FINGERPRINTS TRAP “SLUG” PASSERS (Jul, 1937)
That seems like an awful lot of work just to catch someone stealing a free subway ride…
FINGERPRINTS TRAP “SLUG” PASSERS
To trap persons who insert worthless metal slugs in turnstiles, slot machines, and coin telephones, an ultra-violet-ray fingerprinting process has been developed.
Since the conventional “powder” method of bringing out latent prints is unsatisfactory for photographing specimens found on metal surfaces, the print is “fixed” with a chemical reagent, and the slug is then soaked in a dye that becomes luminous under ultra-violet light. Placed in a special viewing apparatus, the slug background glows under the invisible light, while the fingerprint stands out in black lines that can be photographed to make a permanent record for checking against prints of suspected persons.