This book contains previously unavailable material on the pioneering work of Nikola Tesla in field of radio frequency electrical engineering. While first delivered under the title On the Streams of Lenard and Roentgen with Novel Apparatus for Their Use the information presented in the lecture goes far beyond this topic. In addition to his opening remarks on X-ray discovery, a major portion of Tesla's commentary deals with the high frequency resonators that were used in conjunction with his work, plus clear descriptions of stroboscopic instruments he designed for measurement of frequency and phase. Other topics addressed include wireless receiving methods and the genesis of Tesla's 1937 particle beam tube. During the talk Tesla had displayed approximately 120 drawings of specially constructed vacuum tubes, many being of the Lenard type and also the single-electrode type of his own design. Among the drawings were also tubes used in his wireless communications experiments. Enhanced photographs of these images are among the 32 illustrations which fill out this fine volume.