312 



A(( KSSOEY APPARATUS 



stops, A:c. (herthi.s is fitted a ring into which screw adapters, which 

 will allou other condensers to be used on the one mechanism. 



The metal disc should have a central aperture as large as the 

 largest hack lens of any of the combinations to be used with the 

 mount. It should be thick enough to receive two stops or .dia- 

 phragm- at a time. This power to alter a diaphragm or stop so as 

 to secure any required arrangement of apertures and stops without 



FIG. '257. Abbe's achromatic condenser (1888). 



in the least disturbing any of the adjustments of the condenser is a 



practical gain of a very valuable kind. 



Diaphragms should be marked with the numerical aperture they 



yield, and stops should be marked with the numerical aperture of 



the cmie they cut out. Empirical numbers are misleading and 



valueless. This special mark- 

 ing need not involve two sets 

 of diaphragms with two con- 

 denser combinations, one for 

 high and the other for low 

 powers ; the different numeri- 

 cal apertures for each may be 

 marked on either side of the 

 diaphragm or stop. Memory 

 cannot fail if we make the 

 lotrer side of the diaphragm 

 indicate the apertures for the 

 lo\\er power condenser, and 

 vice versa. 



We may note that for 

 dark-ground \\ork. stops 



.-honld lie placed close to the back lens of the condenser, and in the 



of a diaphragm \\hich is le>s important an inch of distance 



should not In- exceeded. This condenser gives dark-ground illuini- 



OD \\ it h objectives of 'Ti N . A . : for such illumination it, is one of 



' he hot illuminators extant . 



FIG. 258. Baker's fitting I'm- Al.l.i-'s 



matic condenser used in English inicro- 



