OBJECTIVES 157 



will work through the thick cover-glasses of the hsemo- 

 cytometer. 



If dark-ground illumination is to be used, the ^-in. 

 oil-iinmersion lens of Zeiss is one of the best. If funds 

 permit, apochromatic objectives will be chosen of 

 similar power to the achromatic ones mentioned above. 

 This will also entail the purchase of a set of compensating 

 oculars. If expense be no object, the Zeiss J-in. (3 mm.) 

 oil-immersion objective of N.A. 1-4 is one of the finest that 

 can be obtained, and may take the place of a -^-in. oil- 

 immersion for all purposes. It must be noted, however, that 

 oil-immersion lenses of 1-4 N.A. are more "fragile" 

 than ones of 1-3 N.A., because, in order to obtain the wide 

 angle necessary for the high N.A., the front lens has to be 

 made hyperspherical and the cell or metal setting of the 

 lens has to be thinner, and the hold on the lens is, there- 

 fore, not so firm. Lenses, known as semi-apochromats, 

 are also to be had. Fluorite is used in their construction, 

 and the corrections are more perfect than in the achro- 

 mats, though not so perfect as in the apochromats. Their 

 price is lower than that of the apochromats, and no compen- 

 sating oculars are necessary. A T V m - oil-immersion semi- 

 apochromat of N.A. 1-3, or a little over, is a very service- 

 able lens. The oil-immersion objectives must be wiped 

 clean after use with a soft silk rag or Japanese paper. 

 Sticky or dry oil may be removed with the same, moistened 

 with xylol (not spirit). 



For convenience of work, most microscopists employ 

 a double or triple nose-piece, so that the objectives may 

 be changed by simply rotating the nose-piece. This adds 

 15-20 mm. to the length of the tube, and must be com- 

 pensated for in fine work by adjustment with the inner 

 draw-tube. A rotating nose-piece, moreover, rarely or 

 never optically centres the objectives attached to it. 

 If absolute centration be required, as it is in critical 



