TheCodeBreakers
Letters were also checked by
infrared and ultraviolet light. Writing in starch, invisible in daylight or
under electric light, will fluoresce under ultraviolet. Infrared can
differentiate colors indistinguishable in ordinary light and so can pick
up, for example, green writing on a green postage stamp. The censorship
field stations tested all suspicious letters and a percentage of ordinary
mail picked at random, and sometimes all letters to and from a certain
286 THB CUJJJiBKliAKERS
city for a week to see if anything suspicious turned up. During the
war, about 4,600 suspicious letters were passed along to the F.B.I, and
other investigative agencies; of these 400 proved to be of some
importance.
Problems that would not yield to the crude approach of the field
stations went back to the T.O.D. laboratory. Here, amid Bunsen burners
and retorts, Pierce and Breon, aided by an expert photographer and
laboratory technicians, cooked up reagents that would reincarnate the
annaabi.ee 
