IMPORTANCE OF APLANATIC CONDENSE!! 309 



and in practice wholly disregarded. Ko doubt a student instructed 

 on these lines would be astonished indeed when he exchanged such a 

 practice for the illumination and improved image afforded by ;ui 

 Abbe condenser. 



Usually such exchange of illuminating method presages an ex- 

 change of instrument, for the scientifically imperfect and wholly 

 unsatisfactory 'tool 'that is in the majority of cases put into the 

 hands of the medical student will not lend itself even to an Abbe 

 condenser. 



The fact is that a large part of the admiration that has been ex- 

 pressed for this condenser has resulted, not from a comparison of its 

 results with those of oilier high-class 

 achromatic condensers, but of images 

 obtained without any sub-stage optical 

 arrangements at all, placed in contrast 

 with the results obtained by using this 

 condenser against the same objective 

 when used without its aid. But that 

 even these images are entirely inferior 

 to the images obtained by the higher FIG. 254. Optical .arrangement of 

 order of achromatic condensers we oillv Abbe's chromatic condenser. 

 require the practical testimony of 



Professor Abbe to prove ; for /ts>. has since produced an achromatic 

 condenser of much merit, to which we give consideration below. 



In its most perfect form this chromatic condenser of Abbe's con- 

 sists of three single lenses, the front being hemispherical, and the 

 two lower lenses form a Herschelan doublet. This combination is 

 shown in fig. 254, and the general form of the instrument, as applied 

 to Zeiss's own microscopes, is shown in fig. 255. 



The power of this condenser is low, and its aperture is very large 

 (1'36); hence, beyond the fact that it is not achromatised, it has 

 enormous spherical aberration. The distance between the foci of 

 the central portion and of a narrow annular zone whose internal 

 diameter is |th inch is Vth inch. Its aplanatic aperture is therefore 

 only - 5. Now, whilst it is a gain of 110 inconsiderable character to 

 have an achromatised condenser, yet the point of vital importance is 

 that it should be aplanatic ; the best condenser is always that which 

 will transmit the largest aplanatic cone. At the close of this section 

 we furnish a table of the relative qualities of the condensers of the 

 best construction now accessible to the microscopist. and a reference 

 to this will show that Powell and Lealand's dry achromatic (fig. 240), 

 with the top removed, is in this respect as efficient as this form of 

 Abbe's. 



This condenser can be used either dry or homogeneously ; but of 

 course with objectives of greater aperture than TO the base of the 

 slide should always lie in oil contact with the condenser. 



It gives the principal modifications from direct to oblique illu- 

 mination with transmitted light by changing and moving a set of 

 diaphragms placed in a movable fitting, and the diaphragm may be 

 moved eccentrically to the optical axis of the condenser by moving 

 the milled head. It gives dark-ground illumination with objectives 



