ILLUMINATION OF OBJECTS; ILLUMINATING DEVICES 33 



In the diagram, Fig. 9, the passage of the light rays is roughly 

 indicated for a position of the Abbe condenser when used with 

 an objective of low numerical aperture. The iris diaphragm 

 is shown well closed. Usually it is advisable to also lower the 

 condenser. Failure to employ the Abbe condenser in the proper 

 manner or to appreciate the fact that a different adjustment is 

 required to meet different prob- 

 lems, is doubtless responsible for 

 more errors in interpretation in 

 microscopic examinations than 

 any cause other than exces- 

 sive magnification. Since very 

 few dry achromatic objectives 

 have a high numerical aperture 

 it is evident that in order to 

 obtain the best results it will be 

 essential with all such optical 

 combinations to close the iris 

 diaphragm of the Abbe con- 

 denser until the numerical aper- 

 ture is no greater than that of 

 the objective. It will be found 

 to be a safe general rule to lower 

 the Abbe condenser and to close FIG. 10. 

 its iris diaphragm to a diameter 

 about two-thirds or one-half that of the rear lens opening of 

 the objective. The size of the diaphragm opening may easily 

 be adjusted by removing the ocular, looking into the tube of 

 the microscope and closing the diaphragm until the bright disk 

 of light is reduced one-half or two-thirds. 



Oblique illumination with the Abbe condenser is quickest 

 and most easily obtained by the method suggested by Wright 

 of holding a finger below and half across the opening of the con- 

 denser; the light rays then take the path roughly indicated in 

 Fig. 10. Or we may drop upon the swing-out ring attached 

 to the bottom of the condenser mounting a half-disk of black 

 paper or cardboard, or a disk provided with a circular opening 



Diagram of Abbe Condenser; 

 Oblique Light. 



