breadth of the slit be left 1*2 metres, but 

 the screens be moved sideways out of the direct line joining 

 the two mirrors, the sparks also disappear. If the producing 

 mirror be turned right or left, through about 10° out of the 

 correct position, the sparks become weaker, and when turned 

 through 15° the sparks disappear. 



The ray has no geometrically sharp limit or shadow, and it 

 is easy to obtain phenomena due to refraction, but I have not 

 yet succeeded in observing maxima and minima at the edge 

 of a shadow. 



Polarization. 

 The mode of production of our ray leaves no doubt that it 



