108 INSTRUMENTATION 



close to the plane of the conjugate area of the diffraction plate as is 

 practicable. 



It has been stressed that the size and location of the image of the 

 opening in the condenser diaphragm with respect to the conjugate area 

 of the diffraction plate is important. If a lamp with a coiled filament 

 is used in Kohler illimiination, a plate of diffusing glass may finally be 

 put in front of the lamp without loss of contrast or definition. Emphasis 

 is laid on the fact that the procedure recommended by the manufacturer 

 should be followed in order to obtain the correct position of the substage 

 condenser and the diaphragm relative to the objective and the diffraction 

 plate. After such instructions have been carried out, the lamp filament 

 can be imaged by means of the lamp condenser so that the proper 

 conditions for either critical or Kohler illumination are satisfied. 



The following enumerated steps are suggested as a useful procedure 

 for the inexperienced but critical microscopist in setting up his phase 

 microscope. However, the experienced microscopist may find it desir- 

 able to modify this procedure. Kohler illumination will be indicated 

 to the extent that the iris diaphragm in front of the lamp will form the 

 field stop and that the lamp filament will be imaged in the plane of the 

 condenser diaphragm. A plate of diffusing glass may later be placed 

 in front of the lamp. It will be assumed that a separate lamp together 

 with a substage mirror directs the light through the substage condenser. 

 The stands of some phase microscopes contain a built-in source of 

 illumination which should be adjusted according to instructions given by 

 the maker. 



1. Mount the substage condenser together with the appropriate diaphragm, the 

 phase objective, and the eyepiece on the microscojK' stand. Place the specimen shde 

 on the stage. Set the himp so that the iris (haphragm which controls its aperture 

 is a suitable distance from the substage mirror. Some manufacturers specify the 

 distance from the iris diaphragm of the lamp to their substage condenser. If the 

 substage condenser is of the Abbe type, N.A. 1.25, then 5 to 8 inches is a nominal 

 distance. Adjust both the lamp and the substage mirror so that light reaches the 

 center of the mirror and is reflected appro.ximately through the center of the sub- 

 stage condenser and of the objective. If the working distance of the substage con- 

 denser is known, it is helpful for the next step to rack the condenser upward to 

 approximately this distance from the specimen slide. The user must learn from the 

 manufacturer whether the substage condenser must be immersed (e.g., in oil) when 

 it is used with an immersion phase objective. The method of phase microscopy 

 requires that the lamp and substage condenser fill only the conjugate area of the 

 diffraction plate with light. Therefore some manufacturers have designed the 

 diaphragm for the immersion phase objective so that the substage condenser is used 

 dry. Open the iris diaphragm of the substage condenser beyond the opening in the 

 condenser diaphragm. Make certain that the tube length of the microscope is that 

 required by the design of the objective. 



