230 INDUSTRIAL APPLICATIONS OF PHASE MICROSCOPY 



6. GLASS AND PLASTICS 



Glass surfaces have proved one of the most interesting and difficult 

 microscopical specimens. However, slight differences in thickness from 

 minute surface irregularities change the phase of light passing through 

 them and may therefore be seen with the phase microscope. Loos et al. 

 (194:1) reported phase useful for the study of the polishing of glass 

 surfaces. Circular polishing marks and stains not visible with other 





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Fig. VI.3. Fine ground glass, 400 X. Left, brightfield. Center, dark-contrast 

 (B — ) phase. Right, dark-contrast (A — ) phase. 



microscopical methods were photographed and described by Bennett 

 et al. (1946), and Lyot and Frangon (1948) examined faults from polish- 

 ing glass by phase microscopy using stereoscopic technic. Since there 

 is no satisfactory explanation of the nature of polishing of optical glass 

 surfaces, phase microscopy promises information of both heuristic and 

 practical value. Bright contrast reveals very small defects in the glass 

 surface, and the 0.2A+0.33X plate has been found best for the 16-mm 

 objective. Slight deposits of metals on the surface are also revealed. 

 On the other hand, for the examination of fine ground surfaces (Fig. 

 VI.3) the 5B— 0.25X reveals more detail than the bright (A+) plate, 

 and the dark (A — ) contrast is not useful for this application. The 

 picture shows a finely ground surface in air with an 0.18-mm cover glass 

 placed over it to maintain the correction of the 4-mm objective. 



Smaller fragments of glass may be examined in immersion oil or other 

 suitable mounting fluid (Fig. IV.l). Seeds such as occasionally plug 

 the spinneret of fiber-making equipment may be studied with the phase 

 microscope for identification and structure, and the information gained 

 should suggest means for minimizing the numbers per melt. Like other 



