ALIGNMENT OF THE PHASE MICROSCOPE 111 



in the field of the centering telescope when the conjugate area is absorbing anJ the 

 condenser diaphragm is not centered. Figure III.4d shows the appearance of this 

 image after the condenser diaphragm has been centered with respect to the conjugate 

 area of the diffraction plate. If the ol)ject specimen is such that it deviates a con- 

 siderable amount of light into the complementary area, the overlap between the 

 edges of the conjugate area and the image of the edges defining the annular opening 

 in the condenser diaphragm can easily be seen. Figure III.4e shows the condenser 

 diaphragm decentered with respect to a difTraction plate in which an absorbing 

 coating has been put on the complementary area of the diiTraction plate, and Fig. 

 111.4/ shows the correct alignment in this case. 



(5. Remove the centering telescope and replace the eyepiece. Close the lamp 

 diaphragm until its image is slightly smaller than the field of the microscope. If 

 necessary, adjust the sul:)stage mirror until the image of the lamp iris is centered in 

 the field of the microscope. With regard to the opening in the iris diaphragm of 

 the lamp, the phase microscope differs in two ways from the ordinary microscope. 

 The diffraction plate will cause a halo to appear around the image of the lamp dia- 

 phragm. For this reason the diaphragm in front of the lamp should not be closed 

 so far that the halo about its image is superimposed on the image of the detail being 

 observed. Further, if a high-power phase objective (e.g., an oil immersion objective) 

 is used with an Abbe-type substage condenser set in the position for Kohler illumina- 

 tion, a check should be made to ascertain that when the lamp diaphragm is closed 

 down'as a field stop the back aperture of the phase objective is uniformly illuminated 

 at least to the outer edge of the conjugate area. Spherical aberration may produce 

 vignetting when an annular opening at high numerical aperture admits light through 

 the condenser unless the source of light at the lamp is made broad. To check this, 

 use the centering telescope to look at the conjugate area of the diffraction plate. 

 It may be helpful to remove the condenser diaphragm temporarily in making this 

 check. This check may indicate the necessity of increasing the opening in the iris 

 diaphragm of the lamp beyond the minimum opening contemplated for the purposes 

 of a field stop. 



The number of refinements which the user of the phase microscope 

 wishes to make depends on his problem and on the type of phase ac- 

 cessories. After the microscopist is famiUar with his equipment, setting 

 up the phase microscope to perform well takes very little more time than 

 setting up the ordinary microscope to perform well. There will always 

 be variations in procedure due to differences in the design of the many 

 available microscope stands. Sometimes the instructions are, simply, 

 instead of step 3, to insert the centering telescope and to move the 

 condenser up or down until the image of the opening in the condenser 

 diaphragm almost fills the conjugate area of the diffraction plate when 

 the condenser diaphragm has been centered with respect to the diffrac- 

 tion plate. This procedure substitutes for steps 3 and 5. 



A separate step to align the iris diaphragm of the lamp, the substage 

 iris diaphragm, and the eyepiece is sometimes recommended. This 

 procedure requires that the lamp be set at a suitable distance from the 

 substage condenser mount and that the filament of the lamp be imaged 



