174 THE TECHNICS OF PHASE MICROSCOPY 



The thickness of specimen examinable with the phase microscope 

 depends on its transparency and optical density. Dense tissues or 

 materials up to about 4 ^ thick may be seen, and less dense ones with 

 thickness of 100 m or more may be studied. Transparent izing an op- 

 tically dense material may make examination possible (see Section 3.2). 

 Larger specimens may be prepared for examination by the usual means 

 of crushing, teasing, sedimentation, etc. Embedding and sectioning 

 with a microtome is possible with many materials (Richards, 1949a). 

 Frozen sections of tissues that are not thin enough for profitable examina- 

 tion may be further thinned by being squeezed under a cover glass or 

 between two slides. 



Smears of tissues or fluids make useful preparations. The smear 

 should be thin and even. The usual technic for blood smears can be 

 applied to other fluid specimens. The drop of material is placed near 

 one end of a slide and is spread by touching it with another slide and 

 letting it follow the movement of the spreader slide. Colloids and 

 emulsions may be so treated, although dilution with a suitable fluid may 

 be required. Touching the cut surface of an organ to a drop of fluid 

 usually dislodges enough cells for study. A wet preparation should 

 stand long enough for the solid components to settle out flat on the 

 surface of the slide before examination. Flotation with albumin solu- 

 tions has been used by Fawcett et al. (1950) for the separation of 

 malignant cells from body fluids, and this method could be extended to 

 other materials. 



Opaque materials of high reflectivity may be polished and examined 

 with the phase vertical illuminator (Section 8 of Chapter VI). Other 

 opaque materials may be studied by making a replica of the surface 

 and examining the replica. Cellulose acetate may be softened with a 

 solvent and held against a metal surface until it hardens, as was done 

 with a piston ring (Fig. IV. 2). Thinner replicas such as those used 

 with the electron microscope may be examined as made or after shadow- 

 casting. (Barnes et al., 1945; Hershman, 1945; Scott and Wyckoff, 

 1949; and Wyckoff, 1947, describe methods.) Further discussion may 

 be found in Section 8 of Chapter V and Section 8 of Chapter VI. Bright 

 contrast (Fig. IV.3C) reveals the surface detail as contour lines. Rep- 

 licas mounted on the 200-mesh screens can be seen only with difficulty 

 with the phase microscope, because of the light reflected from the wires 

 of the screen. A screen of flattened, oxidized or darker wire would be 

 better when examination with both phase and electron microscopes is 

 contemplated. With the 4-mm objective a cover glass should be placed 

 over the preparation or the objective should be corrected for use without 



