Nikola Tesla Books
Along with comments on photograph No. 55, Tesla displayed other possible ways of producing bright spots on streamers. He assumed that along current streamers acertain kind of moving standing wave is produced which is made by means of two (or more) waves of different frequencies. He even displayed how two waves of adjacent frequencies could produce parameter variations of the oscillator constants at various operating conitions. Tesla recalls some previous experiments and does not give sufficient explanations to allow the understanding of it all.
Continuing his thinking, he described the future experiments with "tamed'' current streamers in a big glass tube, which would prove the existence of bright spots on the current streamers, along which two waves of close frequencies propagate.