GV—48 PHOTOGRAMMETRIC ENGINEERING 
  
Fic. 2. Further examples of typical electron micrographs made from shadowed replicas. 
A. Negative collodion replica showing the calcium fluoride cyrstals which formed on the outer 
enamel surface of a tooth immersed for fifteen days in a sodium fluoride solution. Micro- 
graphs of this type must be carefully oriented for examination, so that elevations and de- 
pressions are seen in their proper perspective, If this picture is rotated through 180 degrees 
the detail appears to be inverted. 11,000 X. 
B. Positive carbon replica of the surface of a planed piece of walnut wood, showing the parallel 
arrangement of the constituent fibers. 5,000 X. 
C. Positive carbon replica of a glazed ceramic building tile, showing the small pits which char- 
acterize the surface. 5,500 X. 
D. Positive carbon replica of the surface of a block of plaster of paris, showing the irregularly 
overlapped arrangement of the component crystals. 8,000 X.