62 Dr. Howard on Electric Radiation. 



second lens and its focal line is of almost uniform intensity in 

 the neighbourhood of the axial plane. At the edge of the 

 cone the intensity falls off very rapidly ; and if the resonator 

 be moved parallel to itself in a plane perpendicular to the 

 axial plane, it shows sharply, by the commencement and 

 stoppage of its sparking, where the boundaries of the cone 

 lie. The cone is a little unsymmetrical on account of dis- 

 turbance at the side nearest the wall, but the convergence 

 of the rays to a focus is placed beyond a doubt. 



The following observations were made on the rays after 

 they had passed the focus of the second lens. The resonator, 

 after having been set to spark at a distance of 80 centim. 

 from the oscillator in the line of reference, was taken to the 

 focus of the second lens, and there gave sparks of fair 

 intensity. Beyond the focus there were traces of a divergence 

 of the cone of rays, which became more evident when the 

 oscillator knobs were quite clean : but in order to make sure 

 of the existence of this divergence a more sensitive resonator 

 would be necessary. In the axial plane itself the resonator 

 used by us gave an effect when the conditions were most 

 favourable, at a distance of 120 centim. beyond the focus of 

 the second lens ; and it would possibly have given an effect 

 still further away, had there not been an iron hot-water pipe 

 9 ♦ centim. in diameter running from floor to ceiling of the 

 passage near this point. The furthest point at which any 

 traces of sparking could be found was in one case 450 centim. 

 from the oscillator, while without the lenses it was only 

 120 centim. This statement has to be taken along with the 

 fact that the lenses were only 180 centim. apart, and that no 

 attempt was made to elongate the parallel portion of the beam 

 by increasing their distance. 



In order to determine experimentally the wave-length of 

 the oscillations, a sheet of tin-plate was set up against the flat 

 (inner) surface of the second lens. The rays reflected from 

 this plate were thus made to interfere with those incident on 

 it so as to give stationary waves, as in some experiments of 

 Hertz. The result was that close to the plate there were 

 no traces of sparking. On taking the resonator further 

 away, however, the sparks appeared, reached a maximum, 

 and then disappeared again at a distance of 50 centim. from 

 the plate. The point of disappearance was very definite. 

 The sparks appeared again when the resonator was still 

 further withdrawn, and as long as it was kept parallel to the 

 oscillator no further disappearance of the sparks could be 

 observed. By rotating it, however, in the axial plane, a 



