312 



ACCESSORY APPARATUS 



stops, &c. Over this is fitted a ring into which screw adapters, which 

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



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

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

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

 phragms 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 cone 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 

 loiver side of the diaphragm 

 indicate the apertures for the 

 lower-power condenser, and 

 vice versa. 



We may note that for 

 dark-ground work, stops 

 should be placed close to the back lens of the condenser, and in the 

 case of a diaphragm which is less important an inch of distance 

 should not be exceeded. This condenser gives dark-ground illumi- 

 nation with objectives of '5 N.A. ; for such illumination it is one of 

 the best illuminators extant. 



FIG. 258. Baker's fitting for Abbe's achro- 

 matic condenser used in English micro- 

 scopes. 



