366 



OSCAR W. RICHARDS 



A brightfield microscope without the condenser is used with mono- 

 chromatic light from a sodium or mercury arc (screened with proper 

 filters to isolate the line desired) , or tungsten light with an interference 

 filter {e.g., 555 mn) (63), for the examination of the specimen mounted 

 between suitable plates with partially metallized surfaces. Merton 

 states that for practical purposes the method is hmited to a specimen 

 a few microns thick, but this will actually depend on the transparency 

 and optical path differences in the specimen. Surface details and 

 internal detail due to optical path differences may be visualized with 

 the interference microscope, but it has been too little used with biolog- 

 ical materials to suggest what will be its ultimate contributions and 

 limitations. 



I. TRANSPARENT AND SLIGHTLY ABSORBING 

 SPECIMENS— PHASE MICROSCOPY 



Phase microscopy utilizes optical path and absorption differences 

 in the microscope to increase or decrease, or reverse and increase or 

 decrease, the contrast in the image from transparent specimens hav- 

 ing optical path differences, or of low absorption contrast (Fig. 9). 



Fig. 9. Wheat chromosomes stained with acetocarmine: (A) brightfield 

 microscope; (B) dark contrast; (C) bright contrast; (D) bright contrast plus 

 red filter with phase microscope. 



The method is applicable to living unstained cells and tissues, crystals, 

 colloids, natural and artificial fibers, surface and internal detail of 

 glass and plastics, minerals, stained or colored materials of low con- 

 trast, and replicas of surfaces (39,40). 



