Smithsonian Report, 1939. — Hoover 



Plate 4 



•^"Y'* ^*4«'" 





mn^mi i i i i i]i| iiii r i On i i i^iiii ii io 





.4.„. 



^•ViTMMVMit 



1. Ordinary Photograph of an Obliterated Return Address. 



2. Photograph of Same Return Address Using Optical Filters and 

 Photographic Plates Sensitized to infrared Radiation. 



