He worked out every detail of the theory splendidly, separately analysing the electric and the magnetic oscillation, using language not always such as we should use now, but himself growing in theoretic insight through the medium of what would have been to most physicists a confusing maze of troublesome facts, and disentangling all their main relations most harmoniously. _Holtz Machine, A and B Sparks; Glass and Quartz Panes in Screen._ While Hertz was observing sparks such as these, the primary or exciting spark and the secondary or excited one, he observed as a bye issue that the secondary spark occurred more easily if the light from the primary fell upon its knobs. He examined this new influence of light in many ways, and showed that although spark light and electric brush light were peculiarly effective, any source of light that gave very ultra-violet rays produced the same result. [Illustration: FIG. 7.--Experiment arranged to show effect on one spark of light from another. The =B= spark occurs more easily when it can see the =A= spark through the window, unless the window is glazed with glass. A quartz pane transmits the effect: glass cuts it off.] The above figure represents my way of showing the experiment. It will be observed that with this arrangement the =B= knobs are at the same potential up to the instant of the flash, and in that case the ultra-violet portion of the light of the =A= spark assists the occurrence of the =B= spark. But it is interesting to note what Elster and Geitel have found (_see_ Appendix IV., Fig. 59), that if the =B= knobs were subjected to steady strain instead of to impulsive rush--_e.g._, if they were connected to the inner coats of the jars instead of the outer coatings--that then the effect of ultra-violet light on either spark gap would exert a deterrent influence, so that the spark would probably occur at the other, or non-illuminated gap. With the 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 Righi 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. [Illustration: FIG. 8.--Zinc Rod in Arc Light, protected by Glass Screen. The lenses are of quartz, but there is no need for any lenses in this experiment; leakage of electricity begins directly the glass plate is withdrawn.] 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.