TRANSPARENT SPECIMENS 



177 



specimen. When the refractive index and dispersion of the specimen 

 and the mounting medium match, the specimen becomes invisible. 

 Hejice, if media of known index are available, the index of the specimen 

 can be determined. In fact, the usual Becke line method is more 

 sensitive when used with the phase microscope. Should this type of 

 investigation be followed to the exclusion of other work, it would be 

 desirable to have a narrow diffraction plate and corresponding annulus 

 so that the observed contrast will be harsh, thereby further increasing 

 the sensitivity of the method. As halftone detail would no longer 

 appear, such a phase system would not be useful for general microscopy. 



As one focuses a little above or below the specimen a bright line is 

 seen to move along the edges of the specimen. This Becke line moves 

 into the region of greater refractive index as the microscope is focused 

 upward and into the region of lower index as the microscope is focused 

 downward. It aids in determining refractive index and interpreting 

 optical path differences. 



When the mounting medium has no adverse effect on the specimen 

 and a range of refracti^'e indices is available, the contrast can be varied 

 by changing the medium until visibility is optimal for the specimen 

 under observation (Table IV. 1, Fig. V.7). 



It is possible to make one or more components of a mixture invisible 

 while intensifying the appearance of another component under observa- 

 tion. Differences in refractive index of 0.05 or less are useful. Com- 

 bined with dispersion staining (Section 3.5), all the advantages of 

 ordinary differential staining become possible for transparent specimens 

 with the phase microscope, without staining or damaging the specimen. 

 This may be important when only microscopic amounts of material are 

 available and the identification must be made without altering the 

 specimen. 



Table IV.l 



Media for mounting specimens for microscopical examination 



A. Liquids 



Water 



Tyrode's solution 



Glycerin and water (equal parts) 



Perfluoro lube oil (du Pont) 



Silicone oil No. 9996-100 



Methyl cellosolve 



Dioxan 



Chamomile oil 



Glycerin 



Corn syrup (Karo) 



Reference 



Thorell (1947) 



Blout and Mellors (1949) 



'^CAl 



lUJ 



