382 HERMANN VON HELMHOLTZ 



also elicited the interesting fact that a large resting surface, 

 emitting weak light, may disappear entirely in the intrinsic light 

 of the retina, while still sending out sufficient light to render 

 visible the moving objects which it illuminates. 



The further development of the theory of electricity on the 

 lines of the Faraday- Maxwell hypothesis had, as we have seen, 

 been handed over entirely by Helmholtz to his friend and pupil 

 Hertz. The latter wrote on November 30 to Helmholtz : 



' When you asked me in Berlin if I had made any further 

 experiments on electric waves, I had nothing important to tell 

 you, but I have now made a farther advance, by which the 

 relation between light and electricity seems to me to be firmly 

 established, and I am anxious to tell you about it. 



' In the first place, I discovered by a happy accident that it is 

 not only possible to produce waves several meters in length, 

 but that one can also work with much shorter waves, which 

 is infinitely more convenient. I have been able to confirm and 

 in part to improve on my earlier results with waves 33 cm. long 

 in air. I have also repeated the experiments with these short 

 waves of sending the force by means of concave mirrors to a 

 distance, and thus producing a beam, and with the best results. 



1 1 place my primary and secondary conductor in the focal line 

 of a parabolically curved tin-plate 2 m. high by 2 m. wide, and 

 then obtain a well-defined beam some ijm. in width from the 

 mirror, which is perceptible in a second concave mirror up to a 

 distance of 16 m., and apparently even farther. The beam can 

 be directed by rotation of the mirror, and one can demonstrate 

 rectilinear propagation and the formation of shadow perfectly 

 by its means. If, for example, a man crosses the path of the 

 beam, the stream of sparks in the induced mirror will entirely 

 disappear. Yesterday I also succeeded in showing the regular 

 reflection of the beam more plainly than I could have hoped. 

 When I put the concave mirrors side by side, there was no 

 effect from A to B ; but if a plane metal screen was placed in 

 front of the concave mirror, sparks passed in B which were still 

 perceptible when the screen was removed 10 m. from the 

 mirrors. I was also able to establish the reflection at 45, by 

 employing two adjacent rooms. Shutting the wooden doors 

 between them in no way hindered the appearance of the 

 secondary spark. On the other hand the sparks ceased when 



