1896.] Generation of Longitudinal Waves in Ether. 271 



vibrations ; with but a very small proportion, practically evanescent, 

 of laminar waves. 



R R ~> 



■ N 



N 





N 





P 



P 







P - 







1 „.£?L,ii, 



n J 





n _ „J 



Let AA be a plane circular metal plate insulated within a metal 

 c ise CCC'C, as indicated in the drawing. Let D be a discharger 

 which can be pushed in so as to make contact with A. 



Let A be charged to begin with, positively for instance as indi- 

 cated by the letters PP^p ; ]$~Nnn showing negative electricity 

 induced by it. Let now the discharger be p ashed in till a spark 

 pisses. The result, as regards the space between AA and the roof 

 RR over it, will be either an instantaneous transmission of com- 

 mencement of diminution of electrostatic force, or a set of electric 

 waves of almost purely longitudinal displacement, according as 

 ether is incompressible or compressible. 



Hence, if the theory of longitudinal waves, suggested by Rontgen 

 as the explanation of his discovery (for the consideration of which he 

 has given strong reasons), be true, it would seem probable that a 

 sensitive photographic plate in the space between A A and RR should 

 be acted on, as sensitive plates are, by Rontgen rays. Either a 

 Wirnshurst electrical machine or an induction-coil, adapted to keep 

 incessantly charging AA. with great rapidity so as to cause an 

 exceedingly rapid succes.-ion of sparks between D and A, might give 

 a practical result. In trying for it, the light of the sparks at D must 

 be carefully screened to prevent general illumination of the interior 

 of the case and ordinary photographic action on the sensitive plate. 



The arrangement may be varied by making the roof of sheet 

 aluminium, perhaps about a millimetre thick, and. placing the sensi- 

 tive photographic plate, or phosphorescent substance, on the outside 

 of this roof, or in any convenient position above it. When a photo- 

 graphic plate is used there must of course, be an outer cover of metal 

 or of wood, to shut out all ordinary light from above. This arrange- 

 ment will allow the spark gap at D to be made wider and wider, 

 until in preference the sparks pass between AA and the aluminium 

 roof above it. The transparency of the aluminium for Rontgen light 

 will allow the photographic plate to be marked, if enough of this kind 

 of light is produced in the space between the roof and AA, whether 

 with or without sparks. 



The new photography has hitherto, so far as generally known, been 

 performed only by light obtained f rom electric action in vacuum ; but 



