EXAMINATION OF SURFACES 215 



changes. The phase microscope is appropriate for preserved as wall as 

 for unstained hving tissues, because poorly staining fibers to the lens are 

 readily seen in mounted sections. 



The information obtained with phase microscopy on unstained 

 preparations has included much that was known previously, and in 

 addition an histology of li^'ing tissues is evoh'ing. By changing the 

 refractive index of the mounting medium, the contrast may be controlled 

 so that certain features show well while the visibility of others is sup- 

 pressed (see Section 3.2 of Chapter IV). Some of the changes possible 

 are shown in Fig. V.7. Several diffraction plates may be required to 

 fully comprehend a specimen. Kef erring to the nephron, Oliver (1948, 

 p. 188) wrote: ". . . these illustrations emphasize the necessity of the 

 use of varying phase contrasts in the examination of a tissue if one is to 

 feel confident that he has seen all of the structural detail within the 

 specimen." 



7. TISSUE CULTURE 



The cells of tissue cultures are especially well suited for examination 

 with the phase microscope because they are too transparent for bright- 

 field and the path differences are rather small for darkfield. Prepara- 

 tions can be studied in many of the standard culture dishes with a 

 long-focus microscope condenser. Hollow-ground slides are contra- 

 indicated. Because of the curvature, only the cells at the top of roUei- 

 tubes can be seen clearly. With the thicker vessels more care is re- 

 qviired in centering the equipment because of the wedge effect caused by 

 local surface irregularities and lack of parallelism of the sides of the 

 flasks (see Section 3.1 of Chapter IV). 



Living spirochetes of syphilis in tissue cultures of testicular cells have 

 been seen with phase by Perry (1948). The increased detail seen with 

 the phase microscope is most effectively demonstrated in the motion 

 pictures by Firor and Gey (1947) and by Danes (1949), which were 

 discussed in Section 4 of Chapter V. 



For the 16-mm objective the 0.2A+0.33X diffraction plate has been 

 preferred for the examination of tissue cultures. With the 4-mm 

 objective, low and medium A± contrast and low B — contrast have been 

 chosen for this application. Some investigators would add the high 

 bright contrast (0.07A+0.25X) to the list for the oil immersion objec- 

 tive. (See also Ludford and Smiles, 1950; Sylven, 1950.) 



8. EXAMINATION OF SURFACES 



Opaque materials including some tissues can be examined with the 

 phase vertical illuminator when they have good reflectivity (see Section 8 



