TflK CONDENSER 81 



circle of the condenser may be centered within the aperture 

 circle of the objective. If the eyepiece circle is magnified 

 by a lens centered over it magnifying 10 or 15 times, the 

 above centering is easier (Beck). (A 15-times compen- 

 sating eyepiece put above will sometimes serve.) {d) If 

 the image of the diaphragm in front of the source of light 

 has been centered by a correctly centered bright-field 

 condenser, a dark-field condenser may then be centered 

 in the same sleeve by this image of the light. 



The condenser should be centered accurately to the 

 high oil-immersion objective. This can be done by cen- 

 tering the image of a S-miUimeter source diaphragm and a 

 small object, with the high oil-immersion objective in 

 question. The low-power objective is then turned on, with- 

 out disturbing light or object. The image of the contracted 

 condenser iris, seen by racking up the low objective, is 

 then made concentric, by the centering collar of the 

 condenser, with the images of the small diaphragm and 

 the object, which have themselves been made concentric 

 with the high oil-immersion objective. If there is no large 

 centering collar to the condenser, which is in a loose sleeve 

 with a tightener (like those of Zeiss) th^re may be screws 

 serving to give a certain amount of adjustment, or the 

 centering can even be adjusted by a piece of paper of the 

 proper thickness gummed in the right place on the side 

 of the condenser. 



It seems best for a centering collar to be small, and 

 either adjusted by eccentric rotation (Zeiss) or by small 

 screws turned by a screwdriver or a watch key. These 

 are less likely to get out of adjustment, when once set, 

 than the large collars with large screw heads. 



The optical part of the condenser, the condenser iris, 

 and the additional achromatic lens (when present) must 

 all be concentric with the high objective, or much light 

 is lost, and the image is abnormal. 



The Uncorrected Condenser. — The uncorrected con- 

 denser, commonly called in English the Abbe condenser, 

 was described by Abbe in 1873. (His firm have made three 



