On that evening in January, 1902, a curious crowd was gathered about the entrance of the Engineers' Club in New York City. Over the doorway a narrow glass tube gleamed with a strange blue-green light of such intensity that print was easily readable across the street, and yet so softly radiant that one could look directly at it without the sensation of blinding discomfort which accompanies nearly all brilliant artificial lights. The hall within, where Mr. Hewitt was making the first public announcement of his discovery, was also illuminated by the wonderful new tubes. The light was different from anything ever seen before, grateful to the eyes, much like daylight, only giving the face a curious, pale-green, unearthly appearance. The cause of this phenomenon was soon evident; the tubes were seen to give forth all the rays except red--orange, yellow, green, blue, violet--so that under its illumination the room and the street without, the faces of the spectators, the clothing of the women lost all their shades of red; indeed, changing the very face of the world to a pale green-blue. It was a redless light. The extraordinary appearance of this lamp and its profound significance as a scientific discovery at once awakened a wide public interest, especially among electricians who best understood its importance. Here was an entirely new sort of electric light. The familiar incandescent lamp, the invention of Thomas A. Edison, though the best of all methods of illumination, is also the most expensive. Mr. Hewitt's lamp, though not yet adapted to all the purposes served by the Edison lamp, on account of its peculiar colour, produces eight times as much light with the same amount of power. It is also practically indestructible, there being no filament to burn out; and it requires no special wiring. By means of this invention electricity, instead of being the most costly means of illumination, becomes the cheapest--cheaper even than kerosene. No further explanation than this is necessary to show the enormous importance of this invention. Mr. Hewitt's announcement at once awakened the interest of the entire scientific world and made the inventor famous, and yet it was only the forerunner of two other inventions equally important. Once discover a master-key and it often unlocks many doors. Tracing out the principles involved in his new lamp, Mr. Hewitt invented: A new, cheap, and simple method of converting alternating electrical currents into direct currents. An electrical interrupter or valve, in many respects the most wonderful of the three inventions. Before entering upon an explanation of these discoveries, which, though seemingly difficult and technical, are really simple and easily understandable, it will be interesting to know something of Mr. Hewitt and his methods of work and the genesis of the inventions.