24 



ELEMENTARY CHEMICAL MICROSCOPY 



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 problems, is doubtless responsible 

 for more errors in interpretation in microscopic examinations 

 than any cause other than excessive 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 theAbbe condenser 

 until the numerical aperture is 

 no greater than that of the ob- 

 jective. It will be found to be 

 a safe general rule to lower the 

 Abbe condenser and to close 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 

 condenser; the light rays then take the path roughly indicated 

 in Fig. 5. 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 

 to one side of the center. The disks furnished with the conden- 

 ser, consisting of a central stop with narrow slots, yield very 



Fig. 5. 



Diagram of Abbe Condenser; 

 Oblique Light. 



