Smithsonian Report, 1936.— Conger 



Plate 12 



Upper, Photomicrograph of surface of smooth plastered wall. This surface echoes sound readily 

 i^ower, Photomicrograph of wall surface covered with diatomaceous earth. This surface catches "sound 



and stops echoes. 

 Magnification, X 100. Both surfaces would look smooth and white to the naked eve 



