168 THE TECHNICS OF PHASE MICROSCOPY 



is operated at full aperture, and it is unnecessary partially to close the 

 condenser diaphragm as must be done with the brightfield microscope 

 in order to obtain enough edge diffraction to see such a specimen. In 

 practical microscopy no decrease in resolution is noted with the phase 

 microscope, and spurious images are less likely to occur. Examination 

 of diatoms and other materials has confirmed this prediction. Oettle 

 (1950) has apparently confused halos with boundaries of cells. 



The phase microscope has proved helpful when combined with other 

 methods of microscopy. Within the resolving power of the light micro- 

 scope, details can be seen in replicas which aid in interpretation and 

 orientation when further study is to be made with the electron micro- 

 scope. Ludford et al. (1948) reported that the phase microscope was 

 useful in preparation of specimens for study with the ultraviolet micro- 

 scope. Direct ultraviolet microscopy is possible with the phase micro- 

 scope, as demonstrated by Bennett et al. (1948), and provides the ad- 

 vantages of greater resolution. The results will depend on how the 

 specific absorptions at specific wavelengths combine with the optical 

 path differences in the specimen. Photographic and photoelectric re- 

 cording will make possible;, quantitative microspectrophotometry. 

 Phase microscopy also is possible with long-wavelength infrared, and 

 the specific absorptions may be used for microchemical analysis, even 

 with specimens such as dark insect exoskeleta, red corals, and minerals 

 that are opaque to light. Photoelectric translation is possible for direct 

 vision with a 1P25 tube, as demonstrated by Bailly (1948), as with 

 ordinary and polarization microscopy. 



The marked clarity of image with great detail is the most impressive 

 aspect of the phase microscope. Although the microscope is not dif- 

 ficult to operate, some knowledge and experience must be gained before 

 it can be used at full efficiency. 



1. COMPARISON OF PHASE MICROSCOPY WITH OTHER METHODS 



When absorption is adequate and reasonably selective, the specimen 

 may be examined with brightfield, ultraviolet, or infrared microscopes, 

 depending on the region of the spectrum absorbed. When absorption 

 is too weak, from inadecjuate staining or slight pigmentation, the phase 

 microscope may add enough contrast to permit examination, and it may 

 also reveal fine details that would be overlooked with brightfield methods 

 on specimens of fairly low contrast. 



Transparent specimens with internal detail due to differences in 

 optical path may be examined with optical staining, darkfield, inter- 

 ference, polarizing, and phase microscopy. When the optical path dif- 

 ferences are small, the phase microscope is preferable. With larger 



