Photomicrography 279 



must have a flat ribbon filament or a homogeneous arc, and a con- 

 denser lens. 



The mechanical set-up and the operation of a particular com- 

 mercial apparatus should be obtained from the directions supplied 

 by the manufacturer. The principles will be outlined on the basis 

 of the apparatus shown in Fig. 17.7. The usual sequence of opera- 

 tions is as follows: 



1. Adjust the lamp condenser to give a beam of parallel rays. 

 This can be done with adequate practical accuracy by focusing the 

 filament upon a wall 8-10 ft. away. Lock the lamp condenser perma- 

 nently into tliis position in relation to the filament. 



2. Remove all optical components from the stage and camera 

 and center the filament image on the ground glass of the camera 

 back. The filament will not be in sharp focus, but do not change 

 the setting of the lamp condenser. 



3. Insert the Micro Tessar objective and center it into the filament 

 image by shifting the objective, not the light beam. 



4. Insert the substage condenser that has the same focal length as 

 the objective. Center the condenser by moving the condenser, not 

 the light beam. 



5. Place the slide on the stage and bring the object into sharp 

 focus on the ground glass. 



6. Try various Wratten filters and use the combination that gives 

 the desired balance between contrast and detail. 



7. Make the exposure. 



The above optical system transmits enough light, if the Wratten 

 filters are removed, to register adequately on a good exposure meter 

 such as the Weston or General Electric. Exposure factors can be 

 worked out for a given optical system, filter combination and type 

 of negative material. 



