76 INSTRUMENTATION 



diaphragm shall be focused upon the diffraction plate by the lenses which 

 lie between the condenser diaphragm and the diffraction plate. The 

 choice of the first focal plane of the substage condenser as the location 

 of the condenser diaphragm makes the most practicable use of the diffrac- 

 tion phenomena. However, it is not essential that the condenser dia- 

 phragm he placed at the first focal plane of the condenser. If the 

 condenser diaphragm is put at the first focal plane of a well-corrected 

 substage condenser and if an image of the light source is formed on the 

 condenser diaphragm, then the object specimen is illuminated by 

 substantially parallel light. With an Abbe-type substage condenser, 

 the spherical aberration necessarily present in this type of condenser 

 destroys to some degree the parallelism of the rays which are incident 

 on the object specimen. However, an Abbe-type condenser is a satis- 

 factory substage condenser for the phase microscope, provided that the 

 condenser has the numerical aperture required by the design of the 

 diffraction plate. It can be shown theoretically that the departure from 

 parallelism of the illuminating rays plays only a secondary part in 

 modifying the contrast produced by the phase microscope. Experi- 

 ment also shows that very little effect is produced on contrast in the 

 image when the rays illuminating the specimen are not parallel. The 

 presence of spherical aberration in the substage condenser can affect 

 the distance between the last surface of the condenser and the specimen 

 slide when the phase microscope is lined up and adjusted for use. This 

 may modify the numerical aperture at which the full aperture of the 

 condenser can fimction if the diaphragm is removed and the position of 

 the condenser is not changed. In the presence of aberrations the magni- 

 fication ratio between the condenser diaphragm and the conjugate area 

 of the diffraction plate depends on the distance between the last surface 

 of the substage condenser and the first surface of the objective. 



In order to minimize any effects which may be due only to the orienta- 

 tion of some specimens with respect to the conjugate area of the diffrac- 

 tion plate, the opening in the condenser diaphragm should be circular or 

 annular and should be centered on the optical axis. Burch and Stock 

 (1942) described an experimental arrangement in which the aperture in 

 the diaphragm and the conjugate area of the diffraction plate were slit- 

 shaped. They reported that the image of long, striated muscle fibers, 

 for example, vanished completely when the fibers were oriented exactly 

 perpendicular to the conjugate area. 



Under conditions necessary for good performance the undeviated light 

 and only a very small part of the deviated light will pass through the 

 conjugate area of the diffraction plate, and the greatest fraction of the 

 deviated light will pass through the complementary area. Since in 



