112 INSTRUMENTATION 



on the substage iris diaphragm before the phase objective, the ocular, 

 the substage condenser, and the condenser diaphragm are mounted on 

 the microscope stand. To continue this method of ahgnment, place a 

 pinhole eyepiece in the body tube of the microscope and close both the 

 substage iris diaphragm and the iris diaphragm of the lamp to their 

 smallest apertures. Adjust the substage mirror so that the aperture of 

 the lamp is seen through the pinhole eyepiece and the closed substage 

 iris diaphragm. Then, without moving the sul)stage mirror again, 

 follow the remainder of the procedure suggested in steps 1, 2, 3, 5, and 6. 

 However, if the condenser to be used is not in a centering mount, it may 

 be necessary to readjust the mirror to center the image of the iris 

 diaphragm of the lamp in the field of the microscope when step 6 is 

 reached. It may also be impossible to insert the condenser in its 

 mount without tilting the substage mirror. Therefore this additional 

 check on alignment was not included in the suggested general procedure. 



If the coating on the diffraction plate consists of only transparent 

 dielectric materials, then it is difficult to distinguish the boundaries of 

 the conjugate area by means of the centering telescope when the diffrac- 

 tion plate is illuminated l\y the entire cone of light admitted by the 

 annular opening in the condenser diaphragm. This introduces a 

 visibility problem in centering the condenser diaphragm. Usually the 

 manufacturer of microscopes of good quality has taken sufficient care in 

 centering the optical components of the objective, including the diffrac- 

 tion plate or coating, with respect to each other and with respect to the 

 mount so that if one phase objective is replaced by another of the same 

 power the opening in the condenser diaphragm may be only slightly 

 decentered with respect to the conjugate area of the diffraction plate. 

 Therefore one may center the opening in the condenser diaphragm with 

 respect to the conjugate area of a phase objective which contains an 

 absorbing material, replace this phase objective w^th the non-absorbing 

 phase objective, insert the eyepiece of the microscope, and adjust the 

 condenser diaphragm until best contrast is obtained in the image of the 

 object specimen. However, it is possible to make the edges of the 

 conjugate area of a transparent diffraction plate visible in the field of 

 the centering telescope by cutting off part of the cone of light which 

 illuminates the conjugate area in order to produce oblique illumination. 

 This can be done either by tilting the substage mirror or by blocking 

 out one side of the light beam which normally enters the substage 

 condenser by sliding any convenient opaque obstacle into the light beam. 

 When the condenser diaphragm has been centered, the mirror is read- 

 justed or the obstacle is removed. 



Some manufacturers supply a phase condenser with a long working 



