PHOTOMICROGRAPHY 187 



the problems of equipment, exposure, and materials in detail, and 

 Ballerini and Scandone have described their 16-mm equipment. 



Photomicrographs for reconstructions or models should be made with 

 the objective of highest numerical aperture appropriate to the specimen 

 in order to obtain optical sections ha^'ing minimal depth of field. When 

 thicker sections are to be photographed an objective with the lowest 

 numerical aperture capable of giving the recjuired resolution should be 

 combined with higher power oculars to achieve the necessary magnifica- 

 tion. A magnification of 1000 times the numerical aperture of the 

 objective is often satisfactory. However, greater magnifications may 

 be needed with the phase microscope to bring out the increased contrast 

 and detail revealed. The ordinary classical Abbe theory of resolution 

 is not adequate as a guide in photomicrography, and it is rarely applicable 

 in practical microscopy in which extended objects are commonly observed 

 rather than the separation of two adjacent geometrical image points. 

 For example, the distance between the divided bacterial cells in Fig. 

 V.IP and V.IQ is much less than the limit of resolution based on the 

 Abbe formula (X/2 N.A.), yet the edges are sharp and the greater 

 resolution of the electron microscope confirms the record. 



One of the main ad^'antages of phase microscopy is that the sharp 

 edges of the specimens are free from indefinite diffraction patterns. 

 Measurements become possible, and this is important with living micro- 

 organisms and colloidal materials, wherein staining methods would so 

 alter the size that the measurements could not be interpreted. How- 

 ever, such measurement is not often possible visually because of the 

 motion of the specimen, and ordinary photomicrographic procedures do 

 not permit exposures fast enough to avoid blurring from movement. 

 This problem was solved in 1945 when Richards, Foster, and Wennemark 

 took phase photomicrographs with an Edgerton type of electronic flash. 

 The newer General Electric FT-230 flash tube has proved even more 

 satisfactory (Richards, 19476, Laporte, 1950). A lens system (Fig. 

 IV. 5) magnifies and images the electrodes of the tube on either side of 

 the opening in the microscope condenser. Discharging an electrical con- 

 denser through the tube provides a rapid flash uniform enough to fill 

 the aperture of the microscope and short enough to effectively stop 

 Brownian and other motions of the specimen. It is necessary to add a 

 source of continuous light for focusing the microscope. This is accom- 

 plished by focusing the aerial image of the filament of a small micro- 

 scope illuminator between the electrodes of the FT-230 tube. This 

 image is automatically focused in the microscope aperture and turned 

 off before the high-speed flash exposure. The light is adequate for 



