246 Trowbridge — Triangulation by Cathode Photography. 



at first, destroyed by disruptive sparks over the surface of the 

 tube which apparently penetrated the glass between the plati- 

 num terminals and the glass. I have lately discovered, how- 

 ever, that if the terminals of the tube are placed in a vessel 

 filled with paraffine oil and if the oil is kept cool by an out- 

 side vessel filled with snow or ice, the entire energy developed 

 by the Tesla coil can be employed, and the tubes are not 

 destroyed. 



I have tried wooden lenses, both double convex and double 

 concave, in order to see whether the rays travel slower or faster 

 in wood than in air, but my results are negative. A copper 

 ring placed on a double convex lens of wood of approximately 

 six inches focus, and one also on a concave lens of the same 

 radius as the surfaces of the double convex lens, gave shadow 

 pictures of the ring which were of the same size and character 

 as those of an equal copper ring placed in air at the same dis- 

 tance from the sensitive plate. 



We naturally turn to Maxwell's great treatise on Electricity 

 and Magnetism, to see if a hint of this new phenomenon can- 

 not be found there : for I believe there is no manifestation of 

 electro-magnetism since the death of Maxwell which has not 

 been predicted or treated by him in one form or another in his 

 remarkable book. In section 792, vol. ii of the treatise on 

 Electricity and Magnetism he says, " Hence the combined effect 

 of the electrostatic and the electrokinetic stresses is a pressure 

 equal to 2p in the direction of the propagation of the wave. 

 Now 2p also expresses the whole energy in unit of volume. 

 Hence in a medium in which waves are propagated there is a 

 pressure in the direction normal to the waves, and numerically 

 equal to the energy in unit of volume. Thus, if in strong sun- 

 light the energy of the light which falls on one square foot is 

 83*4 foot-pounds per second, the mean energy in one cubic foot 

 of sunlight is about 0*0000000882 of a foot-pound, and the 

 mean pressure on a square foot is 0-0000000882 of a pound 

 weight. A flat body exposed to sunlight would experience 

 this pressure on its illuminated side only, and would therefore 

 be repelled from the side on which light falls. It is probable 

 that a much greater energy of radiation might be obtained by 

 means of the concentrated rays of the electric lamp. Such 

 rays falling on a thin metallic disc, delicately suspended in a 

 vacuum, might perhaps produce an observable mechanical 

 effect." 



Jefferson Physical Laboratory. 



