METAL SURFACES 231 



specimens, small glass particles reciuire higher contrast objectives than 

 larger ones, and the contrast can be adjusted for the best visibility by 

 choosing the proper kind and contrast of diffraction plate and the proper 

 immersion medium. 



Transparent plastics may be examined for regions of inhomogeneity 

 and for impurities. Cotton fibers in the sheet can be recognized with a 

 5B— 0.25X objective. The "dope" may be examined before processing, 

 and the processed product examined either directly or after sectioning to 

 obtain preparations thin enough for microscopic examination. With a 

 long-focus condenser, sheets up to about Y2 inch in thickness may be 

 examined. The 0.2A+, 0.14A+, and 5B— 0.25X diffraction plates are 

 preferable for this. Further information for rayons will be given below. 

 The extent of dyeing of cellophane-like materials may be determined 

 from the examination of cross sections. Medium contrast A+, A — , or 

 B— are suitable, depending on the material and the pigment. 



Replica technics may be applied to the examination of glass and plastic 

 surfaces, and some of these surfaces have enough reflectivity to make 

 the phase vertical illuminator worth while (see Section 8). 



7. PAPER 



The phase microscope is useful to show the breakdown and condition 

 of the fibers, freedom from lignin and other undesirable materials, and 

 for examination and size determination of baryta and other materials 

 for smoothing the surface finish. The color reactions of the special dyes 

 are more useful for fiber identification and show better in brightfield. 

 Typical changes in two common types of papers with different kinds of 

 phase objectives are illustrated in Fig. VI.4. 



Sections of paper may be examined for a study of composition and 

 for the measurement of ink penetration below the surface. Low- 

 contrast (0.2A±0.25X) and high-contrast (5B— 0.25X) objectives are 

 helpful in paper microscopy. 



8. METAL SURFACES 



Two methods are available with the phase microscope. When the 

 surface has considerable specular reflection, the vertical phase il- 

 luminator may be used. The replica technic is applicable to metal as 

 well as to other surfaces. Marx and Diehl (1948) have used phase 

 microscopy with replicas from anodized aluminum. 



Jupnik et al. (1946, 1948) described a phase vertical illuminator 

 equipped with both bright- and dark-contrast diffraction plates and the 

 examination of metals with it. With monochromatic green light from the 

 mercury arc, considerable detail could be seen on specularly reflecting 



