94 INSTRUMENTATION 



photomicrography is used in front of the hght source, although the green 

 filter is preferred. 



2.2. Diflfraction plate with 3X/4 optical path step 



In Section 1 the diffraction plate recommended for producing dark 

 contrast when the object specimen has an optical path greater than that 

 of its surround is one in which the optical path through the comple- 

 mentary area exceeds the optical path through the conjugate area by 

 3^ wavelength. A diffraction plate in which the optical path through 

 the conjugate area exceeds the optical path through the complementary 

 area by ^ wavelength will also produce dark contrast in the image of 

 the same specimen. If the diffraction plate is made of materials which 

 do not form an achromatic diffraction plate when the optical path step 

 is X/4 at some wavelength, then the departure from achromatism is even 

 greater when the optical path step is 3X/4 at that same wavelength. 



If a diffraction plate is incorporated into an objective and the con- 

 denser diaphragm is removed, the phase objective will function as an 

 ordinary objective, but the definition may be impaired by a small 

 amount. Just as the spherical aberration of the objective affects the 

 performance of the diffraction plate, so the coating of the diffraction 

 plate alters the spherical aberration of the objective as it was originally 

 designed. A 3X/4 step in a diffraction plate would alter the spherical 

 aberration of the ordinary objective more than would a X/4 step. For 

 very critical work with the ordinary objective, it is in general advisable 

 not to use an objective containing a diffraction plate. 



2.3. Principles of the achromatic diffraction plate 



The advantages of an achromatic diffraction plate are seen best when 

 the optical path difference between the object specimen and its surround 

 is so small that the diffraction plate is producing low contrast, especially 

 if photomicrographs of the low contrast image are taken with colored 

 filters in front of the light source, and when some detail in a strongly 

 colored object specimen can be made better visible by the phase micro- 

 scope. 



To simplify the considerations for the achromatic and the color 

 diffraction plates in the remaining subsections, reference to a dark- 

 contrast diffraction plate will imply that 5 = —90°, and reference to a 

 bright-contrast diffraction plate will imply that 5 = +90°. Again, it 

 will be assumed that any absorbing material incorporated into the 

 diffraction plate produces only a negligible change in the optical path 

 differences introduced by the dielectric materials. It is also to be as- 



