Popular Science, syyskuu 1927
Invisible rays that read secret writing, test food, and detect forgeries issue from a remarkable new type of lamp developed by a manufacturer in Hanau, Germany. Made of quartz, instead of glass, to permit the passage of ultra-violet light, the lamp is equipped with a filter, which absorbs all the visible rays of the spectrum. When its powerful "black light" falls on nay one of a huge class of substances, the substance "tells its sotry" by giving off a deep purple, green, or brilliant orange light.
Paper, for instance, glows with different colors under ultra-violet light, depending on how, and of what, it is made. The genuineness of bank notes or rare postage stamps is detected by comparing under the rays the ones in doubt with others known to be authentic. If one turns yellow, say, and the other blue, forgery is proved. Secret writing and erasures, too, glare out strikingly.
Artificial tannin, sometimes used to adulterate the vegetable tannin of the leather and dyeing industries, is revealed in tests devised by Professor O. Grengross of the Charlottenburg Engineering College, by its brilliant glowing light under the rays. The natural tannin does now glow.
Examination of food by the new lamp has been conducted by Dr. Popp, legal chemist at Frankfort-on-Main. A bluish glow shows the amount of tendons in a sample of sausage, and decay is revealed by a violet shine. Natural fruit juices identify themselves. Wool shows a different fluorescence than cotton or silk.