60 Dr. Howard on Electric Radiation, 



In making the experiments the lenses were placed one at 

 each end of a wooden table 2J metres long, with their plane 

 surfaces turned towards each other, and as nearly as 

 possible parallel. The distance between them was 180 centim., 

 and remained the same throughout the experiments. On 

 one side of the table close to the edges of the lenses was a 

 brick wall about 40 centim. thick ; and on the other side was 

 a residue of gangway 54 centim. wide between the lens and a 

 laboratory-apparatus cupboard, which has had to be set up in 

 the corridor for want of space elsewhere. The oscillator 

 stood, together with its exciting coil, on a small table whose 

 height was adjustable ; the plane of its disks was parallel to 

 the flat surfaces of the lenses in all cases. It was intended to 

 be placed in the focal line of the first lens ; but apparently 

 the index of refraction had been assumed too high, and a 

 position 51 centim. from the vertex of the lens seemed to do 

 best. We shall speak of the vertical plane through the focal 

 lines of the two lenses as the " axial planed It contains the 

 axes of the lenses and of the oscillator. Waves seem to be 

 emitted more powerfully in this plane normal to the disks of 

 the oscillator than in the plane containing them. 



The direct effect from the oscillator could be perceived by 

 the resonator at a distance of 120 centim. in the axial plane 

 in the most favourable case ; that is to say, in a very dark 

 room and just after cleaning the knobs of the oscillator. 

 Under similar circumstances resonance was only just obtain- 

 able at the vertical edge of the first lens, viz. 85 centim. from 

 the oscillator in a direction making an angle of about 30° with 

 the axial plane. To get a rough measure of the intensity of the 

 radiation at any point, the resonator was placed there, and its 

 spark-gap arranged so as to just give a continuous stream of 

 sparks ; it was then brought to the line joining the oscillator 

 and the edge of the first lens (line of reference), and the dis- 

 tance from the oscillator observed at which the sparks ceased 

 to be continuous. When the intensity of the radiation was 

 very small, however, the converse of this method was adopted; 

 the resonator was adjusted at the line of reference and then 

 taken to the point at which the intensity was to be observed. 



The following are the phenomena observed in the space 

 between the two lenses when the oscillator coincides with the 

 focal line of the first one. The resonator gives brilliant 

 sparks in the axial plane near the first lens so long as it 

 is held parallel to the oscillator. On rotating it in a plane 

 perpendicular to the axial plane the sparks decrease^ »in 

 brilliancy and length, and become entirely obliterated when 

 the resonator and oscillator are at right angles. If the rota- 



