156 A MANUAL OF BACTERIOLOGY 



therefore, special eye-pieces or " compensating oculars " 

 must be used. In ordinary bacteriological work apochro- 

 matic objectives, which are relatively costly, are not 

 essential, and achromatic objectives of considerable 

 perfection are obtainable. But for the best work, involv- 

 ing resolution of the finest microscopic structure, apochro- 

 matic objectives surpass achromatic ones ; they are also 

 better for photomicrography. Another advantage of 

 apochromatic lenses is that, being more perfectly corrected 

 than achromatic ones, higher eye-pieces may be employed 

 and the same magnification obtained with a lower-power 

 objective. Thus, a ^-in. or J-in. oil-immersion apochro- 

 matic lens will do all the work that a y^-in. oil-immersion 

 achromatic one will do (provided, of course, they have 

 the same N.A.), with the added advantages of greater 

 working distance, larger and flatter field, more penetra- 

 tion and more illumination. 



The bacteriological microscope is usually supplied for 

 ordinary work with three achromatic objectives, at least 

 a | -in. and a J-in. dry, and a -^-in. (2 mm.) oil-immersion, 

 the last-named of N.A. of about 1-3. Two eye-pieces 

 or oculars are required, a 1 and a 3, or an A and a C, 

 according to the designation employed, magnifying about 

 four, and six or eight, times respectively. It is probable 

 that in the future, oculars will be marked in accordance 

 with their magnifying power (e.g. x 4), instead of being 

 designated by numbers or letters. While objectives 

 nowadays are always provided with the standard screw 

 of the Royal Microscopical Society and will fit on to any 

 microscope, oculars vary a good deal in diameter ; the 

 R.M.S. has, however, proposed to standardise these to 

 three sizes. Oculars that are too small to fit the tube 

 of a particular microscope may be provided with an 

 adapter. 



The J-in. objective should be of long focus, so that it 



