90 Modern Microscopy 



optical portion is shown in Eig. 24. It gives a brilliant 

 illumination, with the highest power objective, while with 

 the lower powers, by removing the top lens, good results can 

 be obtained. Its price is low, but it has the great disad- 

 vantage of not being achromatic, and having so large an 

 amount of spherical aberration as to be almost useless for 

 critical work, for although its total aperture is large, its 

 aplanatic aperture is less than '50. Nevertheless, it fills 

 a distinct position in microscopical work, and is very easy 

 to use. 



The Abbe illuminator was followed some years later by 

 an achromatic condenser, having a numerical aperture of 1*0, 

 which also originated with the firm of Carl Zeiss, and was 

 subsequently made by several London opticians. This was 

 a substantial improvement on the older chromatic pattern. 

 It, however, had the disadvantage of being somewhat large 

 and heavy, and yielded an aplanatic aperture of less than 

 '70. The necessity for using a condenser that yields a solid 

 cone of illumination at least f of the total aperture of the 

 objective with which it is to be worked has become univer- 

 sally recognised, and has resulted in the introduction by 

 all the leading London houses of condensers which possess 

 no disadvantage as compared with the Abbe condensers, 

 while their efficiency is enormously greater. 



One reason why the Abbe condensers have enjoyed 

 popularity has been the facility with which they can be 

 used, and this was in a considerable measure due to the 

 field lens being of a large size. Several condensers with 

 large aplanatic apertures, but with comparatively small 

 field lens, have been introduced, but Mr. C. Baker, of 244, 

 High Holborn, was the first to offer to the microscopist a 

 sub-stage condenser, which he described as a modification 

 of the Abbe achromatic, with a field lens of sufficiently 

 large diameter (|-inch) for quick and easy use. In this 

 condenser the aplanatic aperture is *90 and the magnifying 

 power iVinch. It is shown in Fig 25. 



This was quickly followed by a condenser of similar 



