Photomicrography 217 



uniform Ijiilliancc. Many laboratories do not have a lanij) suital)lc 

 for this system, and it is not used extensively. 



The KoJiler system of illumination is the most practical and 

 widely used method. In the Kohler method the image of the filament 

 is focused on the substage condenser diaphragm, and the image of 

 the lamp diaphragm is focused in the plane of the specimen. The 

 operations usually are performed in the following order: 



1. Direct the beam of light upon the mirror, with no filters or 

 other screens in the beam. Open the substage diaphragm completely, 

 reduce the lamp diaphragm aperture, and manipulate the mirror 

 until the light reflected back from the lower lens of the substage 

 condenser to the mirror is projected by the mirror as a spot of light, 

 exactly centered on the lamp diaphragm. This position of the mirror 

 must not be altered. In a horizontal apparatus a mirror is not 

 used, and this step is omitted. If the filter holder is adjustable, insert 

 any dense filter and adjust the holder until (a) the beam of light 

 is centered in the filter and (b) the light that is reflected from the 

 back surface of the filter is centered on the lamp diaphragm. 



2. Open the lamp diaphragm, close the substage diaphragm and 

 focus the lamp condenser until the filament image is sharply defined 

 on the substage diaphragm. This is the permanent setting of the 

 lamp condenser. 



3. Bring the object into focus with the objective that is to be 

 used to take the photograph. Use a neutral filter to reduce the light 

 enough for visual comfort. 



4. Open the substage diaphragm completely and partly close the 

 lamp diaphragm. Rack the substage condenser up and down until the 

 lamp diaphragm, with its edges sharply defined, is superimposed on 

 the sharply focused specimen. 



5. Replace the ocular with a pinhole ocular, look down into the 

 tube and bring the spot of light in the back lens of the objective 

 into the exact center by manipulating the centering screws of the 

 substage condenser. (Not by moving the mirror.) Open the substage 

 diaphragm fully. If the polygonal disk of illumination — which rep- 

 resents the lamp diaphragm — does not cover the desired area of 

 the object, remove the upper element of the substage condenser and 

 repeat operation 4. It may be necessary to remove the upper two 

 elements of the substage condenser to obtain a large enough illumin- 

 ated field. 



6. Replace the ocular. Hold a 3X to 6X magnifier above the 



