LIGHT (OPTICAL) MICROSCOPY 



SOURCE IMAGED IN 

 SPECIMEN PLANE 



CRITICAL ILLUMINATION 



LAMP CONDENSER IMAGED 

 IN SPECIMEN PLANE 



FIELD DIAPHRAGM 



SOURCE IMAGED IN 

 APERTURE DIAPHRAGM 



KOEHLER ILLUMINATION 



Fig. 4. Ray-paths in critical and Koehler illumination. The Koehler form of illumination is most 

 widely used, since it gives even illumination despite lack of uniformity in the source. 



<::T:ri 



FOCUSABLE 

 ELEMENT 



ABBE CONDENSER 



VARIABLE FOCUS CONDENSER 



ACHROMATIC CONDENSER 



Fig. 5. Condenser types. The simple Abbe type at the left and the modified variable focus Abbe 

 in the center are the most commonlj' used forms. The more complex achromatic form however gives 

 more color-free illumination and higher N.A. 



450 



