CONDENSERS 111 



sensitive to errors in centring of the lenses, and any 

 alteration would be liable to upset the accuracy with 

 which they were constructed. 



Choice of a Condenser. It is not at all easy to make 

 selection of a condenser, when, as is so often the case, a 

 single one has to be used for all purposes. It is generally 

 best to get a moderately high-power condenser, as its 

 front lens can be removed and the back combination then 

 forms a useful low-power illuminator with some sacrifice 

 of correction however. There is considerable difference 

 of opinion over the extent of the advantages presented 

 by aplanatic and achromatic condensers over that of 

 Abbe, apart from the shorter exposures required if the 

 former are used ; and if delicate work, done with an Abbe 

 condenser, is compared with similar work with the more 

 expensive corrected types, it is extremely difficult to find 

 any difference. The superiority of the more highly 

 corrected types is, however, now generally recognised as 

 a factor in the production of a perfect image, and on 

 that account it is certainly best to select one of them, 

 but if cost is a material consideration an Abbe will serve 

 very well. 



Although the aplanatic aperture of an Abbe illuminator 

 is, as a rule, only about 0-5, this does not mean that it 

 will restrict the N.A. of any of the high-power objectives 

 that can be used with it to 0-5. A measure of the effect of 

 the aperture of the condenser on the performance of the 

 objective is given by the mean of the aplanatic apertures 

 of the objective and condenser, and the figure is known 

 as the working aperture. Thus an objective of N.A. 1-4 

 can only be used to its full advantage with an oil-immer- 

 sion condenser having an aplanatic cone of 1-4. If an 

 achromatic condenser of N.A. 1-0 and an aplanatic cone 

 of 0'9 is used, the working aperture is reduced to (1*4 + -9) 

 ^2 = 1' 15, while an Abbe condenser gives a working 

 aperture of only (1-4-j- -5) -=-2 =0-95 with the same objec- 



