PHASE OBJECTIVES 113 



distance for use with specimens mounted on slides of greater than 

 standard thickness or for use with specimens in cells or flasks. Curved 

 surfaces on a specimen cell or flask can distort the image of the con- 

 denser diaphragm enough to make the diffraction plate useless. If 

 the slide or cell has a sufficiently great wedge angle or if it has a wavy 

 surface, it may be necessary to keep recentering the condenser diaphragm 

 with respect to the conjugate area of the diffraction plate as the slide or 

 cell is moved across the field of the microscope. If the mounted specimen 

 acts as a lens and changes the magnification of the image of the con- 

 denser diaphragm, this can be compensated for by substituting a new 

 condenser diaphragm or by introducing a substage condenser with 

 variable power. 



4. SOME FEATURES OF PHASE EQUIPMENT OF DIFFERENT MANU- 

 FACTURE 



Brief descriptions of various makes of phase accessories available at 

 present will illustrate some of the means adopted to satisfy the require- 

 ments of phase microscopy. The starting point for each has been the 

 objectives, the condensers, and the stands designed previously for 

 ordinary microscopy. Some manufacturers have released for publica- 

 tion photographs of their phase microscope stands, and reproductions of 

 these photographs are included in Fig. III. 5. Most of the photographs 

 and much of the descriptive information were obtained during the 

 summer of 1949. The notation (1949) either on the photograph or in 

 the text indicates this fact. Reference will be made to achromatic or 

 apochromatic objectives for phase microscopy. This describes the 

 objective before the diffraction plate or coating was added and does not 

 imply, for example, that the diffraction plate itself has been achromatized. 



4.1. Phase objectives 



Phase objectives have been made by adding diffraction plates or 

 coatings to objectives of existing design. The standard phase accessories 

 now being sold form neutral images, in general, if white light illuminates 

 a thin, transparent specimen. Also, the standard phase objectives 

 are supplied as units which contain a diffraction plate, or the equivalent, 

 that is not separable from the objective. There is nothing in the ex- 

 ternal appearance of the phase objective to distinguish it from the 

 ordinary objective except that the maker takes care to engrave identifica- 

 tion marks on the mount. The diffraction plate or coating can be seen 

 only by looking through the objective. 



All manufacturers offer a series of phase objectives which produce dark 

 contrast with a transparent object specimen that has an optical path 



