National Geographic : 1939 Nov
THE NATIONAL GEOGRAPHIC MAGAZINE TESTING WHISTLES IN SINGING KETTLES IS THIS GIRL'S SOFT JOB Through a tube, she feeds air pressure into the aluminum kettle, with a whistle over its spout. If no pleasant sound issues, or if the result is cacophonic, a new whistle is put on. H(Q) National IUeographic society KJodachromes by Willard K. Lulver NOW INVISIBLE ULTRAVIOLET RAYS ARE CHANGED INTO COLORED LIGHTS These new fluorescent lamps give 50 to 200 times as much colored light, per unit of energy used, as do ordinary incandescent lamps. A "white fluorescent" permits the closest approach to natural daylight ever invented.