328 



Professor Oliver Lodge 



[June I, 



gap. With these altered connections it is, of course, not feasible to 

 illuminate one spark by the light of the other ; the sparks are then 

 alternative, not successive. 



Wiedemann and Ebert, and a number of experimenters, have 

 repeated and extended this discovery, proving that it is the cathode 

 knob on which illumination takes effect; and Hallwachs and Eighi 

 independently made the important observation, which Elster and 

 Geitel, Stoletow, Branly and others have extended, that a freshly- 

 polished zinc or other oxidisable surface, if charged negatively, is 

 gradually discharged by ultra-violet light. 



It is easy to fail in reproducing this experimental result if the 

 right conditions are not satisfied ; but if they are it is absurdly easy, 

 and the thing might have been observed nearly a century ago, 



Fig. 8. 



Zinc Knob in Arc Light, protected by Glass Screen. The lenses arc 

 of quartz, but there is no need for any lenses in this experiment ; 

 leakage begins directly the glass plate is withdrawn. 



Take a piece of zinc, clean it with emery paper, connect it to a 

 gold leaf electroscope, and expose it to an arc lamp, Fig. 8. If 

 charged positively nothing appears to happen, the action is very slow ; 

 but a negative charge leaks away in a few seconds if the light is bright. 

 Any source of light rich in ultra-violet rays will do; the light 

 from a spark is perhaps most powerful of all. A pane of glass cuts 

 off all the action ; so does atmosj)heric air in sufficient thickness (at 

 any rate, town air), hence sunlight is not powerful. A pane of quartz 

 transmits the action almost undiminished, but fluor-spar may be more 

 transparent still. Condensing the arc rays with a quartz lens and 

 analysing them with a quartz prism or reflection grating, we find 

 that the most effective part of the light is high up in the ultra- 



