EMULSIONS AND SUSPENSIONS 181 



When the specimen is incompatil:)le with or would be damaged by 

 oily materials, it is necessary to devise aqueous or other mixtures. 

 Some of these will be described under special applications of phase 

 microscopy. For living materials the possibilities are more limited in 

 that mountants must be non-toxic and not alter the specimen by 

 osmotic or other effects. Some change in refractive index may be 

 obtained by dissolving proteins or other materials of such large mole- 

 cules that they are inactive osmotically in the mountant. Fortunately, 

 small differences are more valuable in phase microscopy, and it may be 

 possible to obtain differences in refractive index without damage to the 

 specimen; e.g., change in index of blood serum is 0.00013 per degree 

 C (du Noiiy, 1929, 1933). Polyvinyl alcohol, methyl cellulose, gelatin, 

 cooked starch, plastics, and other materials are possible mounting media 

 in which the concentration can be varied to produce differences in re- 

 fractive index. 



Inhomogeneous specimens may be examined in several different media 

 to obtain the l)est picture of each component. Moimting small known 

 materials, microscopic glass spheres, threads, etc., with the specimen 

 may make estimation of the path differences possible by direct observa- 

 tion or by photometering as suggested by Martin (19-47). When vari- 

 able phase becomes available it will be possible to investigate local 

 gradients in specimens. 



Aqueous and other mounting fluids change in refractive index with 

 changes in temperature; the use of a warm stage with them provides 

 small changes that are helpful in observing small details. 



Establishing concentration gradients by flowing a more or less con- 

 centrated media under the cover glass brings out differences in some 

 specimens. This is readily done by placing the drop in contact with 

 one side of the cover glass and a piece of dry filter paper against the 

 opposite side. Small changes in refractive index from slight differences 

 in concentration are visible as a form of microschlieren. 



3.3. Emulsions and suspensions 



Suspensions of opaque pigments or strongly colored materials are seen 

 to advantage with the brightfield microscope, but when they are colorless 

 or relatively transparent they may be seen with the phase microscope 

 even though invisible with other methods. Suspensions and emulsions 

 may be prepared as wet mounts (diluting when necessary) or as dry 

 smears. When a dry smear is to be examined a standard cover glass, 

 0.18 mm thick, should be placed over it; otherwise the spherical aber- 

 ration introduced will degrade the image unduly. If uncovered prepara- 

 tions are to be routine, objectives corrected for use without a cover 



