176 THE TECHNICS OF PHASE MICROSCOPY 



refractive index as the wall of the capillary and flattened by a cover 

 glass on the surface of the immersion medium. These methods are 

 helpful when it is necessary to determine whether a local difference in 

 a specimen is due to a difference in thickness or in index (see Section 

 3.2). 



Most of the mountants for permanent preparations have survived 

 over a long period of years because they were good for stained prepara- 

 tions. Others have been devised to give large differences in index, as 

 was necessary with brightfield methods (e.g., Hyrax for diatoms). 

 Many of these are less applicable to phase microscopy, and we may 

 expect discovery of a new series of materials, especially from the re- 

 cently made plastic and synthetic materials. Glycerin jelly mounts are 

 better with many biological materials for phase microscopy than balsam 

 or damar. Tanning the exposed edges with bichromate solution and 

 sealing with Gold Size or synthetic varnish will preserve such prepara- 

 tions for several years. (See Table IV. IC) 



3.2. Transparent specimens 



Transparent specimens may be composed of regions differing from 

 each other in size or in index of refraction; these regions can be ex- 

 amined with the phase microscope when they are above a minimum size 

 (see Section 18 of Chapter II). The present discussion is confined to 

 nearly colorless specimens. Lightly colored specimens will be con- 

 sidered later. 



Since the optical path is the product of the thickness of the object 

 and its refractive index, it is possible for regions of different size to 

 have the same optical path. For example, one region of given thick- 

 ness and index can have the same path as another of half the thickness 

 and twice the refractive index. For these objects the phase microscope 

 will show no difference unless appropriate changes are made in thickness 

 or in mounting medium, or unless other treatment is applied that will 

 affect one component more than the other. Fortunately such regions 

 are not commonly adjacent in many properly prepared specimens. 

 Nevertheless this possibility of misinterpretation should always be kept 

 in mind. 



The optical path differences of transparent specimens are shown in 

 the phase microscope by shades of brightness or darkness. The greater 

 the difference, within the limits of the method, the greater will be the 

 contrast in the image. If the refractive index of the mounting medium 

 is greater than that of the specimen, the specimen will appear dark with 

 an A+ and l)right with an A— diffraction plate, and vice versa when 

 the refractive index of the mounting medium is less than that of the 



