204 PHASE MICROSCOPY IN BIOLOGY AND MEDICINE 



metachromatic and volutin granules, while the harsher contrast of the 

 A— type obscures the detail below the upper layers (Fig. V.3, J and K). 

 Nuclear behavior in stained and unstained yeast has been reported 

 by the Lindegrens (1947). 



Pathogenic fungi in preparations from skin and hair may be seen to 

 advantage with the phase microscope. Alkali clearing is not always 

 necessary. Most pathologists working in this field have preferred the 

 medium dark contrast although the high bright contrast is useful for 

 locating bits of mycelium. The flagellum on the spore of Ashbya 

 (Fig. V.3, A and B) does not show with brightfield because the micro- 

 scope condenser has to be closed too much for resolution, but it does 

 show with the full aperture of the phase microscope. 



Mycelium in tissues may or may not show with the phase micro- 

 scope, depending on the similarity of the optical paths. Granted equal 

 visibility with the tissue cells, identification of fungi is usually easier 

 because the granulation and other structure of the mycelium are clearly 

 seen. Sometimes light staining and the phase microscope are preferable 

 to brightfield microscopy. 



A special diffraction plate (5B— 0.15X) and narrower annulus were 

 developed for counting mold fragments in tomato products. The 

 change in the annulus was necessary to reduce the numerical aperture 

 of the 16-mm objective to include the greater depth of the Howard 

 mold chamber. This was found helpful by the mycologists who depend 

 on the structure of the mycelium for identification, but others who 

 depend solely on the diffraction image of the mycelium could not use 

 phase because diffraction patterns were not visible. 



3.3. Algae and higher plants 



Considerable detail can be seen in living diatoms, desmids, and 

 similar forms and in some even after formalin fixation (Fig. V.4, C and E). 

 Mounted diatoms show better when differences in refractive index and 

 the siliceous shell are not too great (Bennett et al., 1946). For details 

 at the limit of resolution of the light microscope, electron microscopy 

 appears more promising. The phase microscope has been used by 

 Bonner (1950) in a study of polarity in a shme mold. 



Different phase objectives are useful in studying the details of the 

 filamentous algae. In living Spirogyra the pyrenoids show better with 

 a 5B-0.25X diffraction plate, the protoplasmic strands with a high 

 bright-contrast (0.07A+0.25X), and the vacuole with either a 

 0.2A±0.25X low-contrast diffraction plate. Chlorophyll-containing 

 regions usually reveal more detail with the B — diffraction plate, as do 

 algae living as symbionts within lichens or protozoa. Detail suggestive 



