Further, the points to be measured must be well illuminated. As a suitable 
instrument the Leitz Ultropak incident light microscope was chosen. The total 
enlargement was 20x. The principles of the illumination are demonstrated in 
Fig. 3 In order to increase the accuracy of the reading of the elevation of the 
microscope a measuring gauge was applied to the microscope. See Fig. 4 The 
microscope is carried by a pillar stand which is supported by a surface plate. 
Consequently elevation differences can be measured after adjusting the micro- 
scope in the actual points. On each point at least three settings should be done 
and recorded. Evidently the accuracy of the supporting surface plate and the 
stability of same will have a great influence upon the accuracy of the measured 
elevation differences. 
  
  
  
  
  
  
  
  
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Fic. 3. Use of Microscope. 
If the surface, the flatness of which is to be determined, is transparent it is 
suitable to place it upon a surface plate and to measure the elevation differences 
between points in the surface and in the underlying plate. This procedure is very 
useful when the flatness of glass plates, in particular exposed and developed photo- 
graphic plates, is to be determined. The settings in points of the emulsion and of 
the surface plate can generally be done with high accuracy. In this procedure 
evidently the plate which carries the pillar stand will not have any influence upon 
the accuracy of the determination of the elevation differences. The accuracy of 
the microscope method can be determined as described above. 
From a great number of repeated settings in one and the same point the 
standard error of the individual settings was found to be of the magnitude 3 microns. 
From measurements of the elevations of a great number of points in thc surface 
273